Chapter 23: The Paleozoic Era

600

What You’ll Learn• How the Appalachian

and OuachitaMountains formed.

• When and on what scalethree mass extinctionsoccurred.

• Why coal is common inPennsylvanian-agedrocks.

• How the development ofseeds and eggs affectedthe evolution of life.

Why It’s ImportantThe tectonic setting ofeastern North Americaprovided a hospitableenvironment not only fora wide variety of animalsand plants, but also forthe vast swamps that ulti-mately formed the richcoal deposits of easternNorth America.

ThePaleozoicEra

ThePaleozoicEra

2323

Otter Creek Wilderness, Monongahela National Forest, WVOtter Creek Wilderness, Monongahela National Forest, WV

To find out more about thePaleozoic Era, visit theEarth Science Web Site at earthgeu.com

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23.1 The Early Paleozoic 601

Many sedimentary rocks containoil and water in the pore spacesbetween their grains. For example, asyou will learn later in this chapter,there is abundant oil in the Paleozoic-aged, sedimentary rocks of WestTexas. In this activity, you willobserve how oil or water can bestored in solid rock.

1. Place an unglazed brick or sand-stone sample on your table.

2. Using a dropper, slowly squeezethree to five drops per minute,of water or oil, for ten minutes.

Infer In your science journal,sketch a cross-section of the rockor brick. Include both before andafter you added the water to it.Include in your sketches whatthe inside of the brick mightlook like. Infer what happenedto the water.

Where is oil found?Discovery LabDiscovery Lab

OBJECTIVES

• Describe the Cambrianpaleogeography ofLaurentia.

• Discuss the concept ofa passive margin.

• Describe the Cambrianfauna.

VOCABULARY

paleogeographypassive margintransgressionregressionBurgess Shale

The Early Paleozoic23.123.1Clues from the Paleozoic Era help us to understand how the diversityof life developed. Today, the Appalachian Mountains of the easternUnited States border a tectonically calm coastline. But this area wasmuch different in the past. The Paleozoic story explains how earlylife-forms moved from water to land, why the Appalachians exist,and why they contain the vast coal deposits that fueled the industrialrevolution. The Paleozoic portion of the geologic time scale is shownin Figure 23-1 on page 602.

CONTINENTAL SETTINGGeologists refer to the ancient geographic setting of an area as itspaleogeography. The supercontinent Rodinia, was present at theend of the Proterozoic. By the Cambrian, the ancient NorthAmerican continent of Laurentia had split off from Rodinia, waslocated near the equator, and was surrounded by ocean. In addition,it was almost completely covered by a shallow, tropical sea.

Throughout the Cambrian Period, there was no plate tectonicactivity on Laurentia. There were no collisional tectonic events, sothere were no mountain ranges actively forming at that time. The edgeof a continent is called a margin; when there is no tectonic activity

along a margin, it is called a passive margin. Laurentiawas completely surrounded by passive margins through-out the Cambrian Period.

Characteristic Sediments The paleogeographicsetting of Laurentia, as shown in Figure 23-2, set thestage for a characteristic pattern of sandstone-shale-limestone deposits that represent increasing water depthfrom shore. The exposed land surface was flat, and thefloor of the shallow sea that covered Laurentia dippedgently away from the shoreline. On land, the exposedPrecambrian rocks were deeply eroded. Large, sandybeaches formed as fragments of quartz were weathered

from the rocks of the Precambrian Shield and transported toLaurentia’s shoreline. Clay-sized sediments were carried by strongtides and deposited in slightly deeper water. Carbonate sedimentaccumulated in even deeper water as organisms whose skeletons werecomposed of calcium carbonate died and fell to the seafloor. Overtime, the sand that had been deposited on the beaches became sand-stone, the clay-sized sediments compacted to form shale, and the car-bonate sediment became limestone. The most famous location wherethe sandstone-shale-limestone deposits of the Cambrian are exposedis the Grand Canyon. This sequence is shown in Figure 23-3.

602 CHAPTER 23 The Paleozoic Era

Figure 23-2 Laurentia waspositioned at the equatorduring the Early Paleozoic.Strong, tropical storms,much like today’s hurri-canes, contributed to ero-sion and formation of sandybeaches. Corals thrived inthe warm ocean waters andultimately contributed tothe formation of limestone.

