Prehistoric beasts

COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC.

2

8

TABLE OF CONTENTS

ScientificAmerican.comexclusive online issue no. 6PREHISTORIC BEASTS

Feathered dinosaurs, walking whales, killer kangaroos—these are but a few of the fantastic creatures that roamed theplanet before the dawn of humans. For more than 200 years, scientists have studied fossil remnants of eons past,painstakingly piecing together the history of life on earth. Through their efforts, not only have long-extinct beasts cometo light, but the origins of many modern animals have been revealed.

In this exclusive online issue, Scientific American authors ponder some of the most exciting paleontological discoveriesmade in recent years. Gregory Erickson reexamines T. rex and reconstructs how the monster lived. Ryosuke Motanidescribes the reign of fishlike reptiles known as ichthyosaurs. Kevin Padian and Luis Chiappe trace today’s birds back totheir carnivorous, bipedal dinosaur forebears. And Stephen Wroe presents the menacing relatives of Australia’s belovedpouched mammals. Other articles document the descent of whales from four-legged landlubbers and recount the chal-lenges and rewards of leading fossil-collecting expeditions to uncharted locales. —the Editors

Breathing Life into Tyrannosaurus rexBY GREGORY M. ERICKSON; SCIENTIFIC AMERICAN, SEPTEMBER 1999By analyzing previously overlooked fossils and by taking a second look at some old finds, paleontologists are providing the first glimpses of the actual behavior of the tyrannosaurs

The Teeth of the Tyrannosaurs BY WILLIAM L. ABLER; SCIENTIFIC AMERICAN, SEPTEMBER 1999Their teeth reveal aspects of their hunting and feeding habits

Madagascar's Mesozoic SecretsBY JOHN J. FLYNN AND ANDRÉ R. WYSS, SIDEBAR BY KATE WONG; SCIENTIFIC AMERICAN, FEBRUARY 2002The world's fourth-largest island divulges fossils that could revolutionize scientific views on the origins of dinosaurs and mammals

Rulers of the Jurassic SeasBY RYOSUKE MOTANI; SCIENTIFIC AMERICAN, DECEMBER 2000Fish-shaped reptiles called ichthyosaurs reigned over the oceans for as long as dinosaurs roamed the land, but only recently have paleontologists discovered why these creatures were so successful

The Origin of Birds and Their FlightBY KEVIN PADIAN AND LUIS M. CHIAPPE; SCIENTIFIC AMERICAN, FEBRUARY 1998Anatomical and aerodynamic analyses of fossils and living birds show that birds evolved from small, predatory dinosaurs that lived on the ground

The Mammals That Conquered the Seas BY KATE WONG; SCIENTIFIC AMERICAN, MAY 2002New fossils and DNA analyses elucidate the remarkable evolutionary history of whales

Killer Kangaroos and Other Murderous Marsupials BY STEPHEN WROE; SCIENTIFIC AMERICAN, MAY 1999Australian mammals were not all as cute as koalas. Some were as ferocious as they were bizarre

1 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE APRIL 2003COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC.

10

18

26

36

45

Scientific American September 1999 1Breathing Life into Tyrannosaurus rex

Breathing Life into Tyrannosaurus rex

By analyzing previously overlooked fossils and by taking a second look at some old finds, paleontologists are providing the first glimpses of the actual behavior of the tyrannosaurs

by Gregory M. Erickson

Originally publishedSeptember 1999


TYRANNOSAURUS REX defends its meal, a Triceratops, from other hungry T. rex. Tro-odontids, the small velociraptors at the bottomleft, wait for scraps left by the tyrannosaurs,while pterosaurs circle overhead on this typ-ical day some 65 million years ago. Trees andflowering plants complete the landscape; grass-es have yet to evolve.K

AZ

UH

IKO

SA

NO


APRIL 20034 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE

Dinosaurs ceased to walk theearth 65 million years ago,yet they still live among us.

Velociraptors star in movies, and Tricer-atops clutter toddlers’ bedrooms. Ofthese charismatic animals, however, onespecies has always ruled our fantasies.Children, Steven Spielberg and profes-sional paleontologists agree that the su-perstar of the dinosaurs was and isTyrannosaurus rex.

Harvard University paleontologistStephen Jay Gould has said that everyspecies designation represents a theoryabout that animal. The very nameTyrannosaurus rex—“tyrant lizardking”—evokes a powerful image of thisspecies. John R. Horner of MontanaState University and science writer DonLessem wrote in their book The Com-plete T. Rex, “We’re lucky to have theopportunity to know T. rex, study it,imagine it, and let it scare us. Most ofall, we’re lucky T. rex is dead.” And pa-leontologist Robert T. Bakker of theGlenrock Paleontological Museum inWyoming described T. rex as a “10,000-pound [4,500-kilogram] roadrunnerfrom hell,” a tribute to its obvious sizeand power.

In Spielberg’s Jurassic Park, whichboasted the most accurate popular de-piction of dinosaurs ever, T. rex was, asusual, presented as a killing machinewhose sole purpose was aggressive,bloodthirsty attacks on helpless prey. T.rex’s popular persona, however, is asmuch a function of artistic license as ofconcrete scientific evidence. A centuryof study and the existence of 22 fairlycomplete T. rex specimens have generat-ed substantial information about itsanatomy. But inferring behavior fromanatomy alone is perilous, and the truenature of T. rex continues to be largelyshrouded in mystery. Whether it waseven primarily a predator or a scavengeris still the subject of debate.

Over the past decade, a new breed ofscientists has begun to unravel some ofT. rex’s better-kept secrets. These paleo-biologists try to put a creature’s remainsin a living context—they attempt to ani-mate the silent and still skeleton of themuseum display. T. rex is thus changingbefore our eyes as paleobiologists usefossil clues, some new and some previ-ously overlooked, to develop fresh ideasabout the nature of these magnificentanimals.

Rather than draw conclusions aboutbehavior solely based on anatomy, pale-obiologists demand proof of actual ac-tivities. Skeletal assemblages of multipleindividuals shine a light on the interac-tions among T. rex and between themand other species. In addition, so-calledtrace fossils reveal activities throughphysical evidence, such as bite marks inbones and wear patterns in teeth. Alsoof great value as trace fossils are copro-lites, fossilized feces. (Remains of a herbi-vore, such as Triceratops or Edmon-tosaurus, in T. rex coprolites certainlyprovide “smoking gun” proof of speciesinteractions!)

One assumption that paleobiologistsare willing to make is that closely relat-ed species may have behaved in similarways. T. rex data are therefore beingcorroborated by comparisons with thoseof earlier members of the family Tyran-nosauridae, including their cousins Al-bertosaurus, Gorgosaurus and Dasple-tosaurus, collectively known asalbertosaurs.

Solo or Social?

Tyrannosaurs are usually depicted assolitary, as was certainly the case in

Jurassic Park. (An alternative excusefor that film’s loner is that the movie’sgenetic wizards wisely created onlyone.) Mounting evidence, however,points to gregarious T. rex behavior, atleast for part of the animals’ lives. TwoT. rex excavations in the Hell CreekFormation of eastern Montana aremost compelling.

In 1966 Los Angeles County Muse-um researchers attempting to exhume aHell Creek adult were elated to findanother, smaller individual restingatop the T. rex they had originallysought. This second fossil was iden-tified at first as a more petite species oftyrannosaur. My examination of thehistological evidence—the micro-structure of the bones—now suggeststhat the second animal was actually asubadult T. rex. A similar discoverywas made during the excavation of“Sue,” the largest and most completefossil T. rex ever found. Sue is perhapsas famous for her $8.36-million auc-tion price following ownership hag-gling as for her paleontological status[see “No Bones about It,” News andAnalysis, Scientific American, De-

cember 1997]. Remains of a secondadult, a juvenile and an infant T. rexwere later found in Sue’s quarry. Re-searchers who have worked the HellCreek Formation, myself included,generally agree that long odds argueagainst multiple, loner T. rex findingtheir way to the same burial. The moreparsimonious explanation is that theanimals were part of a group.

An even more spectacular find from1910 further suggests gregarious behav-ior among the Tyrannosauridae. Re-searchers from the American Museumof Natural History in New York Cityworking in Alberta, Canada, found abone bed—a deposit with fossils ofmany individuals—holding at least nineof T. rex’s close relatives, albertosaurs.

Philip J. Currie and his team from theRoyal Tyrrell Museum of Paleontologyin Alberta recently relocated the 1910find and are conducting the first de-tailed study of the assemblage. Such ag-gregations of carnivorous animals canoccur when one after another getscaught in a trap, such as a mud hole orsoft sediment at a river’s edge, in whicha prey animal that has attracted them isalready ensnared. Under those circum-stances, however, the collection of fos-sils should also contain those of thehunted herbivore. The lack of such her-bivore remains among the albertosaurs(and among the four–T. rex assemblagethat included Sue) indicates that theherd most likely associated with oneanother naturally and perished togetherfrom drought, disease or drowning.

From examination of the remains col-lected so far, Currie estimates that theanimals ranged from four to almostnine meters (13 to 29 feet) in length.This variation in size hints at a groupcomposed of juveniles and adults. Oneindividual is considerably larger andmore robust than the others. Althoughit might have been a different species ofalbertosaur, a mixed bunch seems un-likely. I believe that if T. rex relatives didindeed have a social structure, thislargest individual may have been the pa-triarch or matriarch of the herd.

Tyrannosaurs in herds, with complexinterrelationships, are in many ways anentirely new species to contemplate. Butscience has not morphed them into a be-nign and tender collection of CretaceousCare Bears: some of the very testimonyfor T. rex group interaction is partially


5 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE APRIL 2003

healed bite marks that reveal nasty in-terpersonal skills. A paper just pub-lished by Currie and Darren Tanke, alsoat the Royal Tyrrell Museum, highlightsthis evidence. Tanke is a leading author-ity on paleopathology—the study of an-cient injuries and disease. He has detect-ed a unique pattern of bite marksamong theropods, the group of carnivo-rous dinosaurs that encompasses T. rexand other tyrannosaurs. These bitemarks consist of gouges and punctureson the sides of the snout, on the sidesand bottom of the jaws, and occasional-ly on the top and back of the skull.

Interpreting these wounds, Tanke andCurrie reconstructed how these dino-saurs fought. They believe that the ani-mals faced off but primarily gnawed atone another with one side of their com-plement of massive teeth rather thansnapping from the front. The workersalso surmise that the jaw-gripping be-havior accounts for peculiar bite marksfound on the sides of tyrannosaur teeth.The bite patterns imply that the com-

batants maintained theirheads at the same levelthroughout a confrontation.Based on the magnitude ofsome of the fossil wounds, T.rex clearly showed little re-serve and sometimes inflict-ed severe damage to its con-specific foe. One tyran-nosaur studied by Tanke andCurrie sports a souvenirtooth, embedded in its ownjaw, perhaps left by a fellowcombatant.

The usual subjects—food,mates and territory—mayhave prompted the vigorousdisagreements among tyran-nosaurs. Whatever the moti-vation behind the fighting,the fossil record demon-strates that the behaviorwas repeated throughout atyrannosaur’s life. Injuriesamong younger individualsseem to have been morecommon, possibly because ajuvenile was subject to attackby members of his own agegroup as well as by largeadults. (Nevertheless, thefossil record may also beslightly misleading and sim-ply contain more evidence ofinjuries in young T. rex.Nonlethal injuries to adults

would have eventually healed, destroy-ing the evidence. Juveniles were morelikely to die from adult-inflicted injuries,and they carried those wounds to thegrave.)

Bites and Bits

Imagine the large canine teeth of a ba-boon or lion. Now imagine a mouth-

ful of much larger canine-type teeth, thesize of railroad spikes and with serratededges. Kevin Padian of the University ofCalifornia at Berkeley has summed upthe appearance of the huge daggers thatwere T. rex teeth: “lethal bananas.”

Despite the obvious potential of suchweapons, the general opinion among pa-leontologists had been that dinosaurbite marks were rare. The few publishedreports before 1990 consisted of briefcomments buried in articles describingmore sweeping new finds, and the cluesin the marred remains concerning be-havior escaped contemplation.

Nevertheless, some researchers specu-

lated about the teeth. As early as 1973,Ralph E. Molnar of the Queensland Mu-seum in Australia began musing aboutthe strength of the teeth, based on theirshape. Later, James O. Farlow of Indi-ana University–Purdue University FortWayne and Daniel L. Brinkman of YaleUniversity performed elaborate mor-phological studies of tyrannosaur denti-tion, which made them confident thatthe “lethal bananas” were robust, thanksto their rounded cross-sectional con-figuration, and would endure bone-shat-tering impacts during feeding.

In 1992 I was able to provide materialsupport for such speculation. Kenneth H.Olson, a Lutheran pastor and superbamateur fossil collector for the Museumof the Rockies in Bozeman, Mont., cameto me with several specimens. One was aone-meter-wide, 1.5-meter-long partialpelvis from an adult Triceratops. Theother was a toe bone from an adultEdmontosaurus (duck-billed dinosaur). Iexamined Olson’s specimens and foundthat both bones were riddled with gougesand punctures up to 12 centimeters longand several centimeters deep. The Tricer-atops pelvis had nearly 80 such indenta-tions. I documented the size and shape ofthe marks and used orthodontic dentalputty to make casts of some of the deep-er holes. The teeth that had made theholes were spaced some 10 centimetersapart. They left punctures with eye-shaped cross sections. They clearly in-cluded carinas, elevated cutting edges,on their anterior and posterior faces.And those edges were serrated. The to-tality of the evidence pointed to theseindentations being the first definitivebite marks from a T. rex.

This finding had considerable behav-ioral implications. It confirmed for thefirst time the assumption that T. rex fedon its two most common contempo-raries, Triceratops and Edmontosaurus.Furthermore, the bite patterns opened awindow into T. rex’s actual feeding tech-niques, which apparently involved twodistinct biting behaviors. T. rex usuallyused the “puncture and pull” strategy,in which biting deeply with enormousforce was followed by drawing theteeth through the penetrated flesh andbone, which typically produced longgashes. In this way, a T. rex appears tohave detached the pelvis found by Ol-son from the rest of the Triceratops tor-so. T. rex also employed a nipping ap-proach in which the front (incisiform)teeth grasped and stripped the flesh in

NIPPING STRATEGY (above) enabled T. rex to removestrips of flesh in tight spots, such as between vertebrae,using only the front teeth.

PATR

ICIA

C.W

YNN

E;G

REG

ORY

M.E

RIC

KSO

N (i

nse

t)

MASSIVE FORCE generated by T. rex in the “punc-ture and pull” biting technique (above) was sufficient tohave created the huge furrows on the surface of the sec-tion of a fossil Triceratops pelvis (inset)


APRIL 20036 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE

tight spots between vertebrae, whereonly the muzzle of the beast could fit.This method left vertically aligned, par-allel furrows in the bone.

Many of the bites on the Triceratopspelvis were spaced only a few centimetersapart, as if the T. rex had methodicallyworked his way across the hunk ofmeat as we would nibble an ear of corn.With each bite, T. rex appears also tohave removed a small section of bone.We presumed that the missing bone hadbeen consumed, confirmation for whichshortly came, and from an unusualsource.

In 1997 Karen Chin of the U.S. Geo-logical Survey received a peculiar, ta-pered mass that had been unearthed bya crew from the Royal SaskatchewanMuseum. The object, which weighed7.1 kilograms and measured 44 by 16by 13 centimeters, proved to be a T. rexcoprolite. The specimen, the first everconfirmed from a theropod and morethan twice as large as any previously re-ported meat-eater’s coprolite, waschock-full of pulverized bone. Onceagain making use of histological meth-ods, Chin and I determined that theshattered bone came from a young her-bivorous dinosaur. T. rex did indeed in-gest parts of the bones of its foodsources and, furthermore, partially di-gested these items with strong enzymesor stomach acids.

Following the lead of Farlow andMolnar, Olson and I have argued vehe-mently that T. rex probably left multi-tudinous bite marks, despite the paucityof known specimens. Absence of evi-dence is not evidence of absence, and webelieve two factors account for thistoothy gap in the fossil record. First, re-searchers have never systematicallysearched for bite marks. Even more im-portant, collectors have had a naturalbias against finds that might displaybite marks. Historically, museums de-sire complete skeletons rather than sin-gle, isolated parts. But whole skeletonstend to be the remains of animals thatdied from causes other than predationand were rapidly buried before beingdismembered by scavengers. The shred-ded bits of bodies eschewed by muse-ums, such as the Triceratops pelvis, areprecisely those specimens most likely tocarry the evidence of feeding.

Indeed, Aase Roland Jacobsen of theRoyal Tyrrell Museum recently sur-veyed isolated partial skeletal remainsand compared them with nearly com-plete skeletons in Alberta. She found

that 3.5 times as many of the indi-vidual bones (14 percent) bore thero-pod bite marks as did the less disrupt-ed remains (4 percent). Paleobiologiststherefore view the majority of the world’snatural history museums as desertsof behavioral evidence when comparedwith fossils still lying in the field waitingto be discovered and interpreted.

Hawk or Vulture?

Some features of tyrannosaur biology,such as coloration, vocalizations or

mating displays, may remain mysteries.But their feeding behavior is accessiblethrough the fossil record. The collectionof more trace fossils may finally settle agreat debate in paleontology—the 80-year controversy over whether T. rexwas a predator or a scavenger.

When T. rex was first found a centuryago, scientists immediately labeled it apredator. But sharp claws and powerfuljaws do not necessarily a predator make.For example, most bears are omnivo-rous and kill only a small proportion oftheir food. In 1917 Canadian paleontol-ogist Lawrence Lambe examined a par-tial albertosaur skull and ascertainedthat tyrannosaurs fed on soft, rottingcarrion. He came to this conclusion af-ter noticing that the teeth were relativelyfree of wear. (Future research wouldshow that 40 percent of shed tyran-nosaur teeth are severely worn and bro-ken, damage that occurs in a mere twoto three years, based on my estimates oftheir rates of tooth replacement.) Lambethus established the minority view thatthe beasts were in fact giant terrestrial“vultures.” The ensuing arguments inthe predator-versus-scavenger disputehave centered on the anatomy and phys-ical capabilities of T. rex, leading to atiresome game of point-counterpoint.

Scavenger advocates adopted the“weak tooth theory,” which maintainedthat T. rex’s elongate teeth would havefailed in predatory struggles or in boneimpacts. They also contended that itsdiminutive arms precluded lethal at-tacks and that T. rex would have beentoo slow to run down prey.

Predator supporters answered withbiomechanical data. They cited my ownbite-force studies that demonstrate thatT. rex teeth were actually quite robust.(I personally will remain uncommittedin this argument until the discovery of di-rect physical proof.) They also note thatKenneth Carpenter of the Denver Muse-um of Natural History and Matthew

Smith, then at the Museum of the Rock-ies, estimate that the “puny” arms of aT. rex could curl nearly 180 kilograms.And they point to the work of Per Chris-tiansen of the University of Copenhagen,who believes, based on limb proportion,that T. rex may have been able to sprintat 47 kilometers per hour. Such speedwould be faster than that of any of T. rex’scontemporaries, although endurance andagility, which are difficult to quantify, areequally important in such considera-tions.

Even these biomechanical studies failto resolve the predator-scavenger de-bate—and they never will. The criticaldeterminant of T. rex’s ecological nicheis discovering how and to what degree itutilized the animals living and dying inits environment, rather than establishingits presumed adeptness for killing. Bothsides concede that predaceous animals,such as lions and spotted hyenas, willscavenge and that classic scavengers,such as vultures, will sometimes kill.And mounting physical evidence leads tothe conclusion that tyrannosaurs bothhunted and scavenged.

Within T. rex’s former range exist bonebeds consisting of hundreds and some-times thousands of edmontosaurs thatdied from floods, droughts and causesother than predation. Bite marks andshed tooth crowns in these edmonto-saur assemblages attest to scavengingbehavior by T. rex. Jacobsen has foundcomparable evidence for albertosaur sca-venging. Carpenter, on the other hand,has provided solid proof of predaceousbehavior, in the form of an unsuccessfulattack by a T. rex on an adult Edmonto-saurus. The intended prey escaped withseveral broken tailbones that later healed.The only animal with the stature, properdentition and biting force to account forthis injury is T. rex.

Quantification of such discoveries canhelp determine the degree to which T.rex undertook each method of obtain-ing food, and paleontologists can avoidfuture arguments by adopting standarddefinitions of predator and scavenger.Such a convention is necessary, as a widerange of views pervades vertebrate pale-ontology as to what exactly makes foreach kind of feeder. For example, someextremists contend that if a carnivorousanimal consumes any carrion at all, itshould be called a scavenger. But such aconstrained definition negates a mean-ingful ecological distinction, as it wouldinclude nearly all the world’s carnivo-rous birds and mammals.



In a definition more consistent withmost paleontologists’ common-sense cat-egorization, a predatory species wouldbe one in which most individuals acquiremost of their meals from animals they ortheir peers killed. Most individuals in ascavenging species, on the other hand,would not be responsible for the deathsof most of their food.

Trace fossils could open the door to asystematic approach to the predator-scavenger controversy, and the resolu-tion could come from testing hypothe-ses about entire patterns of tyrannosaurfeeding preferences. For instance, Ja-cobsen has pointed out that evidence ofa preference for less dangerous or easilycaught animals supports a predatorniche. Conversely, scavengers would beexpected to consume all species equally.

Within this logical framework, Jacob-sen has compelling data supporting pre-dation. She surveyed thousands of di-nosaur bones from Alberta and learnedthat unarmored hadrosaurs are twice aslikely to bear tyrannosaur bite marks asare the more dangerous horned ceratop-sians. Tanke, who participated in thecollection of these bones, relates that nobite marks have been found on the heavi-ly armored, tanklike ankylosaurs.

Jacobsen cautions, though, that otherfactors confuse this set of findings. Mostof the hadrosaur bones are from isolat-ed individuals, but most ceratopsians inher study are from bone beds. Again,these beds contain more whole animalsthat have been fossilized unscathed, cre-ating the kind of tooth-mark bias dis-cussed earlier. A survey of isolated cer-atopsians would be enlightening. Andanalysis of more bite marks that reveal

failed predatory attempts, such as thosereported by Carpenter, could also revealpreferences, or the lack thereof, for lessdangerous prey.

Jacobsen’s finding that cannibalismamong tyrannosaurs was rare—only 2percent of albertosaur bones had alber-tosaur bite marks, whereas 14 percentof herbivore bones did—might also sup-port predatory preferences instead of ascavenging niche for T. rex, particularlyif these animals were in fact gregarious.Assuming that they had no aversion toconsuming flesh of their own kind, itwould be expected that at least as manyT. rex bones would exhibit signs of T.rex dining as do herbivore bones. A sca-venging T. rex would have had to stum-ble on herbivore remains, but if T. rextraveled in herds, freshly dead conspe-cifics would seem to have been a guar-anteed meal.

Coprolites may also provide valuableevidence about whether T. rex had anyfinicky eating habits. Because histologi-cal examination of bone found in copro-lites can give the approximate stage oflife of the consumed animal, Chin and Ihave suggested that coprolites may re-veal a T. rex preference for feeding onvulnerable members of herds, such asthe very young. Such a bias would pointto predation, whereas a more impartialfeeding pattern, matching the normalpatterns of attrition, would indicatescavenging. Meaningful questions maylead to meaningful answers.

Over this century, paleontologists haverecovered enough physical remains ofTyrannosaurus rex to give the world anexcellent idea of what these monsterslooked like. The attempt to discover

what T. rex actually was like relies onthose fossils that carry precious cluesabout the daily activities of dinosaurs.Paleontologists now appreciate the needfor reanalysis of finds that were former-ly ignored and have recognized the bias-es in collection practices, which haveclouded perceptions of dinosaurs. Theintentional pursuit of behavioral datashould accelerate discoveries of dino-saur paleobiology. And new technolo-gies may tease information out of fossilsthat we currently deem of little value.The T. rex, still alive in the imagination,continues to evolve.

GRE

GO

RY M

.ERI

CKS

ON

BONE MICROSTRUCTURE reveals the maturity of the animal under study. Older indi-viduals have bone consisting of Haversian canals (large circles, left), bone tubules thathave replaced naturally occurring microfractures in the more randomly oriented bone ofjuveniles (right). Microscopic examination of bone has shown that individuals thoughtto be members of smaller species are in fact juvenile T. rex.

The Author

GREGORY M. ERICKSON has studieddinosaurs since his first expedition to theHell Creek Formation badlands of easternMontana in 1986. He received his master’sdegree under Jack Horner in 1992 at Mon-tana State University and a doctorate withMarvalee Wake in 1997 from the Universityof California, Berkeley. Erickson is currentlyconducting postdoctoral research at Stan-ford and Brown universities aimed at under-standing the form, function, developmentand evolution of the vertebrate skeleton.Tyrannosaurus rex has been one of his fa-vorite study animals in this pursuit. He haswon the Romer Prize from the Society ofVertebrate Paleontology, the Stoye Awardfrom the American Society of Ichthyologistsand Herpetologists, and the Davis Awardfrom the Society for Integrative and Com-parative Biology. He will shortly become afaculty member in the department of biolog-ical science at Florida State University.

Further Reading

Carnosaur Paleobiology. Ralph E.Molnar and James O. Farlow in Di-nosauria. Edited by David B. Weishampel,Peter Dodson and Halszka Osmolska.University of California Press, 1990.

The Complete T. REX. John Horner andDon Lessem. Simon & Schuster, 1993.

Bite-Force Estimation for TYRAN-NOSAURUS REX from Tooth-MarkedBones. Gregory M. Erickson, Samuel D.van Kirk, Jinntung Su, Marc E. Levenston,William E. Caler and Dennis R. Carter inNature, Vol. 382, pages 706–708; August22, 1996.

Incremental Lines of von Ebner in Di-nosaurs and the Assessment of ToothReplacement Rates Using GrowthLine Counts. Gregory M. Erickson inProceedings of the National Academy ofSciences USA, Vol. 93, No. 25, pages14623–14627; December 10, 1996.

A King-Sized Theropod Coprolite.Karen Chin, Timothy T. Tokaryk, GregoryM. Erickson and Lewis C. Calk in Nature,Vol. 393, pages 680–682; June 18, 1998.


Understanding the teeth is es-sential for reconstructing thehunting and feeding habits of

the tyrannosaurs. The tyrannosaur toothis more or less a cone, slightly curvedand slightly flattened, so that the crosssection is an ellipse. Both the narrow an-terior and posterior surfaces bear rowsof serrations. Their presence has ledmany observers to assume that the teethcut meat the way a serrated steak knifedoes. My colleagues and I, however,were unable to find any definitive studyof the mechanisms by which knives,smooth or serrated, actually cut. Thus,the comparison between tyrannosaurteeth and knives had meaning only as animpetus for research, which I decided toundertake.

Trusting in the logic of evolution, Ibegan with the assumption that tyran-nosaur teeth were well adapted for theirbiological functions. Although investi-gation of the teeth themselves might ap-pear to be the best way of uncoveringtheir characteristics, such direct study islimited; the teeth cannot really be usedfor controlled experiments. For example,doubling the height of a fossil tooth’s ser-rations to monitor changes in cuttingproperties is impossible. So I decided tostudy steel blades whose serrations orsharpness I could alter and then com-

pare these findings with the cutting ac-tion of actual tyrannosaur teeth.

The cutting edges of knives can beeither smooth or serrated. A smoothknife blade is defined by the angle be-tween the two faces and by the radiusof the cutting edge: the smaller the ra-dius, the sharper the edge. Serratedblades, on the other hand, are charac-terized by the height of the serrationsand the distance between them.

To investigate the properties of kniveswith various edges and serrations, I cre-ated a series of smooth-bladed kniveswith varying interfacial angles. I stan-dardized the edge radius for comparablesharpness; when a cutting edge was nolonger visible at 25 magnifications, Istopped sharpening the blade. I alsoproduced a series of serrated edges.

To measure the cutting properties ofthe blades, I mounted them on a butch-er’s saw operated by cords and pulleys,which moved the blades across a seriesof similarly sized pieces of meat thathad been placed on a cutting board. Us-ing weights stacked in baskets at theends of the cords, I measured the down-ward force and drawing force requiredto cut each piece of meat to the samedepth. My simple approach gave consis-tent and provocative results, includingthis important and perhaps unsurprising

one: smooth and serrated blades cut intwo entirely different fashions.

The serrated blade appears to cut meatby a “grip and rip” mechanism. Eachserration penetrates to a distance equalto its own length, isolating a small sec-tion of meat between itself and the adja-cent serration. As the blade moves, eachserration rips that isolated section. Theblade then falls a distance equal to theheight of the serration, and the processrepeats. The blade thus converts a pullingforce into a cutting force.

A smooth blade, however, concen-trates downward force at the tiny cuttingedge. The smaller this edge, the greaterthe force. In effect, the edge crushes themeat until it splits, and pulling or push-ing the blade reduces friction betweenthe blade surface and the meat.

After these discoveries, I mounted ac-tual serrated teeth in the experimentalapparatus, with some unexpected re-sults. The serrated tooth of a fossilshark (Carcharodon megalodon) indeedworks exactly like a serrated knife bladedoes. Yet the serrated edge of even thesharpest tyrannosaur tooth cuts meatmore like a smooth knife blade, and adull one at that. Clearly, all serrationsare not alike. Nevertheless, serrationsare a major and dramatic feature oftyrannosaur teeth. I therefore began to

TheTeeth of the

Tyrannosaursby William L. Abler

Their teeth reveal aspects of their hunting and feeding habits


Originally published in September 1999


wonder whether these serrations serveda function other than cutting.

The serrations on a shark tooth have apyramidal shape. Tyrannosaur serra-tions are more cubelike. Two features ofgreat interest are the gap between serra-tions, called a cella, and the thin slot towhich the cella narrows, called a diaph-ysis. Seeking possible functions of thecellae and diaphyses, I put tyrannosaurteeth directly to the test and used themto cut fresh meat. To my knowledge, thiswas the first time tyrannosaur teeth haveripped flesh in some 65 million years.

I then examined the teeth under themicroscope, which revealed strikingcharacteristics. (Although I was able toinspect a few Tyrannosaurus rex teeth,my cutting experiments were done withteeth of fossil albertosaurs, which aretrue tyrannosaurs and close relatives ofT. rex.) The cellae appear to make ex-cellent traps for grease and other fooddebris. They also provide access to thedeeper diaphyses, which grip and holdfilaments of the victim’s tendon. Tyran-nosaur teeth thus would have harboredbits of meat and grease for extendedperiods. Such food particles are recep-tacles for septic bacteria—even a nipfrom a tyrannosaur, therefore, mighthave been a source of a fatal infection.

Another aspect of tyrannosaur teethencourages contemplation. Neighboringserrations do not meet at the exterior ofthe tooth. They remain separate inside itdown to a depth nearly equal to the ex-terior height of the serration. Wherethey finally do meet, the junction, calledthe ampulla, is flask-shaped rather thanV-shaped. This ampulla seems to haveprotected the tooth from cracking whenforce was applied. Whereas the narrowopening of the diaphysis indeed puthigh pressure on trapped filaments oftendon, the rounded ampulla distribut-ed pressure uniformly around its sur-

face. The ampulla thuseliminated any point ofconcentrated force where acrack might begin.

Apparently, enormouslystrong tyrannosaurs did notrequire razorlike teeth butinstead made other de-mands on their dentition.The teeth functioned lesslike knives than like pegs,which gripped the foodwhile the T. rex pulled it topieces. And the ampullaeprotected the teeth duringthis process.

An additional feature ofits dental anatomy leads tothe conclusion that T. rexdid not chew its food. The teeth haveno occlusal, or articulating, surfacesand rarely touched one another. After itremoved a large chunk of carcass, thetyrannosaur probably swallowed thatpiece whole.

Work from an unexpected quarteralso provides potential help in recon-structing the hunting and feeding habitsof tyrannosaurs. Herpetologist WalterAuffenberg of the University of Floridaspent more than 15 months in Indone-sia studying the largest lizard in theworld, the Komodo dragon [see “TheKomodo Dragon,” by Claudio Ciofi;Scientific American, March].(Paleontologist James O. Farlow ofIndiana University–Purdue UniversityFort Wayne has suggested that the Ko-modo dragon may serve as a livingmodel for the behavior of the tyran-nosaurs.) The dragon’s teeth are re-markably similar in structure to thoseof tyrannosaurs, and the creature iswell known to inflict a dangerously sep-tic bite—an animal that escapes an at-tack with just a flesh wound is often liv-ing on borrowed time. An infectious

bite for tyrannosaurs would lend cre-dence to the argument that the beastswere predators rather than scavengers.As with Komodo dragons, the victim ofwhat appeared to be an unsuccessful at-tack might have received a fatal infec-tion. The dead or dying prey wouldthen be easy pickings to a tyrannosaur,whether the original attacker or merelya fortunate conspecific.

If the armamentarium of tyrannosaursdid include septic oral flora, we can pos-tulate other characteristics of its anato-my. To help maintain a moist environ-ment for its single-celled guests, tyran-nosaurs probably had lips that closedtightly, as well as thick, spongy gumsthat covered the teeth. When tyran-nosaurs ate, pressure between teeth andgums might have cut the latter, causingthem to bleed. The blood in turn may have been a source of nourishmentfor the septic dental bacteria. In thisscenario, the horrific appearance of thefeeding tyrannosaur is further exagger-ated—their mouths would have run redwith their own bloodstained salivawhile they dined.

The Author

WILLIAM L. ABLER received a doctorate in linguistics from the University of Pennsylvania in 1971. Following a postdoctoral appoint-ment in neuropsychology at Stanford University, he joined the faculty of linguistics at the Illinois Institute of Technology. His interests in hu-man origins and evolution eventually led him to contemplate animal models for human evolution and on to the study of dinosaurs, partic-ularly their brains. The appeal of dinosaurs led him to his current position in the Department of Geology at the Field Museum, Chicago.

Further Reading

The Serrated Teeth of Tyrannosaurid Dinosaurs, and Biting Structures in Other Animals. William Abler in Paleobiology, Vol.18, No. 2, pages 161–183; 1992.

Tooth Serrations in Carnivorous Dinosaurs. William Abler in Encyclopedia of Dinosaurs. Edited by Philip J. Currie and Kevin Padi-an. Academic Press, 1997.

EXPERIMENTAL DEVICE (above) for measuring cut-ting forces of various blades: weights attached to cords atthe sides and center cause the blade to make a standardcut of 10 millimeters in a meat sample (represented hereby green rubber).

PHO

TOG

RA

PH C

OU

RTE

SY O

F W

ILLI

AM

L.A

BLER


THE WORLD’S FOURTH-LARGEST ISLAND DIVULGES FOSSILSTHAT COULD REVOLUTIONIZE SCIENTIFIC VIEWS ON THE

ORIGINS OF DINOSAURS AND MAMMALS

M E S O Z O I CS E C R E T S

By John J. Flynn and André R. Wyss

MADAGASCAR’S

T H R E E W E E K S IN TO our first fossil-hunting expedition in Madagascar in 1996, we werebeginning to worry that dust-choked laundry might be all we would have to show for our efforts. We had turned up onlya few random teeth and bones—rough terrain and other logistical difficulties had encumbered our search. With ourfield season drawing rapidly to a close, we finally stumbled on an encouraging clue in the southwestern part of theisland. A tourist map hanging in the visitor center of Isalo National Park marked a local site called “the place of animalbones.” We asked two young men from a neighboring village to take us there right away.


Originally published in February 2002


Our high hopes faded quickly as we realized the bleachedscraps of skeletons eroding out of the hillside belonged tocattle and other modern-day animals. This site, though po-tentially interesting to archaeologists, held no promise of har-boring the much more ancient quarry we were after. Laterthat day another guide, accompanied by two dozen curiouschildren from the village, led us to a second embankmentsimilarly strewn with bones. With great excitement we spot-ted two thumb-size jaw fragments that were undoubtedly an-cient. They belonged to long-extinct, parrot-beaked cousinsof the dinosaurs called rhynchosaurs.

The rhynchosaur bones turned out to be a harbinger of aspectacular slew of prehistoric discoveries yet to come. Sincethen, the world’s fourth-largest island has become a prolificsource of new information about animals that walked theland during the Mesozoic era, the interval of the earth’s histo-ry (from 250 million to 65 million years ago) when both di-nosaurs and mammals were making their debut. We have un-earthed the bones of primitive dinosaurs that we suspect areolder than any found previously. We have also stirred up con-troversy with the discovery of a shrewlike creature that seemsto defy a prominent theory of mammalian history by being inthe “wrong” hemisphere. These exquisite specimens, amongnumerous others collected over five field seasons, have en-abled us to begin painting a picture of ancient Madagascarand to shape our strategy for a sixth expedition this summer.

Much of our research over the past two decades has beenaimed at unraveling the history of land-dwelling animals onthe southern continents. Such questions have driven other pa-leontologists to fossil-rich locales in South Africa, Brazil,Antarctica and India. Rather than probing those establishedsites for additional finds, we were lured to Madagascar: theisland embraces vast swaths of Mesozoic age rocks, but untilrecently only a handful of terrestrial vertebrate fossils fromthat time had been discovered there. Why? We had a hunchthat no one had looked persistently enough to find them.

Persistence became our motto as we launched our 1996 ex-pedition. Our team consisted of a dozen scientists and studentsfrom the U.S. and the University of Antananarivo in Mada-gascar. Among other benefits, our partnership with the coun-try’s leading university facilitated the acquisition of collectingand exporting permits—requisite components of all paleonto-logical fieldwork. Before long, however, we ran headlonginto logistical obstacles that surely contributed to earlier fail-ures to find ancient fossils on the island. Mesozoic deposits inwestern Madagascar are spread over an area roughly the sizeof California. Generations of oxcarts and foot travel havecarved the only trails into more remote areas, and most ofthem are impassable by even the brawniest four-wheel-drivevehicles. We had to haul most of our food, including hun-dreds of pounds of rice, beans and canned meats, from thecapital. Fuel shortages sometimes seriously restricted mobili-ty, and our work was even thwarted by wildfires, which occurfrequently and rage unchecked. New challenges often aroseunexpectedly, requiring us to adjust our plans on the spot.

Perhaps the most daunting obstacle we faced in pros-pecting such a large region was deciding where to begin. For-tunately, we were not planning our search blindly. The pio-neering fieldwork of geologists such as Henri Besairie, whodirected Madagascar’s ministry of mines during the mid-1900s, provided us with large-scale maps of the island’sMesozoic rocks. From those studies we knew that a fortu-itous combination of geologic factors had led to the accumu-lation of a thick blanket of sediments over most of Madagas-car’s western lowlands—and gave us good reason to believethat ancient bones and teeth might have been trapped andpreserved there.

Mostly MammalsAT THE DAWN OF THE MESOZOIC ERA 250 millionyears ago, it would have been possible to walk from Madagas-car to almost anywhere else in the world. All of the planet’slandmasses were united in the supercontinent Pangea, andMadagascar was nestled between the west coast of what is nowIndia and the east coast of present-day Africa (see map). Theworld was a good deal warmer than at present—even the poleswere free of ice. In the supercontinent’s southern region, calledGondwana, enormous rivers coursed into lowland basins thatwould eventually become the Mozambique Channel, which to-day spans the 250 miles between Madagascar and easternAfrica.

These giant basins represent the edge of the geologic gashcreated as Madagascar began pulling away from Africa morethan 240 million years ago. This seemingly destructive pro-cess, called rifting, is an extremely effective way to accumu-late fossils. (Indeed, many of the world’s most important fos-sil vertebrate localities occur in ancient rift settings—includ-ing the famous record of early human evolution in the muchyounger rift basins of east Africa.) The rivers flowing into thebasins carried with them mud, sand, and occasionally thecarcasses or bones of dead animals. Over time the rivers de-posited this material as a sequence of vast layers. Continuedrifting and the growing mass of sediment caused the floors ofthe basins to sink ever deeper. This depositional process per-sisted for nearly 100 million years, until the basin floorsthinned to the breaking point and molten rock ascendedfrom the planet’s interior to fill the gap as new ocean crust.

Up to that point nature had afforded Madagascar threecrucial ingredients required for fossil preservation: dead or-ganisms, holes in which to bury them (rift basins), and mate-rial to cover them (sand and mud). But special conditionswere also needed to ensure that the fossils were not destroyedduring the subsequent 160 million years. Again, geologic cir-cumstances proved fortuitous. As the newly separated land-masses of Africa and Madagascar drifted farther apart, theirsediment-laden coastlines rarely experienced volcanic erup-tions or other events that could have destroyed buried fossils.Also key for fossil preservation is that the ancient rift basinsended up on the western side of the island, which today isdotted with dry forests, grasslands and desert scrub. In amore humid environment, such deposits would have eroded



away or would be hidden under dense vegetation like thekind that hugs much of the island’s eastern coast.

Initially Madagascar remained attached to the other Gon-dwanan landmasses: India, Australia, Antarctica and SouthAmerica. It did not attain islandhood until it split from Indiaabout 90 million years ago. Sometime since then, the islandacquired its suite of bizarre modern creatures, of whichlemurs are the best known. For more than a century, re-searchers have wondered how long these modern creaturesand their ancestors have inhabited the island. Illuminatingdiscoveries by another team of paleontologists indicate thatalmost all major groups of living vertebrates arrived onMadagascar since sometime near the end of the Mesozoic era65 million years ago [see “Modern-Day Mystery,” on page17]. Our own probing has focused on a more ancient interval of Madagascar’s history—the first two periods of the Meso-zoic era.

Pay DirtONE OF THE JOYS OF WORKING in little-charted terrain hasbeen that if we manage to find anything, its scientific signifi-cance is virtually assured. That’s why our first discoveries nearIsalo National Park were so exciting. The same evening in 1996that we found the rhynchosaur jaw fragments, University ofAntananarivo student Léon Razafimanantsoa spotted the six-inch-long skull of another interesting creature. We immediate-ly identified the animal as a peculiar plant eater, neither mam-mal nor reptile, called a traversodontid cynodont.

The rhynchosaur jaws and the exquisite traversodontid

skull—the first significant discoveries of our ongoing U.S.-Malagasy project—invigorated our expedition. The first fos-sil is always the hardest one to find; now we could hunkerdown and do the detailed collecting work necessary to beginpiecing together an image of the past. The white sandstoneswe were excavating had formed from the sand carried by therivers that poured into lowlands as Madagascar unhingedfrom Africa. Within these prehistoric valleys rhynchosaursand traversodontids, both four-legged creatures ranging fromthree to 10 feet in length, probably grazed together much thesame way zebras and wildebeests do in Africa today. Thepresence of rhynchosaurs, which are relatively common incoeval rocks around the world, narrowed the date of this pic-ture to sometime within the Triassic period (the first of threeMesozoic time intervals), which spans from 250 million to205 million years ago. And because traversodontids were muchmore diverse and abundant during the first half of the Triassic

SAR

A C

HE

N

JOHN J. FLYNN and ANDRÉ R. WYSS have collaborated for nearly20 years. Their expeditions have taken them to the Rocky Moun-tains, Baja California, the Andes of Chile, and Madagascar. To-gether they also study the evolutionary history of carnivores, in-cluding dogs, cats, seals, and their living and fossil relatives. Flynnis MacArthur Curator of Fossil Mammals at the Field Museum inChicago, associate chair of the University of Chicago’s committeeon evolutionary biology doctoral program, and adjunct professorat the University of Illinois at Chicago. Wyss is a professor of geo-logical sciences at the University of California, Santa Barbara, anda research associate at the Field Museum. The authors thank theNational Geographic Society, the John C. Meeker family and theWorld Wildlife Fund for their exceptional support of this research.