Permian Period

Pennsylvanian Period

Mississippian Period

Devonian Period

Silurian Period

Ordovician Period

Cambrian Period

Pal

eozo

ic E

ra 245 M.Y.B.P.

408 M.Y.B.P.

439 M.Y.B.P.

510 M.Y.B.P.

540 M.Y.B.P.

290 M.Y.B.P.

323 M.Y.B.P.

362 M.Y.B.P.

Figure 23-1 The PaleozoicEra spans 290 million yearsand is divided into sevenperiods. The rocks of thePaleozoic Era, tell of thegreat changes in both thetectonic setting and the life-forms that developedduring that era.

Equator

Laurentia TranscontinentalArch

CanadianShield

Present dayHudson Bay

CHANGES IN SEA LEVELThe sandstone-shale-limestone sequence deposited during theCambrian was a side-by-side, or lateral, sequence from shallow todeeper water. Why, then, are the layers of sandstone, shale, and lime-stone stacked one on top of the other in the Grand Canyon? Thesediments themselves reflect the energy of the water and often, thewater depth. Thus, any changes in sediments may indicate changesin sea level.

A transgression occurs when sea level rises and the shorelinemoves further inland. Think of a sandy beach. In water slightlydeeper than that at the beach, there is clay-rich sediment, andbeyond this, in even deeper water, is carbonate-rich sediment. As sealevel rises, the water floods inland and the shoreline moves inland.The beach is now located where dry land had been. The area ofslightly deeper water where clay-sized sediments are deposited alsomoves shoreward on top of the old beach. The result of the trans-gression is deeper-water deposits overlying shallower-water deposits,as shown in Figure 23-4.

A regression occurs when sea level falls and causes the shoreline tomove seaward. This results in shallow-water deposits overlying deeper-water deposits. A stacked sequence of limestone-shale-sandstone is evidence of a regression. As sea level rises or falls, sediments that arelateral to each other become stacked one on top of another.

23.1 The Early Paleozoic 603

Figure 23-3 This sequenceof rocks is evidence of a risein sea level during the Cam-brian. The beach environ-ment that existed duringthe Cambrian is preservedin the Tapeats Sandstone(A). Slightly deeper water is represented by the BrightAngel Shale (B). The fossil-iferous Muav Limestonerepresents the deepest, off-shore environment (C).

Originalshoreline position

Sandstone

ShaleLimestone

Time 1

Time 2

Time 3

Figure 23-4 Time 1—sandis deposited at the shore-line and shale and lime-stone are deposited fartherout (A). Time 2—sea levelhas risen, the shoreline hasmoved inland and shale isdeposited at the originalshoreline position (B).Time 3—sea level has risenagain and limestone isdeposited at the originalshoreline position. Theresult is a sedimentarysequence of sandstone–shale–limestone (C).

A

B

C

A

BC

You have learned that these distinct sequences of rock are the resultof changes in sea level. While sea level was changing throughout thePaleozoic, so were the types of plants and animals that lived in the seas.

EARLY PALEOZOIC LIFEThere was such an increase in diversity and abundance of life-formsnear the beginning of the Cambrian Period that some call this theCambrian “explosion.” Organisms representing all but one of themajor marine groups appeared at this time. The development of min-eralized skeletons or hard parts also mark the Cambrian explosion.

Some of the best fossilized Cambrian organisms come from theBurgess Shale, in the Canadian Rocky Mountains. A spectaculararray of soft-bodied organisms, as well as organisms with hard parts,are preserved. This deposit includes wormlike animals of unknownaffinity and many other animals that do not fit into any living phylum,as shown in Figure 23-5.


1. What was the paleogeography ofLaurentia during the Cambrian?

2. What is a passive margin?

3. Briefly describe the significance of theCambrian fauna.

4. Describe how sedimentary sequenceschange when sea level lowers.

5. Thinking Critically If there were a majorice age today, explain the effect it would

have on the sediments that are currentlybeing deposited in the oceans and at sea level.

SKILL REVIEW

6. Communicating In your science journal,explain the relationship between themeanings of the words Cambrian explo-sion and the explosion of an object. Formore help, refer to the Skill Handbook.

A

C

B

Figure 23-5 The BurgessShale fauna were found inan isolated quarry near the town of Field, BritishColumbia. The unusualorganisms lived a quiet lifein the absence of predators(A). Waptia fieldensis wasan arthropod (B). Canadiaspinosa was a polychaeteor bristle worm (C).

earthgeu.com/self_check_quiz

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23.2 The Middle Paleozoic 605

OBJECTIVES

• Describe the MiddlePaleozoic paleogeography.