THE

AU

THO

RS

Jurassic Site: Early tribosphenic mammals

Triassic Site:Early dinosaurs,

rhynchosaurs,traversodontids,chiniquodontids

MADAGASCAR THEN AND NOW

PangeaEarly Triassic Period(240 Million Years Ago)

Isalo Group MesozoicSedimentary Rocks

OtherSedimentary Rocks

CrystallineBasement Rocks

Kilometers

Present-Day Africa

FOSSIL-BEARING ROCKSdrape western Madagascar.These rocks formed from thesand, mud, and occasionalremnants of dead animalsthat accumulated in valleyswhen the island began toseparate from Africa.

0 100 200 300



than during the second, we thought initially that this sceneplayed out sometime before about 230 million years ago.

During our second expedition, in 1997, a third type ofanimal challenged our sense of where we were in time. Short-ly after we arrived in southwestern Madagascar, one of ourfield assistants, a local resident named Mena, showed ussome bones that he had found across the river from our pre-vious localities. We were struck by the fine-grained red rockadhering to the bones—everything we had found until thatpoint was buried in the coarse white sandstone. Mena led usabout half a mile north of the rhynchosaur and traversodon-tid site to the bottom of a deep gully. Within a few minuteswe spotted the bone-producing layer from which his unusualspecimens had rolled. A rich concentration of fossils was en-tombed within the three-foot-thick layer of red mudstones,which had formed in the floodplains of the same ancient riversthat deposited the white sands. Excavation yielded about twodozen specimens of what appeared to be dinosaurs. Our teamfound jaws, strings of vertebrae, hips, claws, an articulatedforearm with some wrist bones, and other assorted skeletalelements. When we examined these and other bones moreclosely, we realized that we had uncovered remains of twodifferent species of prosauropods (not yet formally named),one of which appears to resemble a species from Moroccocalled Azendohsaurus. These prosauropods, which typicallyappear in rocks between 225 million and 190 million yearsold, are smaller-bodied precursors of the long-necked sauro-pod dinosaurs, including such behemoths as Brachiosaurus.

When we discovered that dinosaurs were foraging amongrhynchosaurs and traversodontids, it became clear that wehad unearthed a collection of fossils not known to coexistanywhere else. In Africa, South America and other parts ofthe world, traversodontids are much less abundant and lessdiverse once dinosaurs appear. Similarly, the most commontype of rhynchosaur we found, Isalorhynchus, lacks advancedcharacteristics and thus is inferred to be more ancient than thegroup of rhynchosaurs that is found with other early di-nosaurs. What is more, the Malagasy fossil assemblage lacksremains of several younger reptile groups usually found withthe earliest dinosaurs, including the heavily armored, croco-dilelike phytosaurs and aetosaurs. The occurrence of dino-saurs with more ancient kinds of animals, plus the lack ofyounger groups, suggests that the Malagasy prosauropods areas old as any dinosaur ever discovered, if not older.

Only one early dinosaur site—at Ischigualasto, Argentina—

contains a rock layer that has been dated directly; all otherearly dinosaur sites with similar fossils are thus estimated to beno older than its radioisotopic age of about 228 million years.(Reliable radioisotopic ages for fossils are obtainable onlyfrom rock layers produced by contemporaneous volcanoes.The Malagasy sediments accumulated in a rift basin with novolcanoes nearby.) Based on the fossils present, we have tenta-tively concluded that our dinosaur-bearing rocks slightly pre-date the Ischigualasto time span. And because prosauropodsrepresent one of the major branches of the dinosaur evolu-

Tiny Bones to PickPaleontologists brave wildfires, parasites and scorchingtemperatures in search of ancient mammal fossils

By Kate Wong

THE THREE LAND ROVERS pause while John Flynn consults thedevice in his hand. “Is the GPS happy?” someone asks him. Flynnconcludes that it is, and the caravan continues slowly through thebush, negotiating trails usually traversed by oxcart. We have beendriving since seven this morning, when we left Madagascar’scapital city, Antananarivo. Now, with the afternoon’s azure skymelting into pink and mauve, the group is anxious to locate asuitable campsite. A small cluster of thatched huts comes intoview, and Flynn sends an ambassador party on foot to ask theinhabitants whether we may camp in the area. By the time wereach the nearby clearing, the day’s last light has disappeared andwe pitch our tents in the dark. Tomorrow the real work begins.

The expedition team of seven Malagasies and six Americans, ledby paleontologists Flynn and André Wyss of the Field Museum inChicago and the University of California at Santa Barbara,respectively, has come to this remote part of northwesternMadagascar in search of fossils belonging to early mammals.Previous prospecting in the region had revealed red and buff-colored sediments dating back to the Jurassic period—the ancientspan of time (roughly 205 million to 144 million years ago) duringwhich mammals made their debut. Among the fossils unearthedwas a tiny jaw fragment with big implications.

Conventional wisdom holds that the precursors of modernplacental and marsupial mammals arose toward the end of theJurassic in the Northern Hemisphere, based on the ages andlocations of the earliest remains of these shrewlike creatures,which are characterized by so-called tribosphenic molars. But theMalagasy jaw, which Flynn and Wyss have attributed to a newgenus and species, Ambondro mahabo, possesses tribosphenicteeth and dates back some 167 million years to the Middle Jurassic.As such, their fossil suggests that tribosphenic mammals arose atleast 25 million years earlier than previously thought and possibly

FOUR-INCH-LONG MAMMAL Ambondro mahabo lived in Madagascarabout 167 million years ago.


in the south rather than the north. No one has disputed the age of A. mahabo, but not everyone

agrees that the finding indicates that tribosphenic mammalsoriginated in the south. Fossil-mammal expert Zhexi Luo of theCarnegie Museum of Natural History in Pittsburgh and several of hiscolleagues recently suggested that A. mahabo and a similarlysurprising fossil beast from Australia named Ausktribosphenosnyktos might instead represent a second line of tribosphenicmammals—one that gave rise to the egg-laying monotremes. ButFlynn and Wyss counter that some of the features that thoseresearchers use to link the Southern tribosphenic mammals tomonotremes may be primitive resemblances and therefore notindicative of an especially close evolutionary relationship.

As with so many other debates in paleontology, much of thecontroversy over when and where these mammal groups firstappeared stems from the fact that so few ancient bones have everbeen found. With luck, this season’s fieldwork will help fill in someof the gaps in the fossil record. And recovering more specimens ofA. mahabo or remains of previously unknown mammals couldbolster considerably Flynn and Wyss’s case for a single, Southernorigin for the ancestors of modern placentals and marsupials.

The next morning, after a quick breakfast of bread, peanutbutter and coffee, we are back in the vehicles, following the GPS’strail of electronic bread crumbs across the grassland to a fossillocality the team found at the end of last year’s expedition. Standsof doum palms and thorny Mokonazy trees dot the landscape,which the dry season has left largely parched. By the time we reachour destination, the morning’s pleasant coolness has given way to arather toastier temperature. “When the wind stops, it cooks,” remarksWilliam Simpson, a collections manager for the Field Museum,coating his face with sunscreen. Indeed, noontime temperaturesoften exceed 90 humid degrees Fahrenheit.

Flynn instructs the group to start at the base of the hillside andwork up. Meanwhile he and Wyss will survey the surrounding area,looking for additional exposures of the fossil-bearing horizon. “If it’ssomething interesting, come back and get me,” he calls. Awls inhand and eyes inches from the ground, the workers begin to scourthe gravel-strewn surface for small bones, clues that delicatemammal fossils are preserved below. They crawl and slither inpursuit of their quarry, stopping only to swig water from sun-warmedbottles. Because early mammal remains are so minute (A.mahabo’s jaw fragment, for example, measures a mere 3.6millimeters in length), such sleuthing rarely leads to instantgratification. Rather the team collects sediments likely to containsuch fossils and ships that material back to the U.S. for closerinspection. Within a few hours, a Lilliputian vertebra and femurfragment turn up—the first indications that the fossil hunters havehit pay dirt. “It’s a big Easter egg hunt,” Wyss quips. “The eggs arehidden pretty well, but we know they’re out there.”

By the third day the crew has identified a number of promisingsites and bagged nearly a ton of sediment for screen washing.

Members head for a dammed-up stream that locals use to water their animals. Despite the scorching heat, those working in thewater must don heavy rubber boots and gloves to protect againstthe parasites that probably populate the murky green pool. Theyspend the next few hours sifting the sediments through screen-bottomed baskets and buckets. Wyss spreads the resultingconcentrate on a big blue plastic tarp to dry. Volunteers at the FieldMuseum will eventually look for fossils in this concentrate under amicroscope, one spoonful at a time, but Wyss has a good feelingabout the washed remains already. “You can actually see bone inthe mix,” he observes. The haul that yielded A. mahabo, in contrast,offered no such hints to the naked eye.

Hot and weary from the screen washing, the researcherseagerly break for lunch. Under the shade of a Mokonazy tree, theymunch their sardine, Gouda and jalapeño sandwiches, joking aboutthe bread, which, four days after leaving its bakery in Antananarivo,has turned rather tough. Wyss ceremoniously deposits a ration ofjelly beans into each pair of upturned palms. Some pocket thetreats for later, others trade for favorite flavors, and a few ruefullyrelinquish their sweets, having lost friendly wagers made earlier.

Usually lunch is followed by a short repose, but today nature hasa surprise in store. A brushfire that had been burning off in thedistance several hours ago is now moving rapidly toward us from thenortheast, propelled by an energetic wind. The crackling sound offlames licking bone-dry grass crescendos, and ashen leafremnants drift down around us. We look on, spellbound, as cattleegrets collect in the fire’s wake to feast on toasted insects, andbirds of prey circle overhead to watch for rodents flushed out bythe flames. Only the stream separates us from the blaze, butreluctant to abandon the screen washing, Flynn and Wyss decide towait it out. Such fires plague Madagascar. Often set by farmers toencourage new grass growth, they sometimes spread out ofcontrol, especially in the tinderbox regions of the northwest.Indeed, the explorers will face other fires that season, includingone that nearly consumes their campsite.

An hour later the flames have subsided, and the team returns tothe stream to finish the screening quickly. Banks once thick with drygrass now appear naked and charred. Worried that the winds mightpick up again, we pack up and go to one of the team’s other fossillocalities to dig for the rest of the afternoon.

Following what has already become the routine, we return tocamp by six. Several people attend to the filtering of the drinkingwater, while the rest help to prepare dinner. During the “cocktailhour” of warm beer and a shared plate of peanuts, Flynn and Wysslog the day’s events and catalogue any interesting specimensthey’ve collected. Others write field notes and letters home by thelight of their headlamps. By nine, bellies full and dishes washed,people have retired to their tents. Camp is silent, the end of anotherday’s efforts to uncover the past.

Kate Wong is a writer and editor for ScientificAmerican.com

FRAN

K I

PP

OLI

TO (

opp

osit

e p

ag

e)


tionary tree, we know that the common ancestor of all di-nosaurs must be older still. Rocks from before about 245 mil-lion years ago have been moderately well sampled around theworld, but none of them has yet yielded dinosaurs. That meansthe search for the common ancestor of all dinosaurs must fo-cus on a relatively poorly known and ever narrowing intervalof Middle Triassic rocks, between about 240 million and 230million years old.

Mostly MammalsDINOSAURS NATURALLY ATTRACT considerable atten-tion, being the most conspicuous land animals of the Mesozoic.Less widely appreciated is the fact that mammals and dinosaurssprang onto the evolutionary stage at nearly the same time. Atleast two factors account for the popular misconception thatmammals arose only after dinosaurs became extinct: Earlymammals all were chipmunk-size or smaller, so they don’t grabthe popular imagination in the way their giant Mesozoic con-temporaries do. In addition, the fossil record of early mammalsis quite sparse, apart from very late in the Mesozoic. To our de-light, Madagascar has once again filled in two mysterious gapsin the fossil record. The traversodontid cynodonts from the Isa-lo deposits reveal new details about close mammalian relatives,and a younger fossil from the northwest side of the island pos-es some controversial questions about where and when a keyadvanced group of mammals got its start.

The Malagasy traversodontids, the first known from theisland, include some of the best-preserved representatives ofearly cynodonts ever discovered. (“Cynodontia” is the nameapplied to a broad group of land animals that includes mam-mals and their nearest relatives.) Accordingly, these bonesprovide a wealth of anatomical information previously un-

documented for these creatures. These cynodonts are identi-fied by, among other diagnostic features, a simplified lowerjaw that is dominated by a single bone, the dentary. Somespecimens include both skulls and skeletons. Understandingthe complete morphology of these animals is crucial for re-solving the complex evolutionary transition from the largecold-blooded, scale-covered animals with sprawling limbs(which dominated the continents prior to the Mesozoic) tothe much smaller warm-blooded, furry animals with an erectposture that are so plentiful today.

Many kinds of mammals, with many anatomical varia-tions, now inhabit the planet. But they all share a commonancestor marked by a single, distinctive suite of features. Todetermine what these first mammals looked like, paleontolo-gists must examine their closest evolutionary relatives withinthe Cynodontia, which include the traversodontids and theirmuch rarer cousins, the chiniquodontids (also known asprobainognathians), both of which we have found in south-western Madagascar. Traversodontids almost certainly wereherbivorous, because their wide cheek teeth are designed forgrinding. One of our four new Malagasy traversodontidspecies also has large, stout, forward-projecting incisors forgrasping vegetation. The chiniquodontids, in contrast, wereundoubtedly carnivorous, with sharp, pointed teeth. Mostpaleontologists agree that some chiniquodontids share amore recent common ancestor with mammals than the her-bivorous traversodontids do. The chiniquodontid skulls andskeletons we found in Madagascar will help reconstruct thebridge between early cynodonts and true mammals.

Not only are Madagascar’s Triassic cynodonts among thebest preserved in the world, they also sample a time periodthat is poorly known elsewhere. The same is true for the

Modern-Day MysteryMADAGASCAR IS FAMOUS for its 40 species of lemurs, none of whichoccurs anywhere else in the world. The same is true for 80 percent ofthe island’s plants and other animals. This biotic peculiarity reflectsthe island’s lengthy geographic isolation. (Madagascar has not beenconnected to another major landmass since it separated from Indianearly 90 million years ago, and it has not been joined with itsnearest modern neighbor, Africa, since about 160 million years ago.)But for decades the scant fossil evidence of land-dwelling animalsfrom the island meant that little was known about the origin andevolution of these unique creatures.

While our research group was probing Madagascar’s Triassic andJurassic age rocks, teams led by David W. Krause of the StateUniversity of New York at Stony Brook were unearthing a wealth ofyounger fossils in the island’s northwestern region. Thesespecimens, which date back some 70 million years, include morethan three dozen species, none of which is closely related to theisland’s modern animals. This evidence implies that most modernvertebrate groups must have immigrated to Madagascarafter this point.

The best candidate for a Malagasy motherland is Africa, and yetthe modern faunas of the two landmasses are markedly distinct.Elephants, cats, antelope, zebras, monkeys and many other modernAfrican mammals apparently never reached Madagascar. The fourkinds of terrestrial mammals that inhabit the island today—rodents,lemurs, carnivores and the hedgehoglike tenrecs—all appear to bedescendants of more ancient African beasts. The route theseimmigrants took from the mainland remains unclear, however. Smallclinging animals could have floated from Africa across theMozambique Channel on “rafts” of vegetation that broke free duringsevere storms. Alternatively, when sea level was lower thesepioneers might have traveled by land and sea along a chain ofcurrently submerged highlands northwest of the island.

Together with Anne D. Yoder of Northwestern University MedicalSchool and others, we are using the DNA structure of modernMalagasy mammals to address this question. These analyses havethe potential to reveal whether the ancestors of Madagascar’smodern mammals arrived in multiple, long-distance dispersalevents or in a single episode of “island hopping.” —J.J.F. and A.R.W. FR

ANK

IP

PO

LITO

(op

pos

ite

pa

ge)



L I V I N G I N M I X E D C O M P A N YPALEONTOLOGISTS DID NOT KNOW until recently that the unusualgroup of ancient animals shown above—prosauropods (1),traversodontids (2), rhynchosaurs (3) and chiniquodontids (4)—

once foraged together. In the past six years, southwesternMadagascar has become the first place where bones of eachparticular type of animal have been unearthed alongside the others,in this case from Triassic rocks about 230 million years old. Then theregion was a lush, lowland basin that was forming as thesupercontinent Pangea began to break up. The long-neckedprosauropods here, which represent some of the oldest dinosaurs

yet discovered, browse on conifers while a parrot-beakedrhynchosaur prepares to sip from a nearby pool. The prosauropodteeth were spear-shaped and serrated—good for slicing vegetation;rhynchosaurs were perhaps the most common group of plant eatersin the area at that time. Foraging among these large reptiles are the peculiar traversodontids and chiniquodontids. Both types ofcreatures are early members of the Cynodontia, a broad group thatincludes today’s mammals. The grinding cheek teeth of thetraversodontids suggest they were herbivores; the chiniquodontidssport the sharp, pointed teeth of carnivores. —J.J.F. and A.R.W.

1

2

34

1

2

34


youngest fossils our expeditions have uncovered—those froma region of the northwest where the sediments are about 165million years in age. (That date falls within the middle of theJurassic, the second of the Mesozoic’s three periods.) Becausethese sediments were considerably younger than our Triassicrocks, we allowed ourselves the hope that we might find re-mains of an ancient mammal. Not a single mammal had beenrecorded from Jurassic rocks of a southern landmass at thatpoint, but this did nothing to thwart our motivation.

Once again, persistence paid off. During our 1996 fieldseason, we had visited the village of Ambondromahabo afterhearing local reports of abundant large fossils of the sauro-pod dinosaur Lapperentosaurus. Sometimes where large ani-mals are preserved, the remains of smaller animals can alsobe found—though not as easily. We crawled over the land-scape, eyes held a few inches from the ground. This uncom-fortable but time-tested strategy turned up a few small thero-pod dinosaur teeth, fish scales and other bone fragments,which had accumulated at the surface of a small mound ofsediment near the village.

These unprepossessing fossils hinted that more significantitems might be buried in the sediment beneath. We baggedabout 200 pounds of sediment and washed it through mos-quito-net hats back in the capital, Antananarivo, while wait-ing to be granted permits for the second leg of our trip—theleg to the southwest that turned up our first rhynchosaurjaws and traversodontid skull.

During the subsequent years back in the U.S., while ourstudies focused on the exceptional Triassic material, the tediousprocess of sorting the Jurassic sediment took place. A dedicatedteam of volunteers at the Field Museum in Chicago—DennisKinzig, Ross Chisholm and Warren Valsa—spent many a week-end sifting through the concentrated sediment under a micro-scope in search of valuable flecks of bone or teeth. We didn’tthink much about that sediment again until 1998, when Kinzigrelayed the news that they had uncovered the partial jawboneof a tiny mammal with three grinding teeth still in place. Wewere startled not only by the jaw’s existence but also by its re-markably advanced cheek teeth. The shapes of the teeth docu-ment the earliest occurrence of Tribosphenida, a group encom-passing the vast majority of living mammals. We named thenew species Ambondro mahabo, after its place of origin.

The discovery pushes back the geologic range of thisgroup of mammals by more than 25 million years and offersthe first glimpse of mammalian evolution on the southerncontinents during the last half of the Jurassic period. It showsthat this subgroup of mammals may have evolved in theSouthern Hemisphere rather than the Northern, as is com-monly supposed. Although the available information doesnot conclusively resolve the debate, this important additionto the record of early fossil mammals does point out the pre-carious nature of long-standing assumptions rooted in a fos-sil record historically biased toward the Northern Hemi-sphere [see “Tiny Bones to Pick,” by Kate Wong, on page 13].

Although our team has recovered a broad spectrum offossils in Madagascar, scientists are only beginning to de-

scribe the Mesozoic history of the Southern continents. Thenumber of species of Mesozoic land vertebrates known fromAustralia, Antarctica, Africa and South America is probablyan order of magnitude smaller than the number of contempo-raneous findings from the Northern Hemisphere. Clearly,Madagascar now ranks as one of the world’s top prospectsfor adding important insight to paleontologists’ knowledgeof the creatures that once roamed Gondwana.

Planning Persistently OFTEN THE MOST SIGNIFICANT HYPOTHESES about ancient life onthe earth can be suggested only after these kinds of new fossil dis-coveries are made. Our team’s explorations provide two cases inpoint: the fossils found alongside the Triassic prosauropods in-dicate that dinosaurs debuted earlier than previously recorded,and the existence of the tiny mammal at our Jurassic site impliesthat tribosphenic mammals may have originated in the Southern,rather than Northern, Hemisphere. The best way to bolster theseproposals (or to prove them wrong) is to go out and uncovermore bones. That is why our primary goal this summer will bethe same as it has been for our past five expeditions: find as manyfossils as possible.

Our agenda includes digging deeper into known sites andsurveying new regions, blending risky efforts with sure bets.No matter how carefully formulated, however, our plans willbe subject to last-minute changes, dictated by such things asroad closures and our most daunting challenge to date, theappearance of frenzied boomtowns.

During our first three expeditions, we never gave a secondthought to the gravels that overlay the Triassic rock outcropsin the southwestern part of the island. Little did we know thatthose gravels contain sapphires. By 1999 tens of thousands ofpeople were scouring the landscape in search of these gems.The next year all our Triassic sites fell within sapphire-miningclaims. Those areas are now off limits to everyone, includingpaleontologists, unless they get permission from both theclaim holder and the government. Leaping that extra set ofhurdles will be one of our foremost tasks this year.

Even without such logistical obstacles slowing our pro-gress, it would require uncountable lifetimes to carefully sur-vey all the island’s untouched rock exposures. But now thatwe have seen a few of Madagascar’s treasures, we are in-spired to keep digging—and to reveal new secrets.

Madagascar: A Natural History. Ken Preston-Mafham. Foreword by Sir David Attenborough. Facts on File, 1991.

Natural Change and Human Impact in Madagascar. Edited by Steven M.Goodman and Bruce D. Patterson. Smithsonian Institution Press, 1997.

A Middle Jurassic Mammal from Madagascar. John J. Flynn, J. MichaelParrish, Berthe Rakotosaminimanana, William F. Simpson and André R.Wyss in Nature, Vol. 401, pages 57–60; September 2, 1999.

A Triassic Fauna from Madagascar, Including Early Dinosaurs. John J.Flynn, J. Michael Parrish, Berthe Rakotosaminimanana, William F.Simpson, Robin L. Whatley and André R. Wyss in Science, Vol. 286, pages763–765; October 22, 1999.

M O R E T O E X P L O R E



Fish-shaped reptiles called ichthyosaurs reigned over the oceans

for as long as dinosaurs roamed the land, but only recently have

paleontologists discovered why these creatures were so successful

icture a late autumn evening some 160 million years ago,during the

Jurassic time period, when dinosaurs inhabited the continents. The

setting sun hardly penetrates the shimmering surface of a vast blue-

green ocean, where a shadow glides silently among the dark crags of a sub-

merged volcanic ridge. When the animal comes up for a gulp of evening air, it

calls to mind a small whale—but it cannot be.The first whale will not evolve for an-

other 100 million years.The shadow turns suddenly and now stretches more than

twice the height of a human being.That realization becomes particularly chilling

when its long,tooth-filled snout tears through a school of squidlike creatures.

The remarkable animal is Ophthalmosaurus,one of more than 80 species now

known to have constituted a group of sea monsters called the ichthyosaurs, or

Rulers of theJurassic Seas

by Ryosuke Motani

P

Originally published in December 2000


KA

REN

CA

RR

ICHTHYOSAURS patrolled the world’s oceans for 155 million years.


fish-lizards. The smallest of these ani-mals was no longer than a human arm;the largest exceeded 15 meters. Oph-thalmosaurus fell into the medium-sizegroup and was by no means the mostaggressive of the lot. Its company wouldhave been considerably more pleasantthan that of a ferocious Temnodonto-saurus, or “cutting-tooth lizard,” whichsometimes dined on large vertebrates.

When paleontologists uncovered thefirst ichthyosaur fossils in the early1800s, visions of these long-vanishedbeasts left them awestruck. Dinosaurshad not yet been discovered, so everyunusual feature of ichthyosaurs seemedintriguing and mysterious. Examina-tions of the fossils revealed that ichthy-osaurs evolved not from fish but fromland-dwelling animals, which them-selves had descended from an ancientfish. How, then, did ichthyosaurs makethe transition back to life in the water?To which other animals were they mostrelated? And why did they evolve bizarre

characteristics, such as backbones thatlook like a stack of hockey pucks andeyes as big around as bowling balls?

Despite these compelling questions,the opportunity to unravel the enigmat-ic transformation from landlubbingreptiles to denizens of the open seawould have to wait almost two cen-turies. When dinosaurs such as Iguan-odan grabbed the attention of paleon-tologists in the 1830s, the novelty ofthe fish-lizards faded away. Intense in-terest in the rulers of the Jurassic seasresurfaced only a few years ago, thanksto newly available fossils from Japanand China. Since then, fresh insightshave come quickly.

Murky Origins

Although most people forgot aboutichthyosaurs in the early 1800s, a

few paleontologists did continue tothink about them throughout the 19thcentury and beyond. What has been ev-

ident since their discovery is that theichthyosaurs’ adaptations for life in wa-ter made them quite successful. Thewidespread ages of the fossils revealedthat these beasts ruled the ocean fromabout 245 million until about 90 mil-lion years ago—roughly the entire erathat dinosaurs dominated the conti-nents. Ichthyosaur fossils were foundall over the world, a sign that they mi-grated extensively, just as whales do to-day. And despite their fishy appearance,ichthyosaurs were obviously air-breath-ing reptiles. They did not have gills, andthe configurations of their skull and jaw-bones were undeniably reptilian. Whatis more, they had two pairs of limbs(fish have none), which implied thattheir ancestors once lived on land.

Paleontologists drew these conclu-sions based solely on the exquisite skele-tons of relatively late, fish-shaped ich-thyosaurs. Bone fragments of the firstichthyosaurs were not found until 1927.Somewhere along the line, those early

FACT: The smallest ichthyosaur was shorter than a human arm;

TOM

O N

AR

ASH

IMA

AN

D C

LEO

VIL

ETT

ORIGINS OF ICHTHYOSAURS baffled paleontologists for nearly twocenturies. At times thought to be closely related to everything from fish tosalamanders to mammals, ichthyosaurs are now known to belong to thegroup called diapsids. New analyses indicate that they branched off fromother diapsids at about the time lepidosaurs and archosaurs diverged fromeach other—but no one yet knows whether ichthyosaurs appeared shortlybefore that divergence or shortly after.

SHARKS AND RAYS

RAY-FINNEDFISHES AMPHIBIANS MAMMALS

LEPIDOSAURS

DINOSAURS

Snakes Lizards Tuatara

ANCESTRALVERTEBRATE

Crocodiles Birds

ARCHOSAURS

ICHTHYOSAURS

DIAPSIDS


animals went on to acquire a decidedlyfishy body: stocky legs morphed intoflippers, and a boneless tail fluke anddorsal fin appeared. Not only were theadvanced, fish-shaped ichthyosaursmade for aquatic life, they were madefor life in the open ocean, far fromshore. These extreme adaptations toliving in water meant that most of themhad lost key features—such as particu-lar wrist and ankle bones—that wouldhave made it possible to recognize theirdistant cousins on land. Without com-plete skeletons of the very first ichthyo-saurs, paleontologists could merelyspeculate that they must have lookedlike lizards with flippers.

The early lack of evidence so con-fused scientists that they proposed al-most every major vertebrate group—not only reptiles such as lizards andcrocodiles but also amphibians andmammals—as close relatives of ichthy-osaurs. As the 20th century progressed,scientists learned better how to deci-pher the relationships among variousanimal species. On applying the newskills, paleontologists started to agreethat ichthyosaurs were indeed reptilesof the group Diapsida, which includessnakes, lizards, crocodiles and di-nosaurs. But exactly when ichthyosaursbranched off the family tree remaineduncertain—until paleontologists in Asiarecently unearthed new fossils of theworld’s oldest ichthyosaurs.

The first big discovery occurred onthe northeastern coast of Honshu, themain island of Japan. The beach isdominated by outcrops of slate, the lay-ered black rock that is often used for

the expensive ink plates of Japanesecalligraphy and that also harbors bonesof the oldest ichthyosaur, Utatsusaurus.Most Utatsusaurus specimens turn upfragmented and incomplete, but agroup of geologists from HokkaidoUniversity excavated two nearly com-plete skeletons in 1982. These speci-mens eventually became available forscientific study, thanks to the devotionof Nachio Minoura and his colleagues,who spent much of the next 15 yearspainstakingly cleaning the slate-encrust-ed bones. Because the bones are so frag-ile, they had to chip away the rock care-fully with fine carbide needles as theypeered through a microscope.

As the preparation neared its end in1995, Minoura, who knew of my inter-est in ancient reptiles, invited me to jointhe research team. When I saw theskeleton for the first time, I knew thatUtatsusaurus was exactly what paleon-tologists had been expecting to find foryears: an ichthyosaur that looked like alizard with flippers. Later that same yearmy colleague You Hailu, then at the In-stitute for Vertebrate Paleontology andPaleoanthropology in Beijing, showedme a second, newly discovered fossil—the world’s most complete skeleton ofChaohusaurus, another early ichthyo-saur. Chaohusaurus occurs in rocks thesame age as those harboring remains ofUtatsusaurus, and it, too, had beenfound before only in bits and pieces.The new specimen clearly revealed theoutline of a slender, lizardlike body.

Utatsusaurus and Chaohusaurus illu-minated at long last where ichthyosaursbelonged on the vertebrate family tree,

because they still retained some key fea-tures of their land-dwelling ancestors.Given the configurations of the skulland limbs, my colleagues and I thinkthat ichthyosaurs branched off fromthe rest of the diapsids near the separa-tion of two major groups of living rep-tiles, lepidosaurs (such as snakes andlizards) and archosaurs (such as croco-diles and birds). Advancing the family-tree debate was a great achievement,but the mystery of the ichthyosaurs’evolution remained unsolved.

From Feet to Flippers

Perhaps the most exciting outcomeof the discovery of these two Asian

ichthyosaurs is that scientists can nowpaint a vivid picture of the elaborateadaptations that allowed their descen-dants to thrive in the open ocean. Themost obvious transformation for aquat-ic life is the one from feet to flippers. Incontrast to the slender bones in the frontfeet of most reptiles, all bones in the front“feet” of the fish-shaped ichthyosaurs arewider than they are long. What is more,they are all a similar shape. In mostother four-limbed creatures it is easy todistinguish bones in the wrist (irregu-larly rounded) from those in the palm(long and cylindrical). Most important,the bones of fish-shaped ichthyosaursare closely packed—without skin in be-tween—to form a solid panel. Havingall the toes enclosed in a single envelopeof soft tissues would have enhanced therigidity of the flippers, as it does in liv-ing whales, dolphins, seals and sea tur-tles. Such soft tissues also improve the

the largest was longer than a typical city bus

NEW FOSSILS of the first ichthyosaurs, including Chaohusaurus, have illuminated how these lizard-shaped creatures evolved intomasters of the open ocean.

RYO

SUKE

MO

TAN

I


hydrodynamic efficiency of the flippersbecause they are streamlined in crosssection—a shape impossible to maintainif the digits are separated.

But examination of fossils rangingfrom lizard- to fish-shaped—especiallythose of intermediate forms—revealedthat the evolution from fins to feet wasnot a simple modification of the foot’sfive digits. Indeed, analyses of ichthyo-saur limbs reveal a complex evolution-ary process in which digits were lost,added and divided. Plotting the shapeof fin skeletons along the family tree ofichthyosaurs, for example, indicatesthat fish-shaped ichthyosaurs lost thethumb bones present in the earliest ich-thyosaurs. Additional evidence comesfrom studying the order in which digitsbecame bony, or ossified, during thegrowth of the fish-shaped ichthyosaurStenopterygius, for which we have spec-

imens representing various growthstages. Later, additional fingers ap-peared on both sides of the preexistingones, and some of them occupied theposition of the lost thumb. Needless tosay, evolution does not always follow acontinuous, directional path from onetrait to another.

Backbones Built for Swimming

The new lizard-shaped fossils havealso helped resolve the origin of the

skeletal structure of their fish-shaped de-scendants. The descendants have back-bones built from concave vertebrae theshape of hockey pucks. This shape,though rare among diapsids, was al-ways assumed to be typical of all ichthy-osaurs. But the new creatures from Asiasurprised paleontologists by having amuch narrower backbone, composed of

vertebrae shaped more like canistersof 35-millimeter film than hockeypucks. It appeared that the verte-brae grew dramatically in diameterand shortened slightly as ichthyo-saurs evolved from lizard- to fish-shaped. But why?

My colleagues and I found the an-swer in the swimming styles of livingsharks. Sharks, like ichthyosaurs,come in various shapes and sizes.Cat sharks are slender and lack atall tail fluke, also known as a cau-dal fin, on their lower backs, as didearly ichthyosaurs. In contrast,mackerel sharks such as the greatwhite have thick bodies and a cres-cent-shaped caudal fin similar to thelater fish-shaped ichthyosaurs.Mackerel sharks swim by swingingonly their tails, whereas cat sharksundulate their entire bodies. Undu-latory swimming requires a flexiblebody, which cat sharks achieve byhaving a large number of backbonesegments. They have about 40 ver-tebrae in the front part of their bod-ies—the same number scientists findin the first ichthyosaurs, representedby Utatsusaurus and Chaohu-saurus. (Modern reptiles and mam-mals have only about 20.)

Undulatory swimmers, such ascat sharks, can maneuver and accel-erate sufficiently to catch prey in therelatively shallow water above thecontinental shelf. Living lizards also

undulate to swim, though not as effi-ciently as creatures that spend all theirtime at sea. It is logical to conclude,then, that the first ichthyosaurs—whichlooked like cat sharks and descendedfrom a lizardlike ancestor—swam inthe same fashion and lived in the envi-ronment above the continental shelf.

Undulatory swimming enables pred-ators to thrive near shore, where food isabundant, but it is not the best choicefor an animal that has to travel long dis-tances to find a meal. Offshore preda-tors, which hunt in the open oceanwhere food is less concentrated, need amore energy-efficient swimming style.Mackerel sharks solve this problem byhaving stiff bodies that do not undulateas their tails swing back and forth. Acrescent-shaped caudal fin, which actsas an oscillating hydrofoil, also improvestheir cruising efficiency. Fish-shaped ich-

ANCIENT SKELETONS have helped scientists trace how the slender, lizardlike bodies ofthe first ichthyosaurs (top) thickened into a fish shape with a dorsal fin and a tail fluke.

ED H

ECK

Chaohusaurus geishanesis0.5 to 0.7 meter • Lived 245 million years ago (Early Triassic)

DORSAL FIN

TAIL FLUKE

Mixosaurus cornalianus0.5 to 1 meter • Lived 235 million years ago (Middle Triassic)

Ophthalmosaurus icenicus3 to 4 meters • Lived from 165 million to 150 million years ago (Middle to Late Jurassic)

FACT: No other reptile group ever evolved a fish-shaped body


thyosaurs had such a caudal fin, andtheir thick body profile implies that theyprobably swam like mackerel sharks.

Inspecting a variety of shark speciesreveals that the thicker the body fromtop to bottom, the larger the diameterof the vertebrae in the animal’s trunk. Itseems that sharks and ichthyosaurssolved the flexibility problem resultingfrom having high numbers of body seg-ments in similar ways. As the bodies ofichthyosaurs thickened over time, thenumber of vertebrae stayed about thesame. To add support to the more volu-minous body, the backbone became atleast one and a half times thicker thanthose of the first ichthyosaurs. As a con-sequence of this thickening, the bodybecame less flexible, and the individualvertebrae acquired their hockey-puckappearance.

Drawn to the Deep

The ichthyosaurs’ invasion of openwater meant not only a wider cov-

erage of surface waters but also a deep-er exploration of the marine environ-ment. We know from the fossilized stom-ach contents of fish-shaped ichthyosaursthat they mostly ate squidlike creaturesknown as dibranchiate cephalopods.Squid-eating whales hunt anywherefrom about 100 to 1,000 meters deepand sometimes down to 3,000 meters.The great range in depth is hardly sur-prising considering that food resourcesare widely scattered below about 200meters. But to hunt down deep, whales

and other air-breathing divers have togo there and get back to the surface inone breath—no easy task. Reducing en-ergy use during swimming is one of thebest ways to conserve precious oxygenstored in their bodies. Consequently,deep divers today have streamlinedshapes that reduce drag—and so didfish-shaped ichthyosaurs.

Characteristics apart from diet andbody shape also indicate that at leastsome fish-shaped ichthyosaurs were deepdivers. The ability of an air-breathingdiver to stay submerged dependsroughly on its body size: the heavier the

diver, the more oxygen it can store in itsmuscles, blood and certain other or-gans—and the slower the consumptionof oxygen per unit of body mass. Theevolution of a thick, stiff body increasedthe volume and mass of fish-shapedichthyosaurs relative to their predeces-sors. Indeed, a fish-shaped ichthyosaurwould have been up to six times heav-ier than a lizard-shaped ichthyosaur ofthe same body length. Fish-shaped ich-thyosaurs also grew longer, further aug-menting their bulk. Calculations basedon the aerobic capacities of today’s air-breathing divers (mostly mammals and

KA

REN

CA

RR

KA

REN

CA

RR;A

DRI

ENN

E SM

UC

KER

(ver

tebr

ae)

SWIMMING STYLES—and thus the hab-itats (above)—of ichthyosaurs changed asthe shape of their vertebrae evolved. Thenarrow backbone of the first ichthyosaurssuggests that they undulated their bodieslike eels (right). This motion allowed forthe quickness and maneuverability neededfor shallow-water hunting. As the back-bone thickened in later ichthyosaurs, thebody stiffened and so could remain still asthe tail swung back and forth (bottom).This stillness facilitated the energy-efficientcruising needed to hunt in the open ocean.

CHAOHUSAURUS

CHAOHUSAURUS CONTINENTAL SHELF

OPHTHALMOSAURUS

OPHTHALMOSAURUS

BACKBONE SEGMENT


birds) indicate that an animal the weightof fish-shaped Ophthalmosaurus, whichwas about 950 kilograms, could holdits breath for at least 20 minutes. A con-servative estimate suggests, then, thatOphthalmosaurus could easily havedived to 600 meters—possibly even1,500 meters—and returned to the sur-face in that time span.

Bone studies also indicate that fish-shaped ichthyosaurs were deep divers.Limb bones and ribs of four-limbed ter-restrial animals include a dense outershell that enhances the strength neededto support a body on land. But thatdense layer is heavy. Because aquatic

vertebrates are fairly buoyant in water,they do not need the extra strength itprovides. In fact, heavy bones(which are little help for oxygenstorage) can impede the ability ofdeep divers to return to the sur-face. A group of French biolo-gists has established that mod-ern deep-diving mammalssolve that problem by makingthe outer shell of their bonesspongy and less dense. Thesame type of spongy layer alsoencases the bones of fish-shaped ichthyosaurs, whichimplies that they, too, benefit-ed from lighter skeletons.

Perhaps the best evidence forthe deep-diving habits of laterichthyosaurs is their remarkably

large eyes, up to 23 centimetersacross in the case of Ophthalmo-

saurus. Relative to body size, thatfish-shaped ichthyosaur had the

biggest eyes of any animal everknown. The size of their eyes also suggests that

visual capacity improved as ichthyosaursmoved up the family tree. These esti-mates are based on measurements of thesclerotic ring, a doughnut-shaped bone

ICHTHYOSAUR EYES were surprisingly large. Analyses of doughnut-shaped eye bones called sclerot-ic rings reveal that Ophthalmosaurus had the largest eyes relative to body size of any adult vertebrate, liv-ing or extinct, and that Temnodontosaurus had the biggest eyes, period. The beige shape in the back-ground is the size of an Ophthalmosaurus sclerotic ring. The photograph depicts a well-preserved ringfrom Stenopterygius.

FACT: Their eyes were the largest of any animal, living or dead

TOM

O N

AR

ASH

IMA

(an

imal

s);E

DW

ARD

BEL

L (s

cler

otic

ring

);RY

OSU

KE M

OTA

NI (

phot

ogra

ph)

APPROXIMATE MAXIMUM DIAMETER OF EYE:

AFRICAN ELEPHANT5 CENTIMETERS

BLUE WHALE15 CENTIMETERS

OPHTHALMOSAURUS23 CENTIMETERS

GIANT SQUID25 CENTIMETERS

TEMNODONTOSAURUS26 CENTIMETERS


that was embedded in their eyes. (Hu-mans do not have such a ring—it waslost in mammalian ancestors—but mostother vertebrates have bones in theireyes.) In the case of ichthyosaurs, the ringpresumably helped to maintain theshape of the eye against the forces ofwater passing by as the animals swam,regardless of depth.

The diameter of the sclerotic ringmakes it possible to calculate the eye’sminimum f-number—an index, used torate camera lenses, for the relativebrightness of an optical system. Thelower the number, the brighter the imageand therefore the shorter the exposuretime required. Low-quality lenses have avalue of f/3.5 and higher; high-qualitylenses have values as low as f/1.0. The f-number for the human eye is about 2.1,whereas the number for the eye of a noc-turnal cat is about 0.9. Calculations sug-gest that a cat would be capable of see-ing at depths of 500 meters or greater inmost oceans. Ophthalmosaurus alsohad a minimum f-number of about 0.9,but with its much larger eyes, it proba-bly could outperform a cat.

Gone for Good

Many characteristics of ichthyo-saurs—including the shape of

their bodies and backbones, the size oftheir eyes, their aerobic capacity, andtheir habitat and diet—seem to havechanged in a connected way duringtheir evolution, although it is not possi-ble to judge what is the cause and whatis the effect. Such adaptations enabledichthyosaurs to reign for 155 millionyears. New fossils of the earliest of

these sea dwellers are now making itclear just how they evolved so success-fully for aquatic life, but still no oneknows why ichthyosaurs went extinct.

Loss of habitat may have clinched thefinal demise of lizard-shaped ichthyo-saurs, whose inefficient, undulatoryswimming style limited them to near-shore environments. A large-scale dropin sea level could have snuffed out thesecreatures along with many others byeliminating their shallow-water niche.Fish-shaped ichthyosaurs, on the otherhand, could make a living in the openocean, where they would have had a

better chance of survival. Because theirhabitat never disappeared, somethingelse must have eliminated them. Theperiod of their disappearance roughlycorresponds to the appearance of ad-vanced sharks, but no one has founddirect evidence of competition betweenthe two groups.

Scientists may never fully explain theextinction of ichthyosaurs. But as pale-ontologists and other investigators con-tinue to explore their evolutionary his-tory, we are sure to learn a great dealmore about how these fascinating crea-tures lived.