• Explain the concept ofan active margin and theformation of a clasticwedge.

• Describe the MiddlePaleozoic fauna.

• Define the concept ofmass extinction.

VOCABULARY

Taconic OrogenyCaledonian OrogenyAcadian OrogenyAntler OrogenyPaleozoic faunavascular plantmass extinction

23.223.2 The Middle Paleozoic

The passive margin that existed around Laurentia continued into theEarly Ordovician Period. The paleogeography of Laurentia, as shownin Figure 23-6A, was still equatorial, with the paleo-equator runningfrom approximately modern-day New Mexico through Minnesota.

SEA LEVEL CHANGES AGAINSea level rose during the Early Ordovician and, once again, a beachenvironment covered much of Laurentia’s margins. The base of therock layers that were deposited is marked throughout much of centralNorth America by the pure quartz sand of the St. Peter Sandstone,shown in Figure 23-7 on page 606. The sandstone is overlain byminor amounts of shale and by extensive limestone deposits. Theselimestone deposits contain a tremendously diverse array of organ-isms, including the first corals that built organic reefs.

Organic reefs are structures composed of carbonate skeletonsmade by living organisms. Corals require warm, clear, shallow waterof normal marine salinity in order to thrive. For this reason, they areconfined to latitudes between 30° north and south of the equator.During the Middle Ordovician, corals and a group of sponges calledstromatoporoids became common and began to build reefs in theshallow sea that covered Laurentia.

Reefs affect the environments in which they grow. They commonlyform in long, linear mounds parallel to islands or continents, wherethey absorb the energy of waves that crash against them on theiroceanward side. This protects the environments behind reefs fromthe waves’ energy. The calm areas behind reefs, are called lagoons.

Inland sea

CordilleranO

rogen

Crat

on

Shallowinland sea

Land

Mountains

TaconicHighlands

Pale

oequ

ator

Siberia

Kazakh-stania

(Future site of Ural Mountains)Baltica

Appalachians

SouthAmerica

Africa

EquatorItaly

Mountains

Laur

entia

Ouachitas

A B

Figure 23-6Laurentia was posi-tioned across theequator during theEarly Ordovician.The Taconic Orog-eny formed theTaconic Highlandsalong Laurentia’seastern margin (A).Orogenic activitycontinued duringthe Pennsylvanian.Mountains formedalong the line ofcollision. All of thecontinents wereassembled as onesupercontinent,Pangaea (B).

Ordovician Paleogeographic setting of Laurentia

Pennsylvanian World Paleogeography

Lagoon Back reef Reef flatOpen sea

Talus

Reefcore

Land

Fore-reefslope

Reefbarrier

Back-reef quiet waterlagoon

Figure 23-8 The sheltered,quiet water behind a coralreef is an ideal place forfragile organisms to thrive.

Here fragile organisms can thrive as shown in Figure 23-8. Reefs alsocan restrict water flow from the lagoon to the ocean. The shallow,quiet water in lagoons warms in the tropical sunlight and water evap-orates at a high rate. When this happens, lagoon waters may becomeoversaturated with calcium and sodium. These elements combinewith other elements or compounds in the water and precipitate out ofsolution as evaporite minerals such as gypsum and halite.

The Great Lakes area of North America contains huge deposits ofSilurian-aged evaporite minerals that are mined commercially.Surrounding many of these evaporite deposits are reefs that almostcertainly played a role in restricting the flow of water, thus makingconditions right for the deposition of evaporite minerals. In theMichigan Basin, periodic restriction and deposition of evaporitesresulted in deposits of halite and anhydrite more than 700 m thick!The Ohio Basin and the Appalachian Basin also were sites of evapor-ite deposition during the Late Silurian. Figure 23-9 shows the loca-tions of these basins. Today, these evaporite deposits are importantsources of gypsum, which is used to make plaster and drywall, andhalite, which is mainly used as road salt.

MIDDLE PALEOZOIC TECTONICSDuring the Middle Ordovician, an ocean-continent collisionalboundary, an active margin, developed in what is now eastern NorthAmerica. As shown in Figure 23-6A, this resulted in a mountainbuilding event called the Taconic Orogeny, named for the Taconic


Figure 23-7 Glass is madefrom silica sand such as thevery pure and clear St. Petersandstone.