The Author

RYOSUKE MOTANI, who was born in Tokuyama, Japan, is a researcher in the department of paleobiology at the Royal Ontario Muse-um in Toronto. As a child he found ichthyosaurs uninteresting. (“They looked too ordinary in my picture books,” he recalls.) But his viewchanged during his undergraduate years at the University of Tokyo, after a paleontology professor allowed him to study the only domesticreptilian fossil they had: an ichthyosaur. “I quickly fell in love with these noble beasts,” he says. Motani went on to explore ichthyosaur evo-lution for his doctoral degree from the University of Toronto in 1997. A fellowship from the Miller Institute then took him to the Universi-ty of California, Berkeley, for postdoctoral research. He moved back to Canada in September 1999.

Further Information

Vertebrate Paleontology and Evolution. R. L. Carroll. Freeman, San Francisco, 1987.Dinosaurs, Spitfires, and Sea Dragons. Christopher McGowan. Harvard University Press, 1991.Eel-like Swimming in the Earliest Ichthyosaurs. Ryosuke Motani, You Hailu and Christopher McGowan in Nature, Vol. 382, pages347–348; July 25, 1996.

Ichthyosaurian Relationships Illuminated by New Primitive Skeletons from Japan. Ryosuke Motani, Nachio Minoura and Tat-suro Ando in Nature, Vol. 393, pages 255–257; May 21, 1998.

Large Eyeballs in Diving Ichthyosaurs. Ryosuke Motani, Bruce M. Rothschild and William Wahl, Jr., in Nature, Vol. 402, page 747;December 16, 1999.

Ryosuke Motani’s Web site: www.ucmp.berkeley.edu/people/motani/ichthyo/

SMALL ISLAND in northeast Japan turned out to harbor two almost complete skeletonsof Utatsusaurus, the oldest ichthyosaur.

RYO

SUKE

MO

TAN

I



ILLU

STR

ATIO

NS

BY E

D H

ECK

The Origin of Birds and Their FlightAnatomical and aerodynamic analyses of fossils and living birds show that birds evolved from small, predatory dinosaurs that lived on the ground

by Kevin Padian and Luis M. Chiappe

Sinornis


Originally published in February 1998


Until recently, the origin of birdswas one of the great mysteries ofbiology. Birds are dramatically dif-

ferent from all other living creatures. Feath-ers, toothless beaks, hollow bones, perchingfeet, wishbones, deep breastbones andstumplike tailbones are only part of the com-bination of skeletal features that no otherliving animal has in common with them.How birds evolved feathers and flight waseven more imponderable.

In the past 20 years, however, new fossildiscoveries and new research methods haveenabled paleontologists to determine thatbirds descend from ground-dwelling, meat-eating dinosaurs of the group known astheropods. The work has also offered a pic-ture of how the earliest birds took to the air.

Scientists have speculated on the evolu-tionary history of birds since shortly afterCharles Darwin set out his theory of evolu-tion in On the Origin of Species. In 1860,the year after the publication of Darwin’streatise, a solitary feather of a bird wasfound in Bavarian limestone deposits datingto about 150 million years ago (just beforethe Jurassic period gave way to the Creta-ceous). The next year a skeleton of an ani-mal that had birdlike wings and feathers—but a very unbirdlike long, bony tail andtoothed jaw—turned up in the same region.These finds became the first two specimens ofthe blue jay–size Archaeopteryx lithographi-ca, the most archaic, or basal, known mem-ber of the birds [see “Archaeopteryx,” by Pe-ter Wellnhofer; Scientific American, May1990].

Archaeopteryx’s skeletal anatomy pro-vides clear evidence that birds descend froma dinosaurian ancestor, but in 1861 scien-tists were not yet in a position to make thatconnection. A few years later, though,Thomas Henry Huxley, Darwin’s staunchdefender, became the first person to connectbirds to dinosaurs. Comparing the hindlimbs of Megalosaurus, a giant theropod,with those of the ostrich, he noted 35 fea-tures that the two groups shared but that

EARLY BIRDS living more than 100 million years ago looked quite different frombirds of today. For instance, as these artist’s reconstructions demonstrate, some re-tained the clawed fingers and toothed jaw characteristic of nonavian dinosaurs. Fossilsof Sinornis (left) were uncovered in China; those of Iberomesornis and Eoalulavis (be-loww) in Spain. All three birds were about the size of a sparrow. Eoalulavis sported thefirst known alula, or “thumb wing,” an adaptation that helps today’s birds navigatethrough the air at slow speeds.

Eoalulavis

Iberomesornis


did not occur as a suite in any other animal. He concludedthat birds and theropods could be closely related, althoughwhether he thought birds were cousins of theropods or weredescended from them is not known.

Huxley presented his results to the Geological Society ofLondon in 1870, but paleontologist Harry Govier Seeleycontested Huxley’s assertion of kinship between theropodsand birds. Seeley suggested that the hind limbs of the ostrich

and Megalosaurus might look similar just because both ani-mals were large and bipedal and used their hind limbs insimilar ways. Besides, dinosaurs were even larger than os-triches, and none of them could fly; how, then, could flyingbirds have evolved from a dinosaur?

The mystery of the origin of birds gained renewed atten-tion about half a century later. In 1916 Gerhard Heilmann, amedical doctor with a penchant for paleontology, published

The family tree at the right traces theancestry of birds back to their early

dinosaurian ancestors. This tree, otherwiseknown as a cladogram, is the product oftoday’s gold standard for analyzing theevolutionary relations among animals—amethod called cladistics.

Practitioners of cladistics determine theevolutionary history of a group of animalsby examining certain kinds of traits. Duringevolution, some animal will display a new, ge-netically determined trait that will be passed to itsdescendants. Hence, paleontologists can conclude thattwo groups uniquely sharing a suite of such novel, or derived, traitsare more closely related to each other than to animals lacking those traits.

Nodes, or branching points (dots), on a cladogram mark the emergenceof a lineage possessing a new set of derived traits. In the cladogram here,the Theropoda all descend from a dinosaurian ancestor that newly pos-sessed hollow bones and had only three functional toes. In this scheme,the theropods are still dinosaurs; they are simply a subset of the saurischi-an dinosaurs. Each lineage, or clade, is thus nested within a larger one(colored rectangles). By the same token, birds (Aves) are maniraptoran,tetanuran and theropod dinosaurs. —K.P. and L.M.C.

Tracking the Dinosaur Lineage Leading to Birds

THREE-FINGERED HAND

TETANURAEAllosaurus

DINOSAURIA SAURISCHIA

Titanosaurus

Creatures on these two pages are not drawn to scale

DINOSAUR LINEAGESTHAT DID NOT LEAD

TO BIRDS

THREE FUNCTIONAL TOES

THEROPODACoelophysis

ED H

ECK


(in Danish) a brilliant book that in 1926 was translated into English as TheOrigin of Birds. Heilmann showed that birds were anatomically more sim-ilar to theropod dinosaurs than to any other fossil group but for one in-escapable discrepancy: theropods apparently lacked clavicles, the two col-larbones that are fused into a wishbone in birds. Because other reptiles hadclavicles, Heilmann inferred that theropods had lost them. To him, this lossmeant birds could not have evolved from theropods, because he was con-vinced (mistakenly, as it turns out) that a feature lost during evolution could

TOM

O N

ARA

SHIM

A

WISHBONE

KEELEDSTERNUM

PYGOSTYLE

REPRESENTATIVE THEROPODSin the lineage leading to birds (Aves)display some of the features thathelped investigators establish the di-nosaurian origin of birds—including,in the order of their evolution, threefunctional toes (purple), a three-

fingered hand (green) and ahalf-moon-shaped wrist-bone (red). Archaeopteryx,the oldest known bird, also

shows some new traits, such as a clawon the back toe that curves toward theclaws on the other toes. As later birdsevolved, many features underwentchange. Notably, the fingers fused to-gether, the simple tail became a py-gostyle composed of fused vertebrae,and the back toe dropped, enablingbirds’ feet to grasp tree limbs firmly.

HALF-MOON-SHAPED WRISTBONE

MANIRAPTORAVelociraptor

AVES (Early)Archaeopteryx

AVES (Living)Columba

THEROPODA Three functional toes; hollow bones

Coelophysis Allosaurus Velociraptor ArchaeopteryxColumba(pigeon)

TETANURAE Three-fingered hand

MANIRAPTORA Half-moon-shaped wristbone

AVES Reversed first toe; fewer than 26 vertebrae in tail

CLAW CURVING TOWARD OTHERS

SCAPULA

CORACOID

STERNUM


not be regained. Birds, he asserted, must have evolved from amore archaic reptilian group that had clavicles. Like Seeleybefore him, Heilmann concluded that the similarities be-tween birds and dinosaurs must simply reflect the fact thatboth groups were bipedal.

Heilmann’s conclusions influenced thinking for a long time,

even though new information told a different story. Two sep-arate findings indicated that theropods did, in fact, have clav-icles. In 1924 a published anatomical drawing of the bizarre,parrot-headed theropod Oviraptor clearly showed a wish-bone, but the structure was misidentified. Then, in 1936,Charles Camp of the University of California at Berkeley

COMPARISONS OF ANATOMICAL STRUCTURES notonly helped to link birds to theropods, they also revealed someof the ways those features changed as dinosaurs became morebirdlike and birds became more modern. In the pelvis (sideview), the pubic bone (brown) initially pointed forward (towardthe right), but it later shifted to be vertical or pointed backward.In the hand (top view), the relative proportions of the bones re-

mained quite constant through the early birds, but the wristchanged. In the maniraptoran wrist, a disklike bone took on thehalf-moon shape (red) that ultimately promoted flapping flightin birds. The wide, boomerang-shaped wishbone (fused clavi-cles) in tetanurans and later groups compares well with that ofarchaic birds, but it became thinner and formed a deeper Ushape as it became more critical in flight.

THEROPODACoelophysis

Segisaurus (Close relative of Coelophysis)

No Coelophysis claviclesare well preserved

Not fused

TETANURAEAllosaurus

PEL

VIS

MANIRAPTORAVelociraptor

AVES (Early)Archaeopteryx

AVES (Living)Columba

WR

IST

AN

D H

AN

D

HALF-MOON-SHAPED WRIST-BONE

PUBIC BONE

REMNANT OF V

IV

III II

I

I

I

I

I

II

IIII

II

III

III

III

III

ISCHIUM

CLA

VIC

LES

ED H

ECK


found the remains of a small Early Jurassic theropod, com-plete with clavicles. Heilmann’s fatal objection had beenovercome, although few scientists recognized it. Recent stud-ies have found clavicles in a broad spectrum of the theropodsrelated to birds.

Finally, a century after Huxley’s disputed presentation tothe Geological Society of London, John H. Ostrom of YaleUniversity revived the idea that birds were related to thero-pod dinosaurs, and he proposed explicitly that birds weretheir direct descendants. In the late 1960s Ostrom had de-scribed the skeletal anatomy of the theropod Deinonychus, avicious, sickle-clawed predator about the size of an adolescenthuman, which roamed in Montana some 115 million yearsago (in the Early Cretaceous). In a series of papers publishedduring the next decade, Ostrom went on to identify a collec-tion of features that birds, including Archaeopteryx, sharedwith Deinonychus and other theropods but not with otherreptiles. On the basis of these findings, he concluded thatbirds are descended directly from small theropod dinosaurs.

As Ostrom was assembling his evidence for the theropodorigin of birds, a new method of deciphering the relationsamong organisms was taking hold in natural history muse-ums in New York City, Paris and elsewhere. This method—called phylogenetic systematics or, more commonly, cladis-tics—has since become the standard for comparative biology,and its use has strongly validated Ostrom’s conclusions.

Traditional methods for grouping organisms look at thesimilarities and differences among the animals and might ex-clude a species from a group solely because the species has atrait not found in other members of the group. In contrast,cladistics groups organisms based exclusively on certainkinds of shared traits that are particularly informative.

This method begins with the Darwinian precept that evo-lution proceeds when a new heritable trait emerges in someorganism and is passed genetically to its descendants. The pre-cept indicates that two groups of animals sharing a set ofsuch new, or “derived,” traits are more closely related to eachother than they are to groups that display only the originaltraits but not the derived ones. By identifying shared derivedtraits, practitioners of cladistics can determine the relationsamong the organisms they study.

The results of such analyses, which generally examinemany traits, can be represented in the form of a cladogram: atreelike diagram depicting the order in which new character-istics, and new creatures, evolved. Each branching point, ornode, reflects the emergence of an ancestor that founded agroup having derived characteristics not present in groupsthat evolved earlier. This ancestor and all its descendantsconstitute a “clade,” or closely related group.

Ostrom did not apply cladistic methods to determine thatbirds evolved from small theropod dinosaurs; in the 1970sthe approach was just coming into use. But about a decadelater Jacques A. Gauthier, then at the University of Californiaat Berkeley, did an extensive cladistic analysis of birds, dino-saurs and their reptilian relatives. Gauthier put Ostrom’s com-parisons and many other features into a cladistic frameworkand confirmed that birds evolved from small theropod di-nosaurs. Indeed, some of the closest relatives of birds includethe sickle-clawed maniraptoran Deinonychus that Ostromhad so vividly described.

Today a cladogram for the lineage leading from theropodsto birds shows that the clade labeled Aves (birds) consists ofthe ancestor of Archaeopteryx and all other descendants of

Bones ofContention

Although many lines of evidence establish that birdsevolved from small, terrestrial theropod dinosaurs, afew scientists remain vocally unconvinced. They havenot, however, tested any alternative theory by cladis-tics or by any other method that objectively analyzesrelationships among animals. Here is a sampling oftheir arguments, with some of the evidence againstthose assertions.

Bird and theropod hands differ: theropods retain fingers I,II and III (having lost the “pinky” and “ring finger”), but birdshave fingers II, III and IV. This view of the bird hand is basedon embryological research suggesting that when digits arelost from the five-fingered hand, the outer fingers (I and V)are the first to go. No one doubts that theropods retain fin-gers I, II and III, however, so this “law” clearly has exceptionsand does not rule out retention of the first three fingers inbirds. More important, the skeletal evidence belies the al-leged difference in the hands of birds and nonaviantheropods. The three fingers that nonavian theropods keptafter losing the fourth and fifth have the same forms, propor-tions and connections to the wristbones as the fingers in Ar-chaeopteryx and later birds [see middle row of illustration onprevious page].

Theropods appear too late to give rise to birds. Proponentsof this view have noted that Archaeopteryx appears in thefossil record about 150 million years ago, whereas the fossilremains of various nonavian maniraptors—the closestknown relatives of birds—date only to about 115 millionyears ago. But investigators have now uncovered bones thatevidently belong to small, nonavian maniraptors and thatdate to the time of Archaeopteryx. In any case, failure to findfossils of a predicted kind does not rule out their existencein an undiscovered deposit.

The wishbone (composed of fused clavicles) of birds isnot like the clavicles in theropods. This objection was rea-sonable when only the clavicles of early theropods had beendiscovered, but boomerang-shaped wishbones that look justlike that of Archaeopteryx have now been uncovered inmany theropods.

The complex lungs of birds could not have evolved fromtheropod lungs. This assertion cannot be supported or falsi-fied at the moment, because no fossil lungs are preserved inthe paleontological record. Also, the proponents of this argu-ment offer no animal whose lungs could have given rise tothose in birds, which are extremely complex and are unlikethe lungs of any living animal. —K.P. and L.M.C.


that ancestor. This clade is a subgroup of a broader cladeconsisting of so-called maniraptoran theropods—itself a sub-group of the tetanuran theropods that descended from themost basal theropods. Those archaic theropods in turnevolved from nontheropod dinosaurs. The cladogram showsthat birds are not only descended from dinosaurs, they aredinosaurs (and reptiles)—just as humans are mammals, eventhough people are as different from other mammals as birdsare from other reptiles.

Early Evolutionary Steps to Birds

Gauthier’s studies and ones conducted more recently dem-onstrate that many features traditionally considered

“birdlike” actually appeared before the advent of birds, intheir preavian theropod ancestors. Many of those propertiesundoubtedly helped their original possessors to survive asterrestrial dinosaurs; these same traits and others were even-tually used directly or were transformed to support flight andan arboreal way of life. The short length of this article doesnot allow us to catalogue the many dozens of details thatcombine to support the hypothesis that birds evolved fromsmall theropod dinosaurs, so we will concentrate mainly onthose related to the origin of flight.

The birdlike characteristics of the theropods that evolvedprior to birds did not appear all at once, and some were pres-ent before the theropods themselves emerged—in the earliest

dinosaurs. For instance, the immediate reptilian ancestor ofdinosaurs was already bipedal and upright in its stance (thatis, it basically walked like a bird), and it was small and carni-vorous. Its hands, in common with those of early birds, werefree for grasping (although the hand still had five digits, notthe three found in all but the most basal theropods and inbirds). Also, the second finger was longest—not the third, asin other reptiles.

Further, in the ancestors of dinosaurs, the ankle joint hadalready become hingelike, and the metatarsals, or foot bones,had became elongated. The metatarsals were held off theground, so the immediate relatives of dinosaurs, and dino-saurs themselves, walked on their toes and put one foot infront of the other, instead of sprawling. Many of the changesin the feet are thought to have increased stride length and run-ning speed, a property that would one day help avian thero-pods to fly.

The earliest theropods had hollow bones and cavities inthe skull; these adjustments lightened the skeleton. They alsohad a long neck and held their back horizontally, as birds dotoday. In the hand, digits four and five (the equivalent of thepinky and its neighbor) were already reduced in the first di-nosaurs; the fifth finger was virtually gone. Soon it was com-pletely lost, and the fourth was reduced to a nubbin. Those

reduced fingers disappeared altogether in tetanuran thero-pods, and the remaining three (I, II, III) became fused togeth-er sometime after Archaeopteryx evolved.

In the first theropods, the hind limbs became more birdlikeas well. They were long; the thigh was shorter than the shin,and the fibula, the bone to the side of the shinbone, was re-duced. (In birds today the toothpicklike bone in the drum-stick is all that is left of the fibula.) These dinosaurs walkedon the three middle toes—the same ones modern birds use.The fifth toe was shortened and tapered, with no joints, andthe first toe included a shortened metatarsal (with a smalljoint and a claw) that projected from the side of the secondtoe. The first toe was held higher than the others and had noapparent function, but it was later put to good use in birds.By the time Archaeopteryx appeared, that toe had rotated tolie behind the others. In later birds, it descended to becomeopposable to the others and eventually formed an importantpart of the perching foot.

More Changes

Through the course of theropod evolution, more featuresonce thought of as strictly avian emerged. For instance,

major changes occurred in the forelimb and shoulder girdle;these adjustments at first helped theropods to capture preyand later promoted flight. Notably, during theropod evolu-tion, the arms became progressively longer, except in such gi-

ant carnivores as Carnotaurus, Allosaurus and Tyrannosau-rus, in which the forelimbs were relatively small. The fore-limb was about half the length of the hind limb in very earlytheropods. By the time Archaeopteryx appeared, the fore-limb was longer than the hind limb, and it grew still more inlater birds. This lengthening in the birds allowed a strongerflight stroke.

The hand became longer, too, accounting for a progressive-ly greater proportion of the forelimb, and the wrist under-went dramatic revision in shape. Basal theropods possessed aflat wristbone (distal carpal) that overlapped the bases of thefirst and second palm bones (metacarpals) and fingers. Inmaniraptorans, though, this bone assumed a half-moon shapealong the surface that contacted the arm bones. The half-moon, or semilunate, shape was very important because it al-lowed these animals to flex the wrist sideways in addition toup and down. They could thus fold the long hand, almost asliving birds do. The longer hand could then be rotated andwhipped forward suddenly to snatch prey.

In the shoulder girdle of early theropods, the scapula (shoul-der blade) was long and straplike; the coracoid (which alongwith the scapula forms the shoulder joint) was rounded, andtwo separate, S-shaped clavicles connected the shoulder tothe sternum, or breastbone. The scapula soon became longer


Birds are not only descended from dinosaurs,

they are dinosaurs


and narrower; the coracoid also thinned and elongated,stretching toward the breastbone. The clavicles fused at themidline and broadened to form a boomerang-shaped wish-bone. The sternum, which consisted originally of cartilage,calcified into two fused bony plates in tetanurans. Togetherthese changes strengthened the skeleton; later this strengthen-ing was used to reinforce the flight apparatus and support theflight muscles. The new wishbone, for instance, probably be-came an anchor for the muscles that moved the forelimbs, atfirst during foraging and then during flight.

In the pelvis, more vertebrae were added to the hip girdle,and the pubic bone (the pelvic bone that is attached in frontof and below the hip socket) changed its orientation. In thefirst theropods, as in most other reptiles, the pubis pointeddown and forward, but then it began to point straight downor backward. Ultimately, in birds more advanced than Ar-chaeopteryx, it became parallel to the ischium, the pelvicbone that extends backward from below the hip socket. Thebenefits derived from these changes, if any, remain unknown,but the fact that these features are unique to birds and othermaniraptorans shows their common origin.

Finally, the tail gradually became shorter and stiffer through-out theropod history, serving more and more as a balancingorgan during running, somewhat as it does in today’s road-runners. Steven M. Gatesy of Brown University has demon-strated that this transition in tail structure paralleled anotherchange in function: the tail became less and less an anchorfor the leg muscles. The pelvis took over that function, and inmaniraptorans the muscle that once drew back the leg nowmainly controlled the tail. In birds that followed Archaeopte-ryx, these muscles would be used to adjust the feathered tailas needed in flight.

In summary, a great many skeletal features that were oncethought of as uniquely avian innovations—such as light, hol-low bones, long arms, three-fingered hands with a long sec-ond finger, a wishbone, a backward-pointing pelvis, and longhind limbs with a three-toed foot—were already present intheropods before the evolution of birds. Those features gen-erally served different uses than they did in birds and wereonly later co-opted for flight and other characteristicallyavian functions, eventually including life in the trees.

Evidence for the dinosaurian origin of birds is not confinedto the skeleton. Recent discoveries of nesting sites in Mon-golia and Montana reveal that some reproductive behaviorsof birds originated in nonavian dinosaurs. These theropodsdid not deposit a large clutch of eggs all at once, as most oth-er reptiles do. Instead they filled a nest more gradually, layingone or two eggs at a time, perhaps over several days, as birdsdo. Recently skeletons of the Cretaceous theropod Oviraptorhave been found atop nests of eggs; the dinosaurs were ap-parently buried while protecting the eggs in very birdlikefashion. This find is ironic because Oviraptor, whose namemeans “egg stealer,” was first thought to have been raidingthe eggs of other dinosaurs, rather than protecting them.Even the structure of the eggshell in theropods shows fea-tures otherwise seen only in bird eggs. The shells consist oftwo layers of calcite, one prismatic (crystalline) and onespongy (more irregular and porous).

As one supposedly uniquely avian trait after another hasbeen identified in nonavian dinosaurs, feathers have contin-ued to stand out as a prominent feature belonging to birdsalone. Some intriguing evidence, however, hints that evenfeathers might have predated the emergence of birds.

In 1996 and 1997 Ji Qiang and Ji Shu’an of the NationalGeological Museum of China published reports on two fossilanimals found in Liaoning Province that date to late in theJurassic or early in the Cretaceous. One, a turkey-size dino-saur named Sinosauropteryx, has fringed, filamentous struc-tures along its backbone and on its body surface. Thesestructures of the skin, or integument, may have been precur-sors to feathers. But the animal is far from a bird. It has shortarms and other skeletal properties indicating that it may berelated to the theropod Compsognathus, which is not espe-cially close to birds or other maniraptorans.

The second creature, Protarchaeopteryx, apparently hasshort, true feathers on its body and has longer feathers at-tached to its tail. Preliminary observations suggest that theanimal is a maniraptoran theropod. Whether it is also a birdwill depend on a fuller description of its anatomy. Neverthe-less, the Chinese finds imply that, at the least, the structuresthat gave rise to feathers probably appeared before birds didand almost certainly before birds began to fly. Whether theiroriginal function was for insulation, display or somethingelse cannot yet be determined.

The Beginning of Bird Flight

The origin of birds and the origin of flight are two distinct,albeit related, problems. Feathers were present for other

functions before flight evolved, and Archaeopteryx wasprobably not the very first flying theropod, although at pre-sent we have no fossils of earlier flying precursors. What canwe say about how flight began in bird ancestors?

Traditionally, two opposing scenarios have been put for-ward. The “arboreal” hypothesis holds that bird ancestorsbegan to fly by climbing trees and gliding down from branch-es with the help of incipient feathers. The height of trees pro-vides a good starting place for launching flight, especiallythrough gliding. As feathers became larger over time, flap-ping flight evolved, and birds finally became fully airborne.

This hypothesis makes intuitive sense, but certain aspectsare troubling. Archaeopteryx and its maniraptoran cousinshave no obviously arboreal adaptations, such as feet fullyadapted for perching. Perhaps some of them could climbtrees, but no convincing analysis has demonstrated how Ar-chaeopteryx would have climbed and flown with its fore-limbs, and there were no plants taller than a few meters inthe environments where Archaeopteryx fossils have beenfound. Even if the animals could climb trees, this ability isnot synonymous with arboreal habits or gliding ability. Mostsmall animals, and even some goats and kangaroos, can climbtrees, but that does not make them tree dwellers. Besides, Ar-chaeopteryx shows no obvious features of gliders, such as abroad membrane connecting forelimbs and hind limbs.

The “cursorial” (running) hypothesis holds that small di-nosaurs ran along the ground and stretched out their armsfor balance as they leaped into the air after insect prey or,perhaps, to avoid predators. Even rudimentary feathers onforelimbs could have expanded the arm’s surface area to en-hance lift slightly. Larger feathers could have increased lift in-crementally, until sustained flight was gradually achieved. Ofcourse, a leap into the air does not provide the accelerationproduced by dropping out of a tree; an animal would have torun quite fast to take off. Still, some small terrestrial animalscan achieve high speeds.

The cursorial hypothesis is strengthened by the fact that


the immediate theropod ancestors of birds were terrestrial.And they had the traits needed for high liftoff speeds: theywere small, active, agile, lightly built, long-legged and goodrunners. And because they were bipedal, their arms were freeto evolve flapping flight, which cannot be said for other rep-tiles of their time.

Although our limited evidence is tantalizing, probably nei-ther the arboreal nor the cursorial model is correct in its ex-treme form. More likely, the ancestors of birds used a combi-nation of taking off from the ground and taking advantageof accessible heights (such as hills, large boulders or fallentrees). They may not have climbed trees, but they could haveused every available object in their landscape to assist flight.

More central than the question of ground versus trees,however, is the evolution of a flight stroke. This stroke gener-ates not only the lift that gliding animals obtain from movingtheir wings through the air (as an airfoil) but also the thrustthat enables a flapping animal to move forward. (In contrast,the “organs” of lift and thrust in airplanes—the wings andjets—are separate.) In birds and bats, the hand part of thewing generates the thrust, and the rest of the wing providesthe lift.

Jeremy M. V. Rayner of the University of Bristol showed inthe late 1970s that the down-and-forward flight stroke ofbirds and bats produces a series of doughnut-shaped vorticesthat propel the flying animal forward. One of us (Padian)and Gauthier then demonstrated in the mid-1980s that themovement generating these vortices in birds is the same ac-tion—sideways flexion of the hand—that was already presentin the maniraptorans Deinonychus and Velociraptor and inArchaeopteryx.

As we noted earlier, the first maniraptorans must have usedthis movement to grab prey. By the time Archaeopteryx andother birds appeared, the shoulder joint had changed its an-gle to point more to the side than down and backward. Thisalteration in the angle transformed the forelimb motion froma prey-catching one to a flight stroke. New evidence from Ar-gentina suggests that the shoulder girdle in the closest mani-raptorans to birds (the new dinosaur Unenlagia) was alreadyangled outward so as to permit this kind of stroke.

Recent work by Farish A. Jenkins, Jr., of Harvard Universi-ty, George E. Goslow of Brown University and their col-leagues has revealed much about the role of thewishbone in flight and about how theflight stroke is achieved. The wish-bone in some living birds acts as aspacer between the shoulder girdles,one that stores energy expendedduring the flight stroke. In the firstbirds, in contrast, it probably wasless elastic, and its main functionmay have been simply to anchor theforelimb muscles. Apparently, too,the muscle most responsible for ro-tating and raising the wing duringthe recovery stroke of flight was notyet in the modern position inArchaeopteryx or other very earlybirds. Hence, those birds wereprobably not particularly skilledfliers; they would have been unableto flap as quickly or as precisely astoday’s birds can. But it was not

long—perhaps just several million years—before birds ac-quired the apparatus they needed for more controlled flight.

Beyond Archaeopteryx

More than three times as many bird fossils from the Cre-taceous period have been found since 1990 than in all

the rest of recorded history. These new specimens—uncov-ered in such places as Spain, China, Mongolia, Madagascarand Argentina—are helping paleontologists to flesh out theearly evolution of the birds that followed Archaeopteryx, in-cluding their acquisition of an improved flying system. Anal-yses of these finds by one of us (Chiappe) and others haveshown that birds quickly took on many different sizes,shapes and behaviors (ranging from diving to flightlessness)and diversified all through the Cretaceous period, which end-ed about 65 million years ago.

A bird-watching trek through an Early Cretaceous forestwould bear little resemblance to such an outing now. Theseearly birds might have spent much of their time in the treesand were able to perch, but there is no evidence that the firstbirds nested in trees, had complex songs or migrated greatdistances. Nor did they fledge at nearly adult size, as birds donow, or grow as rapidly as today’s birds do. Scientists canonly imagine what these animals looked like. Undoubtedly,however, they would have seemed very strange, with theirclawed fingers and, in many cases, toothed beaks.

Underneath the skin, though, some skeletal features cer-tainly became more birdlike during the Early Cretaceous andenabled birds to fly quite well. Many bones in the hand andin the hip girdle fused, providing strength to the skeleton forflight. The breastbone became broader and developed a keeldown the midline of the chest for flight muscle attachment.The forearm became much longer, and the skull bones andvertebrae became lighter and more hollowed out. The tail-bones became a short series of free segments ending in afused stump (the familiar “parson’s nose” or “Pope’s nose”of roasted birds) that controlled the tail feathers. And thealula, or “thumb wing,” a part of the bird wing essential forflight control at low speed, made its debut, as did a long firsttoe useful in perching.

Inasmuch as early birds could fly, they certainly had higher

OVIRAPTOR, a maniraptoran theropod that evolved be-fore birds, sat in its nest to protect its eggs (left drawing),just as the ostrich (right drawing) and other birds do to-day. In other words, such brooding originated before birdsdid.


metabolic rates than cold-blooded reptiles; at least they wereable to generate the heat and energy needed for flying with-out having to depend on being heated by the environment.But they might not have been as fully warm-blooded as to-day’s birds. Their feathers, in addition to aiding flight, pro-vided a measure of insulation—just as the precursors of feath-ers could have helped preserve heat and conserve energy innonavian precursors of birds. These birds probably did notfly as far or as strongly as birds do now.

Bird-watchers traipsing through a forest roughly 50 mil-lion years later would still have found representatives of veryprimitive lineages of birds. Yet other birds would have beenrecognizable as early members of living groups. Recent re-search shows that at least four major lineages of livingbirds—including ancient relatives of shorebirds, seabirds,loons, ducks and geese—were already thriving several millionyears before the end of the Cretaceous period, and new pale-ontological and molecular evidence suggests that forerunnersof other modern birds were around as well.

Most lineages of birds that evolved during the Cretaceousdied out during that period, although there is no evidence

that they perished suddenly. Researchers may neverknow whether the birds that disappeared were

outcompeted by newer forms, were killed byan environmental catastrophe or were just

unable to adapt to changes in theirworld. There is no reasonable doubt,

however, that all groups of birds, living and extinct, are de-scended from small, meat-eating theropod dinosaurs, asHuxley’s work intimated more than a century ago. In fact,living birds are nothing less than small, feathered, short-tailed theropod dinosaurs.

CLADOGRAM OF BIRD EVOLUTION indicates that birds(Aves) perfected their flight stroke gradually after they first ap-peared approximately 150 million years ago. They became ar-

boreal (able to live in trees) relatively early in their history, how-ever. Some of the skeletal innovations that supported theiremerging capabilities are listed at the bottom.

Prey-seizing forelimb stroke Flapping flightMore powerful flight

stroke; arborealityEnhanced flightmaneuverability

Essentially modernflight abilities

For most, nest-ing in trees and

migration

Velociraptor Archaeopteryx Iberomesornis Enantiornithes Ichthyornithiformes Living birds

AVES

EVOLVING CAPABILITIES

Long, grasping arms;swivel wrist

Flight feathers; lengthened arms;

shortened tail

Strutlike bones thatbrace shoulder to chest; pygostyle (tail stump);

perching feet

More skeletal fusion; alula

(“thumb wing”)

Shorter back and tail;deeply keeled breast-bone (sternum); morecompact back and hip

ILLU

STR

ATIO

NS

BY E

D H

ECK

ED H

ECK

The Authors

KEVIN PADIAN and LUIS M. CHIAPPE are frequent collabo-rators. Padian is professor of integrative biology and curator in theMuseum of Paleontology at the University of California, Berkeley.He is also president of the National Center for Science Education.Chiappe, who has extensively studied the radiation of birds duringthe Cretaceous period, is Chapman Fellow and research associateat the American Museum of Natural History in New York Cityand adjunct professor at the City University of New York.

Further Reading

Archaeopteryx and the Origin of Birds. John H. Ostrom inBiological Journal of the Linnaean Society (London), Vol. 8, No.1, pages 91–182; 1976.

Saurischian Monophyly and the Origin of Birds. JacquesA. Gauthier in Memoirs of the California Academy of Sciences,Vol. 8, pages 1–55; 1986.

The First 85 Million Years of Avian Evolution. Luis M.Chiappe in Nature, Vol. 378, pages 349–355; November 23,1995.

The Encyclopedia of Dinosaurs (see entries “Aves” and “BirdOrigins”). Edited by Philip J. Currie and Kevin Padian. Academ-ic Press, 1997.

The Origin and Early Evolution of Birds. Kevin Padian andLuis M. Chiappe in Biological Reviews (in press).


Mammals ThatConquered the The

New fossils and DNA analyses elucidate the remarkable

Originally published in May 2002


Seas By Kate WongBy Kate Wongevolutionary history of whales

“They say the sea is cold, but the sea contains the hottest blood of all, and the wildest, the most urgent.”

—D. H. Lawrence, “Whales Weep Not!”

Dawn breaks over

the Tethys Sea, 48 million

years ago, and the blue-

green water sparkles with

the day’s first light. But for

one small mammal, this

new day will end almost as

soon as it has started.

ANCIENT WHALE Rodhocetus (right and left front)feasts on the bounty of the sea, while Ambulocetus(rear) attacks a small land mammal some 48 millionyears ago in what is now Pakistan.


Tapir-like Eotitanops has wandered perilously close to thewater’s edge, ignoring its mother’s warning call. For the brutelurking motionless among the mangroves, the opportunity issimply too good to pass up. It lunges landward, propelled bypowerful hind limbs, and sinks its formidable teeth into the calf,dragging it back into the surf. The victim’s frantic strugglingsubsides as it drowns, trapped in the viselike jaws of its cap-tor. Victorious, the beast shambles out of the water to devourits kill on terra firma. At first glance, this fearsome predator re-sembles a crocodile, with its squat legs, stout tail, long snoutand eyes that sit high on its skull. But on closer inspection, ithas not armor but fur, not claws but hooves. And the cusps onits teeth clearly identify it not as a reptile but as a mammal. Infact, this improbable creature is Ambulocetus, an early whale,and one of a series of intermediates linking the land-dwellingancestors of cetaceans to the 80 or so species of whales, dol-phins and porpoises that rule the oceans today.

Until recently, the emergence of whales was one of the mostintractable mysteries facing evolutionary biologists. Lacking furand hind limbs and unable to go ashore for so much as a sip offreshwater, living cetaceans represent a dramatic departurefrom the mammalian norm. Indeed, their piscine form led Her-man Melville in 1851 to describe Moby Dick and his fellowwhales as fishes. But to 19th-century naturalists such as CharlesDarwin, these air-breathing, warm-blooded animals that nursetheir young with milk distinctly grouped with mammals. Andbecause ancestral mammals lived on land, it stood to reasonthat whales ultimately descended from a terrestrial ancestor.Exactly how that might have happened, however, eluded schol-ars. For his part, Darwin noted in On the Origin of Species thata bear swimming with its mouth agape to catch insects was aplausible evolutionary starting point for whales. But the propo-sition attracted so much ridicule that in later editions of thebook he said just that such a bear was “almost like a whale.”

The fossil record of cetaceans did little to advance the studyof whale origins. Of the few remains known, none were suffi-ciently complete or primitive to throw much light on the mat-ter. And further analyses of the bizarre anatomy of livingwhales led only to more scientific head scratching. Thus, evena century after Darwin, these aquatic mammals remained anevolutionary enigma. In fact, in his 1945 classification of mam-mals, famed paleontologist George Gaylord Simpson notedthat whales had evolved in the oceans for so long that nothinginformative about their ancestry remained. Calling them “onthe whole, the most peculiar and aberrant of mammals,” he in-serted cetaceans arbitrarily among the other orders. Wherewhales belonged in the mammalian family tree and how theytook to the seas defied explanation, it seemed.

Over the past two decades, however, many of the pieces ofthis once imponderable puzzle have fallen into place. Paleon-tologists have uncovered a wealth of whale fossils spanning theEocene epoch, the time between 55 million and 34 million yearsago when archaic whales, or archaeocetes, made their transi-tion from land to sea. They have also unearthed some cluesfrom the ensuing Oligocene, when the modern suborders ofcetaceans—the mysticetes (baleen whales) and the odontocetes(toothed whales)—arose. That fossil material, along with analy-ses of DNA from living animals, has enabled scientists to painta detailed picture of when, where and how whales evolved fromtheir terrestrial forebears. Today their transformation—fromlandlubbers to Leviathans—stands as one of the most profoundevolutionary metamorphoses on record.

Evolving IdeasAT AROUND THE SAME TIME that Simpson declared therelationship of whales to other mammals undecipherable on thebasis of anatomy, a new comparative approach emerged, onethat looked at antibody-antigen reactions in living animals. Inresponse to Simpson’s assertion, Alan Boyden of Rutgers Uni-versity and a colleague applied the technique to the whale ques-tion. Their results showed convincingly that among living ani-mals, whales are most closely related to the even-toed hoofedmammals, or artiodactyls, a group whose members includecamels, hippopotamuses, pigs and ruminants such as cows.Still, the exact nature of that relationship remained unclear.Were whales themselves artiodactyls? Or did they occupy theirown branch of the mammalian family tree, linked to the artio-dactyl branch via an ancient common ancestor?

Support for the latter interpretation came in the 1960s,from studies of primitive hoofed mammals known as condy-larths that had not yet evolved the specialized characteristics ofartiodactyls or the other mammalian orders. PaleontologistLeigh Van Valen, then at the American Museum of NaturalHistory in New York City, discovered striking resemblancesbetween the three-cusped teeth of the few known fossil whalesand those of a group of meat-eating condylarths called mesony-chids. Likewise, he found shared dental characteristics betweenartiodactyls and another group of condylarths, the arctocy-onids, close relatives of the mesonychids. Van Valen conclud-ed that whales descended from the carnivorous, wolflikemesonychids and thus were linked to artiodactyls through thecondylarths. K

ARE

N C

ARR

(p

rece

din

g p

ag

es)

CETACEA is the order of mammals that comprises livingwhales, dolphins and porpoises and their extinct ancestors,the archaeocetes. Living members fall into two suborders: theodontocetes, or toothed whales, including sperm whales, pilotwhales, belugas, and all dolphins and porpoises; and themysticetes, or baleen whales, including blue whales and finwhales. The term “whale” is often used to refer to all cetaceans.

MESONYCHIDS are a group of primitive hoofed, wolflikemammals once widely thought to have given rise to whales.

ARTIODACTYLA is the order of even-toed, hoofed mammalsthat includes camels; ruminants such as cows; hippos;and, most researchers now agree, whales.

EOCENE is the epoch between 55 million and 34 millionyears ago, during which early whales made their transitionfrom land to sea.

OLIGOCENE is the epoch between 34 million and 24 millionyears ago, during which odontocetes and mysticetesevolved from their archaeocete ancestors.

Guide to Terminology



Walking WhalesA DECADE OR SO PASSED before paleontologists finally be-gan unearthing fossils close enough to the evolutionary branch-ing point of whales to address Van Valen’s mesonychid hy-pothesis. Even then the significance of these finds took a whileto sink in. It started when University of Michigan paleontolo-gist Philip Gingerich went to Pakistan in 1977 in search ofEocene land mammals, visiting an area previously reported toshelter such remains. The expedition proved disappointing be-cause the spot turned out to contain only marine fossils. Find-ing traces of ancient ocean life in Pakistan, far from the coun-

try’s modern coast, is not surprising: during the Eocene, the vastTethys Sea periodically covered great swaths of what is now theIndian subcontinent. Intriguingly, though, the team discoveredamong those ancient fish and snail remnants two pelvis frag-ments that appeared to have come from relatively large, walk-ing beasts. “We joked about walking whales,” Gingerich re-calls with a chuckle. “It was unthinkable.” Curious as the pelvispieces were, the only fossil collected during that field season thatseemed important at the time was a primitive artiodactyl jawthat had turned up in another part of the country.

Two years later, in the Himalayan foothills of northern Pak-istan, Gingerich’s team found another weird whale clue: a par-

climate systems brought about radical changes in thequantity and distribution of nutrients in the sea, generating a whole new set of ecological opportunities for the cetaceans.

As posited by paleontologist Ewan Fordyce of the Universityof Otago in New Zealand, that set the stage for thereplacement of the archaeocetes by the odontocetes andmysticetes (toothed and baleen whales, respectively). Theearliest known link between archaeocetes and the moderncetacean orders, Fordyce says, is Llanocetus, a 34-million-year-old protobaleen whale from Antarctica that may well havetrawled for krill in the chilly Antarctic waters, just as livingbaleen whales do. Odontocetes arose at around the same time, he adds, specializing to become echolocators that couldhunt in the deep.

Unfortunately, fossils documenting the origins ofmysticetes and odontocetes are vanishingly rare. Low sealevels during the middle Oligocene exposed most potentialwhale-bearing sediments from the early Oligocene to erosivewinds and rains, making that period largely “a fossilwasteland,” says paleontologist Mark Uhen of the CranbrookInstitute of Science in Bloomfield Hills, Mich. The later fossilrecord clearly shows, however, that shortly after, by about 30million years ago, the baleen and toothed whales haddiversified into many of the cetacean families that reign overthe oceans today. —K.W.

It might seem odd that 300 million years after vertebratesfirst established a toehold on land, some returned to the sea.But the setting in which early whales evolved offers hints as

to what lured them back to the water. For much of the Eoceneepoch (roughly between 55 million and 34 million years ago), a sea called Tethys, after a goddess of Greek mythology,stretched from Spain to Indonesia. Although the continents andocean plates we know now had taken shape, India was stilladrift, Australia hadn’t yet fully separated from Antarctica, andgreat swaths of Africa and Eurasia lay submerged underTethys. Those shallow, warm waters incubated abundantnutrients and teemed with fish. Furthermore, the spacevacated by the plesiosaurs, mosasaurs and other large marinereptiles that perished along with the dinosaurs created roomfor new top predators (although sharks and crocodiles stillprovided a healthy dose of competition). It is difficult toimagine a more enticing invitation to aquatic life for a mammal.