Wisconsin

Illinois Indiana

Pennsylvania

MichiganBasin

Kokomo Sea

LakeMichigan

IllinoisBasin

LakeHuron

OhioBasin

LakeErie

AppalachianBasin

Wills Creek Tonoloway

Sea

Pinnacle reefs

Reef

Evaporites

Salt Deposits in the Great Lakes Region Figure 23-9 The barrierreefs that grew withinthese basins most likelyrestricted the flow of waterfrom time to time. This,coupled with increasedevaporation, resulted in theformation of halite andother evaporite minerals.

Mountains of eastern New York State. A series of volcanic islandsalso formed as a result of this mountain building event.

How do we know that this Taconic Orogeny occurred? Geologistslook for clues and evidence in the rocks like detectives at a crimescene. One form of evidence is angular unconformities. Rocks thatare older than the Middle Ordovician of eastern New York are tiltedat a different angle than those of the younger Silurian rocks. Thismeans that something, such as a tectonic collision, tilted the rocksafter the Middle Ordovician but before the Silurian. Volcanic activityalso provides evidence of the Taconic Orogeny. Lava flows and vol-canic ash deposits that are Middle-to-Late Ordovician in age exist inpresent-day eastern North America. There are also igneous intru-sions and regional metamorphic features that have been radiometri-cally dated as being between 480 and 440 million years old. The finalpiece of evidence of the Taconic Orogeny is the wedge of sediment,some of which is shown in Figure 23-10, that formed as the moun-tains from the Taconic Orogeny eroded. In a cross section, such adeposit has a triangular wedge shape, and is called a clastic wedge.

Orogenies and Deformation Tectonism continued during theLate Silurian and into the Devonian when Laurentia collided withBaltica, an ancient continent that consisted of what are now north-ern Europe and Russia west of the Ural Mountains. This collisionjoined Laurentia and Baltica into a larger continent known asLaurasia, pictured in Figure 23-6B on page 605.

Figure 23-10 Erosion andtransport of material fromrising mountains to the eastresulted in the formation of these coarse-grained, layered deposits. They arepart of a large clastic wedgecalled the Queenston Deltaand are exposed in GeneseeGorge in Letchworth StatePark, NY.

This collisional tectonic event, known as the Caledonian Orogeny,closed the ocean that had separated these two continents. Shortly afterBaltica collided with Laurentia, a microcontinent called Avalonia,which is now Newfoundland, collided with the southeastern marginof Laurasia. This tectonic event, known as the Acadian Orogeny,affected the same general area as the Ordovician Taconic Orogeny.The deformation caused by the Acadian Orogeny added folds, faults,and igneous intrusions to the already deformed rocks of the TaconicOrogeny. Figure 23-11 shows some of the complex folds that resulted.During the Late Devonian and into the early Mississippian, the passivewestern margin of Laurentia was affected by a collisional tectonicevent called the Antler Orogeny. It was most likely caused by a micro-continent or island arc that collided with what is now western NorthAmerica, as shown in Figure 23-12. As you will learn in the next chap-ter, one of the problems with interpreting the Paleozoic geology ofwestern North America is that it has been complicated by numerousyounger orogenic events.


Figure 23-11 These rockswere deformed and faultedduring the collisional eventsof the Taconic and AcadianOrogenies.

Figure 23-12 The moun-tains of the Antler Orogenybordered an inland sea tothe east. The collision of a microcontinent withLaurentia is thought to bethe cause of the orogeny.

LandMountains

EvaporitesInland sea

Barrier islands

Barrierislands

Ellesmere Mountains

basinevaporites

Paleoequator

Deep muddy bottom

Antle

r Hig

hlan

ds

Shallow muddy inland seaA

ppal

achi

an

Mou

ntai

nsShallow carbonateinland

sea

CordilleranO

rogen

Ouachita Orogen

Laurentia

MIDDLE PALEOZOIC LIFEThe Middle Paleozoic seas were domi-nated by animals that are collectivelycalled the Paleozoic fauna. The ani-mals that dominated the Cambrianseas were replaced during the EarlyOrdovician by a variety of new organ-isms. Representatives of the Paleozoicfauna are shown in Figure 23-13.