During the Oligocene epoch that followed, sea levels sankand India docked with the rest of Asia, forming the crumpledinterface we know as the Himalayas. More important,University of Michigan paleontologist Philip Gingerich notes,Australia and Antarctica divorced, opening up the SouthernOcean and creating a south circumpolar current thateventually transformed the balmy Eocene earth into the ice-capped planet we inhabit today. The modern current and

SAR

A C

HE

N A

ND

ED

WAR

D B

ELL

50 Million Years Ago Present

PROTO-INDIA

PROTO-AUSTRALIA

BASILOSAURIDSFOSSIL LOCATIONS

PROTOCETIDS

THE WHALE’S CHANGING WORLD

LLANOCETUSPAKICETIDS AMBULOCETIDS REMINGTONOCETIDS

TETHYS SEA



PO

RTI

A SL

OAN

AN

D E

DW

ARD

BE

LL

HIPPOS = HIPPOPOTAMIDSARTIOS = ARTIODACTYLS OTHER THAN HIPPOS MESOS = MESONYCHIDS

OLD MESONYCHID HYPOTHESIS

MESOS ARTIOS HIPPOS WHALES

ARTIOS HIPPOS MESOS WHALES

HIPPOPOTAMID HYPOTHESIS

ARTIOS HIPPOS MESOS WHALES

NEW MESONYCHID HYPOTHESIS

MESOS ARTIOS HIPPOS WHALES

ARTIODACTYL HYPOTHESIS

FAMILY TREE OF CETACEANS shows the descent of the two modernsuborders of whales, the odontocetes and mysticetes, from theextinct archaeocetes. Representative members of each archaeocetefamily or subfamily are depicted (left). Branching diagrams illustratevarious hypotheses of the relationship of whales to other mammals(right). The old mesonychid hypothesis, which posits that extinctwolflike beasts known as mesonychids are the closest relatives ofwhales, now seems unlikely in light of new fossil whale discoveries.The anklebones of those ancient whales bear the distinctivecharacteristics of artiodactyl ankles, suggesting that whales are

themselves artiodactyls, as envisioned by the artiodactylhypothesis. Molecular studies indicate that whales are more closelyrelated to hippopotamuses than to any other artiodactyl group.Whether the fossil record can support the hippopotamid hypothesis,however, remains to be seen. A fourth scenario, denoted here asthe new mesonychid hypothesis, proposes that mesonychids couldstill be the whale’s closest kin if they, too, were included in theartiodactyl order, instead of the extinct order Condylarthra, in whichthey currently reside. If so, they would have to have lost the ankletraits that characterize all known artiodactyls. —K.W.

CETACEAN RELATIONS

BASILOSAURUS18.2 meters

DORUDON4.5 meters

RODHOCETUS3 meters

KUTCHICETUS1.75 meters

AMBULOCETUS4.15 meters

PAKICETUS1.75 meters

Millions of Years Ago55 50 45 40 35

PAKICETIDAE

AMBULOCETIDAE

PROTOCETIDAE

BASILOSAURIDAEODONTOCETES

MYSTICETES

CETACEA

DORUDONTINAE

BASILOSAURINAE

REMINGTONOCETIDAE


tial braincase from a wolf-size creature—found in the companyof 50-million-year-old land mammal remains—that bore somedistinctive cetacean characteristics. All modern whales have fea-tures in their ears that do not appear in any other vertebrates.Although the fossil skull lacked the anatomy necessary for hear-ing directionally in water (a critical skill for living whales), itclearly had the diagnostic cetacean ear traits. The team had dis-covered the oldest and most primitive whale then known—onethat must have spent some, if not most, of its time on land. Gin-gerich christened the creature Pakicetus for its place of originand, thus hooked, began hunting for ancient whales in earnest.

At around the same time, another group recovered addi-tional remains of Pakicetus—a lower jaw fragment and someisolated teeth—that bolstered the link to mesonychids throughstrong dental similarities. With Pakicetus showing up around 50million years ago and mesonychids known from around thesame time in the same part of the world, it looked increasinglylikely that cetaceans had indeed descended from the mesonychidsor something closely related to them. Still, what the earliestwhales looked like from the neck down was a mystery.

Further insights from Pakistan would have to wait, how-ever. By 1983 Gingerich was no longer able to work there be-cause of the Soviet Union’s invasion of Afghanistan. He decid-ed to cast his net in Egypt instead, journeying some 95 milessouthwest of Cairo to the Western Desert’s Zeuglodon Valley,so named for early 20th-century reports of fossils of archaicwhales—or zeuglodons, as they were then known—in the area.Like Pakistan, much of Egypt once lay submerged underTethys. Today the skeletons of creatures that swam in that an-cient sea lie entombed in sandstone. After several field seasons,Gingerich and his crew hit pay dirt: tiny hind limbs belongingto a 60-foot-long sea snake of a whale known as Basilosaurusand the first evidence of cetacean feet.

Earlier finds of Basilosaurus, a fully aquatic monster thatslithered through the seas between some 40 million and 37 mil-lion years ago, preserved only a partial femur, which its discov-erers interpreted as vestigial. But the well-formed legs and feetrevealed by this discovery hinted at functionality. Although atless than half a meter in length the diminutive limbs probablywould not have assisted Basilosaurus in swimming and certain-ly would not have enabled it to walk on land, they may well havehelped guide the beast’s serpentine body during the difficult ac-

tivity of aquatic mating. Whatever their purpose, if any, the lit-tle legs had big implications. “I immediately thought, we’re 10million years after Pakicetus,” Gingerich recounts excitedly. “Ifthese things still have feet and toes, we’ve got 10 million yearsof history to look at.” Suddenly, the walking whales they hadscoffed at in Pakistan seemed entirely plausible.

Just such a remarkable creature came to light in 1992. Ateam led by J.G.M. (Hans) Thewissen of the Northeastern OhioUniversities College of Medicine recovered from 48-million-year-old marine rocks in northern Pakistan a nearly completeskeleton of a perfect intermediate between modern whales andtheir terrestrial ancestors. Its large feet and powerful tail be-spoke strong swimming skills, while its sturdy leg bones andmobile elbow and wrist joints suggested an ability to locomoteon land. He dubbed the animal Ambulocetus natans, the walk-ing and swimming whale.

Shape ShiftersSINCE THEN, Thewissen, Gingerich and others have uneartheda plethora of fossils documenting subsequent stages of thewhale’s transition from land to sea. The picture emerging fromthose specimens is one in which Ambulocetus and its kin—them-selves descended from the more terrestrial pakicetids—spawnedneedle-nosed beasts known as remingtonocetids and the intre-pid protocetids—the first whales seaworthy enough to fan outfrom Indo-Pakistan across the globe. From the protocetidsarose the dolphinlike dorudontines, the probable progenitorsof the snakelike basilosaurines and modern whales [see box onprevious page].

In addition to furnishing supporting branches for the whalefamily tree, these discoveries have enabled researchers to chartmany of the spectacular anatomical and physiological changesthat allowed cetaceans to establish permanent residency in theocean realm. Some of the earliest of these adaptations to emerge,as Pakicetus shows, are those related to hearing. Sound travelsdifferently in water than it does in air. Whereas the ears of hu-mans and other land-dwelling animals have delicate, flat ear-drums, or tympanic membranes, for receiving airborne sound,modern whales have thick, elongate tympanic ligaments thatcannot receive sound. Instead a bone called the bulla, which inwhales has become quite dense and is therefore capable of trans-mitting sound coming from a denser medium to deeper partsof the ear, takes on that function. The Pakicetus bulla showssome modification in that direction, but the animal retained aland mammal–like eardrum that could not work in water.

What, then, might Pakicetus have used its thickened bul-lae for? Thewissen suspects that much as turtles hear by

picking up vibrations from the ground through theirshields, Pakicetus may have employed its bullaeto pick up ground-borne sounds. Taking newpostcranial evidence into consideration alongwith the ear morphology, he envisions Pakicetus

as an ambush predator that may have lurkedaround shallow rivers, head to the ground, preying

on animals that came to drink. Ambulocetus is even


more likely to have used such inertial hearing, Thewissen says,because it had the beginnings of a channel linking jaw and ear.By resting its jaw on the ground—a strategy seen in modern croc-odiles—Ambulocetus could have listened for approaching prey.The same features that allowed early whales to receive soundsfrom soil, he surmises, preadapted them to hearing in the water.

Zhe-Xi Luo of the Carnegie Museum of Natural History inPittsburgh has shown that by the time of the basilosaurines anddorudontines, the first fully aquatic whales, the ropelike tym-panic ligament had probably already evolved. Additionally, airsinuses, presumably filled with spongelike tissues, had formedaround the middle ear, offering better sound resolution and di-rectional cues for underwater hearing. Meanwhile, with the ex-ternal ear canal closed off (a prerequisite for deep-sea diving),he adds, the lower jaw was taking on an increasingly importantauditory role, developing a fat-filled canal capable of conduct-ing sound back to the middle ear.

Later in the evolution of whale hearing, the toothed andbaleen whales parted ways. Whereas the toothed whales evolvedthe features necessary to produce and receive high-frequencysounds, enabling echolocation for hunting, the baleen whalesdeveloped the ability to produce and receive very low frequen-cy sounds, allowing them to communicate with one another overvast distances. Fossil whale ear bones, Luo says, show that byaround 28 million years ago early odontocetes already had someof the bony structures necessary for hearing high-pitched soundand were thus capable of at least modest echolocation. The ori-gin of the mysticete’s low-frequency hearing is far murkier, eventhough the fossil evidence of that group now dates back to asearly as 34 million years ago.

Other notable skull changes include movement of the eyesockets from a crocodilelike placement atop the head in Pa-kicetus and Ambulocetus to a lateral position in the moreaquatic protocetids and later whales. And the nasal opening mi-grated back from the tip of the snout in Pakicetus to the top ofthe head in modern cetaceans, forming the blowhole. Whaledentition morphed, too, turning the complexly cusped, grind-

ing molars of primitive mammalian ancestors into the simple,pronglike teeth of modern odontocetes, which grasp and swal-low their food without chewing. Mysticetes lost their teeth al-together and developed comblike plates of baleen that hangfrom their upper jaws and strain plankton from the seawater.

The most obvious adaptations making up the whale’s pro-tean shift are those that produced its streamlined shape and un-matched swimming abilities. Not surprisingly, some bizarre am-phibious forms resulted along the way. Ambulocetus, for one, re-tained the flexible shoulder, elbow, wrist and finger joints of itsterrestrial ancestors and had a pelvis capable of supporting itsweight on land. Yet the creature’s disproportionately large hindlimbs and paddlelike feet would have made walking somewhatawkward. These same features were perfect for paddling aroundin the fish-filled shallows of Tethys, however.

Moving farther out to sea required additional modifications,many of which appear in the protocetid whales. Studies of onemember of this group, Rodhocetus, indicate that the lower armbones were compressed and already on their way to becominghydrodynamically efficient, says University of Michigan paleon-tologist Bill Sanders. The animal’s long, delicate feet were prob-ably webbed, like the fins used by scuba divers. Rodhocetus alsoexhibits aquatic adaptations in its pelvis, where fusion betweenthe vertebrae that form the sacrum is reduced, loosening up thelower spine to power tail movement. These features, says Gin-gerich, whose team discovered the creature, suggest that Rod-hocetus performed a leisurely dog paddle at the sea surface anda swift combination of otterlike hind-limb paddling and tailpropulsion underwater. When it went ashore to breed or perhapsto bask in the sun, he proposes, Rodhocetus probably hitcheditself around somewhat like a modern eared seal or sea lion.

By the time of the basilosaurines and dorudontines, whaleswere fully aquatic. As in modern cetaceans, the shoulder re-mained mobile while the elbow and wrist stiffened, forming flip-pers for steering and balance. Farther back on the skeleton, onlytiny legs remained, and the pelvis had dwindled accordingly.Analyses of the vertebrae of Dorudon, conducted by Mark D. P

OR

TIA

SLO

AN (

illu

stra

tion

s)

BECOMING LEVIATHAN

REPRESENTATIVE ARCHAEOCETES in the lineage leading to modern odontocetesand mysticetes trace some of the anatomical changes that enabled theseanimals to take to the seas (reconstructed bone appears in lavender). In just 15million years, whales shed their terrestrial trappings and became fully adaptedto aquatic life. Notably, the hind limbs diminished, the forelimbs transformedinto flippers, and the vertebral column evolved to permit tail-powered swimming.Meanwhile the skull changed to enable underwater hearing, the nasal openingmoved backward to the top of the skull, and the teeth simplified into pegs forgrasping instead of grinding. Later in whale evolution, the mysticetes’ teethwere replaced with baleen.

PAKICETUS AMBULOCETUS

MODERN MYSTICETE


Uhen of the Cranbrook Institute of Science in Bloomfield Hills,Mich., have revealed one tail vertebra with a rounded profile.Modern whales have a similarly shaped bone, the ball vertebra,at the base of their fluke, the flat, horizontal structure capping thetail. Uhen thus suspects that basilosaurines and dorudontineshad tail flukes and swam much as modern whales do, using so-called caudal oscillation. In this energetically efficient mode oflocomotion, motion generated at a single point in the vertebralcolumn powers the tail’s vertical movement through the water,and the fluke generates lift.

Exactly when whales lost their legs altogether remains un-known. In fact, a recent discovery made by Lawrence G. Barnesof the Natural History Museum of Los Angeles County hints atsurprisingly well developed hind limbs in a 27-million-year-oldbaleen whale from Washington State, suggesting that whale legspersisted far longer than originally thought. Today, however,some 50 million years after their quadrupedal ancestors first wad-ed into the warm waters of Tethys, whales are singularly sleek.Their hind limbs have shrunk to externally invisible vestiges, andthe pelvis has diminished to the point of serving merely as an an-chor for a few tiny muscles unrelated to locomotion.

Making WavesTHE FOSSILS UNCOVERED during the 1980s and 1990s ad-vanced researchers’ understanding of whale evolution by leapsand bounds, but all morphological signs still pointed to amesonychid origin. An alternative view of cetacean roots wastaking wing in genetics laboratories in the U.S., Belgium andJapan, however. Molecular biologists, having developed so-phisticated techniques for analyzing the DNA of living creatures,took Boyden’s 1960s immunology-based conclusions a step fur-ther. Not only were whales more closely related to artiodactylsthan to any other living mammals, they asserted, but in factwhales were themselves artiodactyls, one of many twigs on thatbranch of the mammalian family tree. Moreover, a number ofthese studies pointed to an especially close relationship betweenwhales and hippopotamuses. Particularly strong evidence for

this idea came in 1999 from analyses of snippets of noncodingDNA called SINES (short interspersed elements), conducted byNorihiro Okada and his colleagues at the Tokyo Institute ofTechnology.

The whale-hippo connection did not sit well with paleontol-ogists. “I thought they were nuts,” Gingerich recollects. “Every-thing we’d found was consistent with a mesonychid origin. I washappy with that and happy with a connection through mesony-chids to artiodactyls.” Whereas mesonychids appeared at theright time, in the right place and in the right form to be consid-ered whale progenitors, the fossil record did not seem to containa temporally, geographically and morphologically plausible ar-tiodactyl ancestor for whales, never mind one linking whalesand hippos specifically. Thewissen, too, had largely dismissedthe DNA findings. But “I stopped rejecting it when Okada’sSINE work came out,” he says.

It seemed the only way to resolve the controversy was to find,of all things, an ancient whale anklebone. Morphologists havetraditionally defined artiodactyls on the basis of certain featuresin one of their anklebones, the astragalus, that enhance mobili-ty. Specifically, the unique artiodactyl astragalus has twogrooved, pulleylike joint surfaces. One connects to the tibia, orshinbone; the other articulates with more distal anklebones. Ifwhales descended from artiodactyls, researchers reasoned, thosethat had not yet fully adapted to life in the seas should exhibitthis double-pulleyed astragalus.

That piece of the puzzle fell into place last fall, when Gin-gerich and Thewissen both announced discoveries of new prim-itive whale fossils. In the eastern part of Baluchistan Province,Gingerich’s team had found partially articulated skeletons ofRodhocetus balochistanensis and a new protocetid genus, Ar-tiocetus. Thewissen and his colleagues recovered from a bonebed in the Kala Chitta Hills of Punjab, Pakistan, much of thelong-sought postcranial skeleton of Pakicetus, as well as thatof a smaller member of the pakicetid family, Ichthyolestes. Eachcame with an astragalus bearing the distinctive artiodactylcharacteristics.

MODERN ODONTOCETE

RODHOCETUS DORUDON


The anklebones convinced both longtime proponents of themesonychid hypothesis that whales instead evolved from artio-dactyls. Gingerich has even embraced the hippo idea. Althoughhippos themselves arose long after whales, their purported an-cestors—dog- to horse-size, swamp-dwelling beasts called an-thracotheres—date back to at least the middle Eocene and maythus have a forebear in common with the cetaceans. In fact, Gin-gerich notes that Rodhocetus and anthracotheres share featuresin their hands and wrists not seen in any other later artiodactyls.

Thewissen agrees that the hippo hypothesis holds much moreappeal than it once did. But he cautions that the morphologicaldata do not yet point to a particular artiodactyl, such as the hip-po, being the whale’s closest relative, or sister group. “We don’thave the resolution yet to get them there,” he remarks, “but Ithink that will come.”

What of the evidence that seemed to tie early whales tomesonychids? In light of the new ankle data, most workers nowsuspect that those similarities probably reflect convergent evo-lution rather than shared ancestry and that mesonychids repre-sent an evolutionary dead end. But not everyone is convinced.Maureen O’Leary of the State University of New York at StonyBrook argues that until all the available evidence—both mor-phological and molecular—is incorporated into a single phylo-genetic analysis, the possibility remains that mesonychids belongat the base of the whale pedigree. It is conceivable, she says, thatmesonychids are actually ancient artiodactyls but ones that re-versed the ankle trend. If so, mesonychids could still be thewhales’ closest relative, and hippos could be their closest livingrelative. Critics of that idea, however, point out that althoughfolding the mesonychids into the artiodactyl order offers an es-cape hatch of sorts to supporters of the mesonychid hypothesis,it would upset the long-standing notion that the ankle makes theartiodactyl.

Investigators agree that figuring out the exact relationshipbetween whales and artiodactyls will most likely require findingadditional fossils—particularly those that can illuminate the be-ginnings of artiodactyls in general and hippos in particular. Yeteven with those details still unresolved, “we’re really getting ahandle on whales from their origin to the end of archaeocetes,”Uhen reflects. The next step, he says, will be to figure out howthe mysticetes and odontocetes arose from the archaeocetes andwhen their modern features emerged. Researchers may never un-ravel all the mysteries of whale origins. But if the extraordinaryadvances made over the past two decades are any indication,with continued probing, answers to many of these lingeringquestions will surface from the sands of time.

Kate Wong is a writer and editor for ScientificAmerican.com

The Emergence of Whales: Evolutionary Patterns in the Origin ofCetacea. Edited by J.G.M. Thewissen. Plenum Publishing, 1998.

Skeletons of Terrestrial Cetaceans and the Relationship of Whales toArtiodactyls. J.G.M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussainin Nature, Vol. 413, pages 277–281; September 20, 2001.

Origin of Whales from Early Artiodactyls: Hands and Feet of EoceneProtocetidae from Pakistan. Philip D. Gingerich, Munir ul Haq, Iyad S.Zalmout, Intizar Hussain Khan and M. Sadiq Malkani in Science, Vol. 293,pages 2239–2242; September 21, 2001.

The Encyclopedia of Marine Mammals. Edited by W. F. Perrin, Bernd G.Würsig and J.G.M. Thewissen. Academic Press, 2002.

M O R E T O E X P L O R E

WATER, WATER EVERYWHEREMOST MAMMALS—big ones in particular—cannot live withoutfreshwater. For marine mammals, however, freshwater isdifficult to come by. Seals and sea lions obtain most of theirwater from the fish they eat (some will eat snow to getfreshwater), and manatees routinely seek out freshwater fromrivers. For their part, cetaceans obtain water both from theirfood and from sips of the briny deep.

When did whales, which evolved from a fairly large (andtherefore freshwater-dependent) terrestrial mammal, develop asystem capable of handling the excess salt load associated withingesting seawater? Evidence from so-called stable oxygenisotopes has provided some clues. In nature, oxygen mainlyoccurs in two forms, or isotopes: 16O and 18O. The ratios of theseisotopes in freshwater and seawater differ, with seawatercontaining more 18O. Because mammals incorporate oxygenfrom drinking water into their developing teeth and bones, theremains of those that imbibe seawater can be distinguishedfrom those that take in freshwater.

J.G.M. (Hans) Thewissen of the Northeastern OhioUniversities College of Medicine and his colleagues thusanalyzed the oxygen isotope ratios in ancient whale teeth togain insight into when these animals might have moved from afreshwater-based osmoregulatory system to a seawater-basedone. Oxygen isotope values for pakicetids, the most primitivewhales, indicate that they drank freshwater, as would bepredicted from other indications that these animals spent muchof their time on land. Isotope measurements from amphibiousAmbulocetus, on the other hand, vary widely, and somespecimens show no evidence of seawater intake. Inexplanation, the researchers note that although Ambulocetus isknown to have spent time in the sea (based on the marinenature of the rocks in which its fossils occur), it may still havehad to go ashore to drink. Alternatively, it may have spent theearly part of its life (when its teeth mineralized) in freshwaterand only later entered the sea.

The protocetids, however, which show more skeletaladaptations to aquatic life, exhibit exclusively marine isotopevalues, indicating that they drank only seawater. Thus, just afew million years after the first whales evolved, theirdescendants had adapted to increased salt loads. Thisphysiological innovation no doubt played an important role infacilitating the protocetids’ dispersal across the globe. —K.W.