Biologists have observed that mod-ern corals deposit thin layers of car-bonate, called growth lines, to theirskeletons each day. Seasonal variationsin length of day cause yearly changes inthe spacing of these growth lines. TheDevonian horn coral, Heliophyllum,pictured in Figure 23-13D, displaysthese types of growth lines. Based uponthe spacing and number of growth lines in a cycle, paleontologistshave deduced that, during the Devonian, the days were about 22 hourslong and there were about 400 days in a year. The corals thus indicatethat Earth was rotating more rapidly during the Paleozoic and, like atop, has been slowing ever since. You will calculate the lengths of daysof other periods in the Problem-Solving Lab on page 610.

LIFE MOVES TO LANDThe seas were not the only places where life was developing during theMiddle Paleozoic. For the first time in the history of life on Earth,land was colonized. Fossilized plant parts and spores suggest thatsome form of land plant existed during the Late Ordovician. Vascularplants have tissue for circulating water and nutrients through theirstems and leaves. It was the development of this tissue that allowedplants to spread out on land. Fossils that undisputedly represent vas-cular plants are found in Early Silurian rocks. These plants were small,leafless, grew along the ground, and reproduced by spores.

New Plants Emerge By the Late Devonian, three importantgroups of vascular spore-bearing plants were living on land. Onegroup, the ferns, still thrive today. The second group, the sphenopsids,were joint-stemmed plants. A modern species of sphenopsid is thehorsetail or scouring rush, shown in Figure 23-14, whose commonname is derived from its use by early plains settlers to scour their dishes.The last group is the lycopods. The leaves of the lycopods grew directlyfrom their stems, leaving diamond-shaped scars on the fossilized stems.


Figure 23-13 Graptoliteslived in shallow, marinewaters (A). The Early Paleo-zoic fishes had internalskeletons of cartilage. ThisDunkleosteus, from Ohiowas a top predator in theDevonian Seas (B). Cono-donts were eel-like swim-mers whose only hard-partswere tooth-like structures.They are excellent indexfossils because they arewidespread, existed duringa short period of time, andhave distinct characteristics(C). The growth lines onthis Devonian horn coral,Heliophyllum, help deter-mine the length of aDevonian year (D).

A B

C D

During the Late Devonian, the seed ferns developed. The mostimportant aspect of these plants was the development of seeds. Seedscontain their own moisture and food source. They are covered by ahard coating that prevents drying and protects them. Seeds allowedplants to spread out and colonize dry land. Until the development ofthe seed, plants required water to complete their reproductive cycles.Thus, with the development of seeds, the surface of the continentswas changed forever.

MASS EXTINCTIONSTwo of the greatest extinction events in Earth’s history occurred dur-ing the Middle Paleozoic. When an unusually large number of organ-isms become extinct over a relatively short period of geologic time, it


Figure 23-14 This modernhorsetail (A) and this fossilhorsetail from Italy (B)exhibit strikingly similar features such as the individ-ual joints and clusters ofleaves along the stem. Thisfossil fern is from Piesberg,Germany (C).

Determine the length of aPaleozoic day Geologists study theshapes and compositions of fossil organ-isms to interpret how and in what typesof environments they lived. Fossils caneven be used to interpret climaticchanges. For example, rugose coralsexhibit lines on their shells that representyearly growth cycles. Ultimately, paleon-tological data such as this can be used tointerpret the past and to predict futureevents. Use the figure at the right toanswer the following questions.

Analysis1. How many hours were in a day 400

million years ago?

2. How many hours were in a day 200million years ago?

Thinking Critically3. When will there be 24.5 hours in

a day?

Ho

urs

per

day

Day

s p

er y

ear

Geologic time (m.y.)

430

0Cenozoic Mesozoic Paleozoic

100 200 300 400 500

420

410

400

390

21

22

23

24

25

380

370

360

350

Interpreting Graphs

A

B C

is called a mass extinction. The first mass extinction occurred at theend of the Ordovician Period. Approximately 57 percent of all marinegenera became extinct. Those that survived suffered large but notcomplete losses in their numbers of species and genera.

What caused the Late Ordovician mass extinction? One clue isthat there are Late Ordovician glacial deposits preserved in what isnow northern Africa. Another clue is that species that were adaptedto warm environments were the hardest hit. Geologists hypothesizethat an episode of global cooling, plus the rapid lowering of sea levelthat accompanied the glaciations, caused the extinction of manyspecies that were adapted to warm environments.

Global Cooling and Overturning A second mass extinctionoccurred during the late Devonian when approximately 50 percent ofthe marine genera were wiped out, including many species of coralsand stromatoporoids. Again, the polar communities were not severelyaffected. Was global cooling again the cause? There is evidence thatglaciation occurred on some continents at this time. The cooling ofEarth’s atmosphere may have caused a tremendous disturbance in theocean system. This created overturning, a process in which oxygen-poor, deep ocean water rises up, creating surface waters that containlittle or no oxygen.

Evidence of overturning exists in the form of black shale. Blacksediment indicates a lack of oxygen. Black shale deposits from theLate Devonian are common worldwide, and therefore, suggests thatthe seawater at that time contained reduced amounts of oxygen. Thelack of oxygen in the water stressed the organisms to the point atwhich many species became extinct.


1. Where was Laurentia positioned on Earthduring the Middle Paleozoic?

2. Explain how evaporite deposits formedduring the Middle Paleozoic.

3. What kinds of evidence do geologists useto determine whether an area was sub-jected to an orogeny?

4. What were the common causes of theLate Ordovician and Late Devonian massextinctions?

5. Thinking Critically In Chapter 19, youlearned about different kinds of faults.What kinds of faults would you expect tohave been associated with the AcadianOrogeny? Why?

SKILL REVIEW

6. Recognizing Cause and Effect Explainhow the appearance of land plants led to the appearance of land animals.


Update To find out moreabout present-day extinc-tions, visit the EarthScience Web Site at earthgeu.com

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23.323.3 The Late Paleozoic


OBJECTIVES

• Describe the formationof Pangaea.

• Explain how cyclothemsformed.

• Identify the importanceof amniote eggs.

• Discuss the causes of theLate Permian massextinction.

VOCABULARY

GondwanacyclothemOuachita OrogenyAncestral RockiesAlleghenian Orogenyamniote egg

Figure 23-15 The moun-tain building and subse-quent erosion that occurredas Gondwana formedresulted in the formation ofclastic wedges in front ofthe mountains. The moun-tain building events aresometimes referred to asthe Gondwanan Orogeny.

+ South pole

The Supercontinent of Gondwana

During the Late Paleozoic, the supercontinent Pangaea formed. Inthe previous section, you learned that Laurentia, Baltica, and themicrocontinent Avalonia were joined together by the end of theDevonian. Figure 23-15 shows how, as a result of similar collisionalevents, South America, Africa, India, and Antarctica joined to formthe large continent in the southern hemisphere called Gondwana.

SEA LEVEL AND DEPOSITIONThe Late Paleozoic began with Laurasia still covered by a shallowtropical sea. This setting provided the perfect environment for thedeposition of carbonate sediment. Thus, Mississippian rocksthroughout North America are predominantly limestone. The end ofthe Mississippian was marked by a major regression of the sea.

The Pennsylvanian Period began with a slow transgression. Thesediments that were deposited have quite different characteristicsfrom those of the underlying Mississippian carbonates. ThePennsylvanian and Permian rocks in central and eastern NorthAmerica are predominantly river and delta deposits.

The Pennsylvanian Period is known for the coal deposits thataccumulated in heavily vegetated lowland swamps. As you havelearned, coal forms through the compaction of plant material. Why


are coal deposits not common in older rocks? Theanswer lies in the development of plant life on land.Not enough plant material had accumulated on landto form significant coal deposits until theMississippian. Because a shallow sea coveredLaurasia during the Mississippian, and because coaldeposits generally form on land, there are fewMississippian-aged coal deposits in North America.

Cyclothems Pennsylvanian rocks in NorthAmerica display a repeating cyclic pattern of sedi-ments stacked one on top of another. Such asequence as pictured in Figure 23-16 is called acyclothem. Cyclothems record two changes in sealevel. A regression is represented by layers of marinelimestone, and sandstone. A transgression is repre-sented by sandstone, clay, coal, shale and marinelimestone. In some parts of Laurentia, as many as 40to 50 of these sequences were stacked one on top ofthe other. What could have caused the seas to trans-gress and regress 50 times? Most geologists hypothe-size that glaciation was responsible. Glaciers grow bythe accumulation of snow. Most of the water thatfreezes into snow comes from the evaporation of seawater. Thus,when glaciers are growing, sea level is falling. Gondwana was locatedin part over the south pole during the Pennsylvanian and large,glacial ice sheets developed on land. It is likely that glaciers grew dur-ing cooler periods during the Pennsylvanian and caused sea level todrop, which, in turn, caused a regression. The glaciers partiallymelted during warmer periods and caused sea level to rise, whichresulted in a transgression. In this way, numerous cyclothems weredeposited one on top of the other.