Killer Kangaroosand Other Murderous MarsupialsAustralian mammals were not all as cute as koalas. Some were as ferocious as they were bizarre

by Stephen Wroe

POWERFUL-TOOTHED GIANT RAT-KANGAROOpounces on a juvenile tube-nosedbandicoot in a rain forest. The sceneis set in Miocene Australia, around15 million years ago. Looking on aretwo marsupial “lions” (Wakaleovanderleuri) and a Thunder Bird(Bullockornis planei).

Originally published in May 1999


Dawn mist blankets the rain forestof Riversleigh in northeasternAustralia, 15 million years ago.

A bandicoot family emerges to dip snouts war-ily into a shallow freshwater pool. Their ears

swivel, ever alert to a sudden crack or rustle in theundergrowth: drinking is always a dangerous activity.

Suddenly, a dark, muscular form explodes from behind anearby bush, colliding with a young bandicoot in one

bound. The shaggy phantom impales its victim on long,daggerlike teeth, carrying the carcass to a quiet nook to be

dismembered and eaten at leisure. ROBE

RTO

OST

I


In nature, many animals will meet a violent death.So the sad end of one small bandicoot seems hardly

worth mention. The demise of this little fellowwould, however, have surprised most modern onlook-ers. Its killer was a kangaroo—the Powerful-ToothedGiant Rat-kangaroo (Ekaltadeta ima), to be exact.

In 20th-century Australia, warm-blooded preda-tors are few and far between. Among our natives, thelargest carnivores are the Spotted-Tailed Quoll(Dasyurus maculatus) and the Tasmanian Devil (Sar-cophilus harrisii). (The doglike dingo, which alsoeats flesh, did not originate in Australia but was in-troduced by humans between 5,000 and 4,000 yearsago.) The Spotted-Tailed Quoll is a marsupial thatweighs up to seven kilograms (15 pounds); it is alsoknown as a native “cat” because of a passing resem-blance to ordinary, placental cats. The TasmanianDevil, another marsupial, is only slightly larger andlooks like a lapdog with a fierce hyena’s head. It isarguably the least fussy eater in the world and willdevour an entire carcass, including the teeth. Thisodd pair is placed in the family Dasyuridae, whichincludes other native cats as well as far smaller, most-ly insectivorous creatures called marsupial mice.

Some scientists have suggested that Australia hasnever supported a healthy contingent of large warm-blooded carnivores. Most recently, Tim Flannery ofHarvard University has argued that their evolutionwas constrained by poor soils and erratic climate forthe past 20 million years or so. His rationale is thatthese constraints limited plant biomass, in turn re-stricting the size and abundance of potential prey ani-mals. Instead, he and others have hypothesized, rep-tiles such as the seven-meter-long (23-foot-long) lizard Mega-lania prisca, which lived in Pleistocene times, took up the roleof large terrestrial carnivores. Cold-blooded predators requireless food than warm-blooded ones and so—the argumentgoes—were more likely to survive difficult conditions.

This claim is challenged by recent developments, notablyspectacular fossil finds in Riversleigh, Queensland. A Euro-pean naturalist, W. E. Cameron, first noted the presence offossils at this remote site in 1900. But Cameron believed thatthe material he had seen was fairly young, less than two mil-lion years old. Moreover, Riversleigh’s extreme inaccessi-bility—summer heat and monsoon rains allow excavationsonly in winter—persuaded paleontologists to neglect the lo-cality for decades. In 1963, however, Richard Tedford of theAmerican Museum of Natural History in New York Cityand Alan R. Lloyd of the Australian Bureau of Mineral Re-sources took a gamble and visited the site. They found thefossils intriguing and older than previously believed but frag-mentary and hard to retrieve.

Still, their findings stimulated other expeditions to Rivers-leigh, and in 1983 my former supervisor Michael Archer,now director of the Australian Museum in Sydney, struckpaleo pay dirt. In an idle moment at the site he looked downat his feet and saw a very large lump of rock that just hap-pened to contain as many new species of Australian Tertiarymammals as had been described in previous centuries. Sincethen, new specimens, including large carnivores, haveemerged at a prodigious rate. Many are exquisitely well pre-served, so much so that some could be mistaken for the re-mains of animals that died only weeks ago.

The ancient creatures appear to have been mostly trappedin limestone caves. Their bones, which were quickly and per-fectly preserved by water rich in calcium carbonate, testify toa lost menagerie of beasts that were every bit as deadly as,but far stranger than, anything known today. Since 1985nine new species from Riversleigh, each the size of the Spot-ted-Tailed Quoll or bigger, have more than doubled the tallyof large Australian carnivores at least five million years old.This bestiary now includes two kinds of giant rat-kangaroo,nine species of marsupial “wolf,” five species of marsupial“lion” and one native cat.

The giant rat-kangaroos (propleopines) are closely relatedto the Musky Rat-kangaroo. This tiny animal, still found inthe rain forests of Queensland, weighs less than a kilogram—small enough to look like a rat. It eats a wide variety of plantstuffs and small animals, and alone among living kangaroosit cannot hop. A living fossil, it is the last and tiniest survivorof a family that included some fearsome, muscle-boundcousins. The giant rat-kangaroos ranged from around 15 to60 kilograms in weight. Like their diminutive descendant,they probably walked on all fours.

The marsupial wolves (thylacinids) and marsupial lions(thylacoleonids) are so named because of their superficialphysical resemblances to canines and felines, although theywere more closely related to kangaroos. The last of the mar-supial wolves, perhaps confusingly called the TasmanianTiger because of the stripes on its rump, was exterminatedearly in this century because of a largely undeserved reputa-tion for preying on sheep. Like cats, the marsupial lions hadshort, broad, powerful skulls, and they probably filled simi-

Predator’s Gallery

Formidable flesh-eaters from ancient Australia included a marsu-pial lion (below), a marsupial wolf (near right), a giant rat-kanga-roo (far right) and an enormous lizard (below, right). The largestrat-kangaroo, Propleopus oscillans (which weighed 60 kilograms),the “lion” and the lizard survived until fairly recent times and mayhave even preyed on humans. —S.W.

ILLU

STR

ATIO

NS

BY R

OBE

RTO

OST

I AN

D A

NN

E M

USS

ER

LARGEST MARSUPIAL LION (Thylacoleo carnifex)

130 TO 260 KILOGRAMS


lar ecological niches as well; their size ranged from that of ahouse cat to that of a lion. Although no fossils contain actu-al traces of a pouch, specialized features of the bones sharedwith living animals leave no doubt that all these creatureswere marsupials.

Fearsome Forest

For much of the Miocene epoch (25 to five million yearsago), Australia was carpeted in wall-to-wall green, and

rain forest covered many areas that are now savanna or desert.These jungles were an evolutionary powerhouse, nurturing afar greater diversity of life than any modern Australian habitatdoes. A day trip through one of these forests would have beenfilled with surprises, many of them potentially dangerous.

One would have been the Powerful-Toothed Giant Rat-kangaroo, among the most ancient of rat-kangaroos (anoth-er five species have been described from younger deposits).E. ima was also the smallest, weighing only about 10 to 20kilograms. It is well represented by two nearly completeskulls. These fossils give us our best shot yet at understand-ing the feeding habits of the giant rat-kangaroos.

Because these animals descended from plant-eating marsu-pials, some controversy surrounds the interpretation of theirbiology. Nevertheless, all recent authors agree that these dis-tinctly uncuddly kangaroos included meat in their diets. Evi-dence supporting this hypothesis comes from both theirskulls and their teeth.

In popular imagination, ferocious meat-eaters usuallycome with large canines. In the main this holds true, but

there are some exceptions. Many humans consume a gooddeal of flesh—more than some so-called carnivores—but wehave small canines, whereas in gorillas, which are vegetari-ans, these teeth are large. The real hallmark of a terrestrialmammalian killer is a set of distinctive cheek teeth used forcutting and shearing.

In less specialized members of the placental carnivore, gi-ant rat-kangaroo and marsupial lion clans, the last two tofour teeth in the upper and lower jaws are broad molars,used primarily for crushing plant material. Immediately infront of these molars are vertical shearing blades, called car-nassials, that can efficiently slice through muscle, hide andsinew. Within each of these three groups of animals, howev-er, the carnassials of the most carnivorous species are greatlyenlarged, whereas the plant-processing teeth are reduced,even lost. In the mouth of a domestic cat, for instance, canbe found the cheek teeth of a highly specialized carnivore.

So the relative importance of the carnassial versus thecrushing teeth in an animal’s jaws offers a good indication ofhow much flesh it devoured. In this respect, the giant rat-kangaroos resembled canids such as foxes, which are oppor-tunistic feeders and retain significant capacity to crush. Butthe skull of E. ima featured a number of other attributes typ-ical of carnivores. Its robust architecture, for instance, un-doubtedly supported the massive neck and jaw muscles thatmany predators need to subdue struggling prey. But it neverevolved long canines in the lower jaw; instead its lower frontincisors became daggerlike blades.

On these grounds, I and others have argued that giant rat-kangaroos were generalists, taking flesh when available but

GIANT MONITOR LIZARD(Megalania prisca)620 KILOGRAMS

POWERFUL-TOOTHED GIANT RAT-KANGAROO

(Ekaltadeta ima)20 KILOGRAMS

LARGEST MARSUPIAL WOLF(Thylacinus potens)

MAXIMUM 45 KILOGRAMS


supplementing their diet with a healthy variety of vegetablematter. These renegades of the kangaroo clan terrorized theAustralian continent for at least 25 million years, going ex-tinct only sometime over the past 40,000 years.

While keeping an eye open for meat-eating kangaroos, ahuman intruder in Miocene Australia would have done wellto avoid low-slung branches. The trees were home to anoth-er unpleasant surprise: marsupial lions. Like the giant rat-kan-garoos, the four species of Miocene “lions” evolved frompeaceable, plant-eating types. The most primitive specieshave generalized molar teeth typical of omnivores, as well ascarnassial blades. In other species the crushing molars are re-duced or lost, and the flesh-shearing teeth become huge.

At least eight species of marsupial lions have been formal-ly described, and two more are being studied by Anna Gil-lespie of the University of New South Wales in Sydney. His-torically, the interpretation of marsupial lion biology hasbeen contentious. As vombatomorphian marsupials, theirclosest living relatives are koalas and wombats. Some earlypaleontologists, prejudiced by the close relationship of these“lions” to herbivorous marsupials, refused to concede thepossibility of a carnivorous way of life for them. They of-fered a variety of unlikely scenarios, culminating in the sug-gestion that the creatures were specialized melon munchers.(Because the teeth could barely grind, the food was assumedto have been rather soft!)

Nowadays scientists agree that marsupial lions were in-deed killers. Many consider that the most recent species,Thylacoleo carnifex, was the most specialized mammaliancarnivore ever known: it effectively dispensed with plant-processing teeth, whereas the elaboration of its carnassials isunparalleled. It did not have big canines and must have usedits long incisors to kill.

T. carnifex is also the only marsupial lion known from acomplete skeleton. Many researchers have suggested that itwas the size of a large wolf or leopard. Others, myself in-cluded, believe that such estimates have not accounted forthe extreme robustness of the skeleton and that this frighten-ing beast could have been as heavy as a modern lion. It wasbuilt for power, not endurance, and had tremendously mus-cular forelimbs. With teeth like bolt-cutters and a huge,sheathed, switchbladelike claw on the end of each semiop-

posable thumb, it would have been an awesome predator onany continent.

Pouched Pouncers

Undoubtedly, T. carnifex was adapted to take relativelylarge prey, probably much larger than itself. The exact

purpose to which it put its thumb-claw is unclear, but onething seems certain: once caught in the overpowering embraceof a large marsupial lion, few animals would have survived.

The kinds of marsupial lion known as Wakaleo weresmaller, about the size of a leopard. Not designed for speedbut immensely powerful, species of Wakaleo (and possiblyThylacoleo) may have specialized in aerial assault. Like theleopard, they could have launched themselves onto unsus-pecting prey from trees. At the other end of the scale, ataround the size of a domestic cat, Priscileo roskellyae mayhave concentrated on taking arboreal prey. Given their sizeand extreme predatory adaptations, I believe the larger mar-supial lions most likely maintained a position at the top ofthe Australian food pyramid. And T. carnifex lived at leastuntil 50,000 years ago—recently enough, perhaps, to havefed on humans.

On the forest floor, the marsupial wolves dominated.When Europeans arrived in Australia more than 200 yearsago, they found only two marsupial families with carnivo-rous representatives. These were the “wolves”—only the Tas-manian Tiger remained—and a far more numerous group,the dasyurids. These mostly diminutive but pugnaciousbeasts are commonly measured in grams, not kilograms, andover 60 living species have been described.

Because in recent times dasyurids have clearly dominatedin terms of species diversity, paleontologists had expected tofind that they were also far more common than thylacinidsin the distant past. We were wrong. Since 1990 seven newspecies of Miocene-age “wolves” have been found, bringingthe total for the family to nine (including the TasmanianTiger). Descriptions of four more species are in the pipeline.On the other hand, only one definite dasyurid has been de-scribed from Miocene deposits. A few species known fromfragmentary material may also turn out to be dasyurids.Even so, the proportion of marsupial wolf to dasyurid spe-

CARNASSIAL TEETH—vertical blades for slicing through meatand hide—are the hallmark of a terrestrial mammalian killer. Inhighly specialized carnivores such as the marsupial lion and theAfrican lion shown, a single tooth on each side of the upper and

lower jaws has been modified for this task; all the molars behindthis carnassial are reduced or lost. (Only the lower jaw is drawn.)Generalized carnivores, such as the giant rat-kangaroos and foxes,which consume much vegetation, retain their crushing molars.

INCISORSMOLARS

GRAY FOX

POWERFUL-TOOTHED GIANT RAT-KANGAROO

AFRICAN LION

LARGEST MARSUPIAL LION (T. carnifex)

CANINE

MOLARS

MOLARINCISORS

INCISORINCISOR

MOLARS

CARNASSIAL

CARNASSIALCANINE

CARNASSIAL

CARNASSIAL

AN

NE

MU

SSER


cies during the Miocene is in stark contrast to that of mod-ern times.

The Tasmanian Tiger is the only thylacinid for which anyfirsthand accounts of biology and behavior are available.Most of these must be taken with a grain of salt. But the fol-lowing is fairly certain: the Tasmanian Tiger was similar tomost canids in that it was fully terrestrial, long-snouted andprobably tended to take prey considerably smaller than it-self. It differed in being relatively poorly adapted for runningand probably was not a pack hunter. It further differed fromthe majority of canids in that its cheek teeth were adapted toa completely carnivorous diet.

In thylacinids and dasyurids the dental layout is differentfrom that of most other flesh-eaters. These animals retainboth a crushing and a vertical-slicing capacity on each indi-vidual molar. Thus, in meat-eating specialists of this type thecrushing surfaces are reduced and the vertical shear is in-creased on each molar tooth.

Indeed, all the marsupial wolves were largely carnivorous,although the smaller, less specialized ones probably also ateinsects. A number of these animals departed still furtherfrom the canid model. Some Miocene “wolves” were smallcompared with the Tasmanian Tiger, and one, Wabulacinusridei, had a short, more catlike skull. We cannot even be surethat all Miocene-age thylacinids were terrestrial, becauseonly fragments of the skulls and jaws are known for most. Amagnificent exception is a 15-million-year-old individual re-cently discovered at Riversleigh; its skull and most of itsskeleton are beautifully preserved. We can be reasonably cer-tain that this animal at least lived on firm ground.

In the past few months Henk Godthelp of the University ofNew South Wales, Archer and I have described a mouse-sizemarsupial from deposits around 55 million years old in Mur-gon in southeastern Queensland. This new species has an ex-tremely generalized dentition, so primitive in fact that its rela-tion to other marsupials is very difficult to ascertain. It mayrepresent an ancestor of thylacinids and dasyurids—or even ofall Australian marsupials. An alternative possibility is that thisnew species does not belong to Australidelphia (a taxonomiccategory that contains all living Australian marsupials) but in-stead to the mostly South American group Ameridelphia.

South America and Australia were once joined together inthe continent of Gondwana, via Antarctica. And marsupialsare believed to have arrived in Australia from South Ameri-ca. Some scientists have suggested that only Australidelphianmammals entered Australia before Gondwana completely

broke up. In light of the new fossil finding, this conclusioncould be premature.

Death to Killers

Having established that Australia’s large marsupial carni-vores were very diverse during the Miocene period, pa-

leontologists are now faced with this question: What hap-pened to them? The last of the marsupial lions and giant rat-kangaroos (T. carnifex and Propleopus oscillans, respectively)died out not so long ago. In fact, they were probably aroundwhen the first Aborigines entered Australia, 50,000 or moreyears ago. Consequently, some scientists have maintainedthat it was the first humans who sounded their death knell.

Human culpability in this matter has been impossible toprove or disprove and remains a very contentious issue. Nodoubt the Aborigines helped to drive the Tasmanian Tiger toextinction by introducing the dingo, but their influence re-

PERAMELEMORPHIA (bandicoot)

MICROBIOTHERIIDAE (monito del monte)

DASYURIDAE (native cat)THYLACINIDAE (marsupial wolf )

MYRMECOBIIDAE (numbat)

NOTORYCTIDAE (marsupial mole)PHASCOLARCTIDAE (koala)

THYLACOLEONIDAE (marsupial lion)OTHER VOMBATOMORPHIANS* (wombat)

MACROPODIDAE (red kangaroo)

POTOROIDAE (potoroo)

HYPSIPRYMNODONTIDAE (giant rat-kangaroo and musky rat-kangaroo)PHALANGEROIDEA (brush-tailed possum)

PETAUROIDEA (ring-tailed possum)

TREE OF DESCENT of Australianmarsupials includes four familieswith carnivorous members (red).Representatives of each family arenoted in parentheses.

LISA

BURN

ETT

FOSSIL SKULL of the Powerful-Toothed Giant Rat-kangaroo displaysthe fearsome incisors and serrated car-nassials (resembling cockleshells) thatwould have enabled it to kill and con-sume its prey efficiently. The skull mea-sures 145 millimeters from end to end,and the lower jaw is 122 millimeters.

STEP

HEN

WRO

E

*Vombatomorphians include koalas and marsupial lions.


garding other species is less clear-cut. These issues may neverbe completely resolved, but the fossil record makes one factclear: marsupial carnivore diversity peaked by the early tomiddle Miocene and was already in steep decline long beforehumans arrived. For example, at least five marsupial wolveslived during the mid-Miocene, and two coexisted in the lateMiocene, but only one was ever known to humans.

Obviously, some factor other than human influence was at

work; perhaps Aborigines simply accelerated an extinctionprocess already long established. The most likely alternativecandidate is drought. From mid-Miocene times onward,Australia was subject to increasingly severe ice age condi-tions as well as declining rainfall and sea levels. This trendpeaked over the past two million years or so, with around20 ice ages exposing the Australian fauna to great stress. Thelast of these was severe, though not the worst.

Many researchers believe some combination of climatechange and pressure imposed by human arrivals extin-guished most of the continent’s surviving larger herbivores.With their favorite meat dishes gone, the clock began to runout on Australia’s marsupial predators. It is now a sad factthat of the dozens of wondrous large marsupial carnivoresthat have existed, not only in Australia but in the Americasas well, only our own Spotted-Tailed Quoll and TasmanianDevil remain. Nonindigenous Australians must accept fullresponsibility for the inexcusable loss of the TasmanianTiger, and posterity will surely never forgive us should we al-low the same fate to befall our last two pouched killers.

The Author

STEPHEN WROE recently received his Ph.D. in paleontol-ogy from the University of New South Wales in Sydney. Hehas published widely on the evolution of Australian marsupi-al carnivores, living and extinct. His areas of special interestinclude all aspects of the giant rat-kangaroo and dasyuromor-phian marsupial radiations, as well as the biology of giantdromornithid birds and marsupial lions. The illustrations arebased on reconstructions by Anne Musser of UNSW.

Further Reading

Riversleigh: The Story of Animals in Ancient Rainforests of In-land Australia. M. Archer, S. Hand and H. Godthelp. Reed Books, 1994.

Killer Kangaroo. S. Wroe in Australasian Science, Vol. 19, No. 6, pages25–28; July 1998.

The Geologically Oldest Dasyurid, from the Miocene of Rivers-leigh, Northwestern Queensland. S. Wroe in Palaeontology (in press).

The Riversleigh Society Australian Paleontology site is at www.ozemail.com.au/~promote1/auspalaeo/index.html on the World Wide Web.

A Killer Bird?

In November 1998 Peter Murray and Dirk Megirian of the Central Aus-tralian Museum described new fossil material from an extinct, terrestrial

bird called Bullockornis planei. This species belongs to the Australian familyDromornithidae, also called Thunder Birds, known since 1839. Dromornithidscould be huge, some weighing perhaps 500 kilograms or more. But with verylimited skull material preserved, little that was certain could be said abouttheir biology. Given the paucity of material and the generally accepted viewthat dromornithids were closely related to predominantly plant-eating birds,most scientists were of the view that these giants were herbivores. But Mur-ray’s excellent reconstruction of B. planei is startling, showing a massivehead possibly more than half a meter long. Furthermore, the muscle attach-ment sites were enormous. What did a half-ton bird with military-grade jawmuscles and a beak that could hide a football eat?

In 1991 Lawrence M. Witmer, now at Ohio University’s College of Osteo-pathic Medicine, and Kenneth D. Rose of the Johns Hopkins UniversitySchool of Medicine convincingly argued that the massive beak and jawmusculature of Diatryma, an extinct bird from North America and Europe,would have constituted serious “overdesign” unless the bird was a carnivore.Following this line of reasoning, I have lately suggested that at least somedromornithids might similarly have eaten vertebrates, killed or scavenged. Ifso, Thunder Birds were the largest carnivores on two legs since the demiseof the meat-eating dinosaurs. —S.W.

ROBE

RTO

OST

I AN

D A

NN

E M

USS

ER