REEFS AND EVAPORITESDuring the Permian, much of what is now eastern North Americawas dry land and a shallow sea covered most of what is now west-ern North America. A spectacular fossilized barrier reef complexwas formed during the Permian in what is now western Texas,southeastern New Mexico, and northern Mexico. Sponges andalgae built this reef complex, known as the Great Permian ReefComplex, the remains of which are shown in Figure 23-17 on page614. Because these barrier reefs restricted the flow of water, largeevaporite deposits formed behind the reefs during the Middle andLate Permian.

23.3 The Late Paleozoic 613

Two typical cyclothems

Disconformity

Disconformity

Disconformity

Regression

Regression

Transgression

Transgression

Offshoremarine

limestone

Offshoremarine

limestone

Nearshoremarine shales

Nearshoremarine shales

Coal

Coal

Underclay

Underclay

Cross-beddedriver sandstone

Cross-beddedriver sandstone

Cyc

loth

em 1

Cyc

loth

em 2

Figure 23-16 The generalrock sequence of a cyclo-them from base to top, is sandstone-clay-coal-shale-limestone.

Figure 23-17 The bestexposure of the GreatPermian Reef Complex is atGuadalupe MountainsNational Park in West Texas.

These evaporites and the Great Permian Reef Complex are veryimportant deposits. As you learned in the Discovery Lab at the begin-ning of this chapter, pore spaces in rocks and sediment can be filledwith oil or water. The pore spaces in Permian reefs and in the sur-rounding coarse-grained rocks are filled with oil making these reefsimportant oil reservoirs. The evaporites associated with these reefs areimportant for a different reason. The thick salt deposits have very lowpermeability and, thus, virtually no liquids move through them. Thismakes them an excellent environment for long-term storage ofnuclear waste. In the thick salt deposits in southeastern New Mexico,a large facility has been built for the disposal of various kinds ofnuclear waste.

CONTINENTAL COLLISIONS AND MOUNTAIN BUILDINGThe Late Paleozoic was a time of active mountain building.Gondwana collided with the southeastern margin of Laurasia duringthe Ouachita Orogeny. The Ouachita Orogeny formed the OuachitaMountains of Arkansas and Oklahoma and the mountains in theMarathon area of West Texas.

The collision between Gondwana and Laurasia was so intensethat it caused the crust to uplift inland as far as present-dayColorado. Geologists call the mountain range that formed theAncestral Rockies. Large vertical faults lifted the Paleozoic rocksmore than 2 km. The uplifted rocks eventually eroded and weredeposited as red sandstone beds. The most famous of these depositsare the beautiful exposures at Garden of the Gods in ColoradoSprings, shown in Figure 23-18.

As the collision between the two supercontinents continued, itclosed the ocean that separated Gondwana from Laurasia. The partof Gondwana that is now Africa began to collide with Laurasia during the Late Pennsylvanian, causing the Alleghenian Orogeny.


Figure 23-18 These rocksare part of the FountainFormation, a unit whose sed-iments were derived fromerosion of Pennsylvanian-aged arkoses. As a result ofcontinued faulting, the layersare standing literally on end.

EnvironmentalConnection

It continued through the Permian along the southeastern margin ofLaurasia. The Alleghenian Orogeny was the last of the three majormountain-building events to affect what is now eastern NorthAmerica and resulted in the formation of the AppalachianMountains. Pangaea had formed. This is summarized in Figure 23-6B on page 605.

LATE PALEOZOIC LIFEThe invertebrate marine organisms that flourished during theMiddle Paleozoic continued to dominate the marine environment.One group, the crinoids, shown in Figures 23-19A and B, becamedominant during the Mississippian. Crinoids were so abundant thatsome Mississippian limestones are made almost entirely of fos-silized crinoid stem fragments.

Changes in the Ocean Following theextinction of many fishes during the LateDevonian, sharks and other ray-finnedfishes, some of which still exist today,became more abundant. Lobe-finnedfishes appeared during the Late Devonian,but became more abundant during theLate Paleozoic. Their club-shaped finswere supported by thick bones that madeit possible for some of these fishes to walkon land. Of the three groups of lobe-finned fishes, two of them, the lungfishesand the coelacanths, are still living today.The third group of lobe-finned fishes gaverise to the amphibians.

On land, the true ferns, sphenopsids,lycopods, and seed ferns that appearedduring the Late Devonian had thoroughly


A B

Figure 23-19 This fossilcrinoid is from the Silurian(A). Living crinoids feed byspreading out their arms tocatch and filter food parti-cles from the water andpass them down to theirmouths through grooves in their arms (B).

Figure 23-20 A reconstruc-tion of the life-forms on landduring the Late Paleozoic.

invaded the low, swampy land of thePennsylvanian as illustrated in Figure 23-20. The warm, wet lowlands that developedat the base of the newly formed AppalachianMountains was a perfect environment forplants to grow in and ultimately gave rise tothe great coal swamps of the Pennsylvanian.They were a breeding ground for insects aswell as plants. In fact, the largest insects thatever lived were preserved in coal swamps,including cockroaches that reached morethan 10 cm in length and centipedes thatwere longer than 30 cm. The largest of allwas a dragonfly with a 74-cm wingspan.

The coal swamps also housed amphib-ians. Although they were most likely slug-gish and ate only fish, insects, vegetation,and smaller amphibians, they were the topcarnivores on the land. Amphibians haveone major limitation: they must lay theireggs in water because their eggs have noprotective coating to prevent them fromdrying out on land. Thus, although earlyamphibians could colonize the land, theyhad to remain close to a source of water.Sometime during the Late Mississippian,this dependence on water was overcome.Reptiles that evolved from the earlyamphibians developed a new type of egg,the amniote egg. It has a shell that protectsthe embryo, which is surrounded by a liquid-filled sac that contains a food sac anda waste sac. Early reptiles were small, agile,and had more advanced teeth and jaws thanamphibians. During the Permian, the morerapidly moving and stronger reptilesbecame abundant on land and displacedthe amphibians as the top land carnivores.

THE PERMIAN MASS EXTINCTIONThe largest mass extinction in the history of life on Earth defines the end of the Paleo-zoic Era. It is called the Permo-TriassicExtinction Event and it marks the end of


Collisions and ShelvesModel the difference in continental shelfarea between individual continents and one supercontinent.

Procedure1. Using 250 g of modeling clay, make a

sphere and flatten it into a disk that is 1/2 cm thick. This represents a craton.

2. Divide another 250 g of clay into 2 equalspheres and flatten them as above.

3. Roll 250 g of modeling clay into 3 longcylinders with 1/2 cm diameters. Wrap thecylinders around the edges of the claydisks. These represent continental shelves.

4. Using the formula area = �r2, calculate thearea of the large craton and the area ofthe large craton plus the continental shelf.Subtract the craton area from the totalarea. This equals the area of the continen-tal shelf.

5. Repeat step 4 for each of the small models.

Analyze and Conclude1. Which has more shelf area, two small con-

tinents or one large continent? Why?2. Tropical oceans contain the greatest diver-

sity of animals. If there is only one super-continent how does this further limit theamount of habitat space?

3. Explain how reduced habitat space,Pangaea, and the mass extinction at theend of the Permian are related.

nearly 95 percent of all species including trilobites. You will learnmore about this event in the Science & the Environment feature at theend of this chapter.

In contrast to the mass extinctions of the Middle Paleozoic, thisextinction did not affect only the marine animals. Many of the ter-restrial organisms that thrived in the swamps of the Late Paleozoicbecame extinct as well. More than 65 percent of the amphibians andreptiles did not survive, nor did almost one third of all insects. Whatcould have caused such a widespread catastrophe?

A Major Marine Regression One possible cause of the Permo-Triassic Extinction Event was a major marine regression. Now, asthen, most marine animals live in the relatively shallow water calledthe continental shelf. When sea level is high, the entire continentalshelf is flooded, and there is quite a large area where marine animalscan live. During regression, however, the continental shelf canbecome very narrow, leaving little space on the continental shelf formarine animals to live. This situation would have been particularlycritical when there was only one continent. You learned about theconsequences of reduced continental shelf space in the MiniLab onthe previous page.

The end of the Paleozoic was a stressful time for life on Earth.Marine habitats shrank as a result of the major regression thatoccurred around Pangaea. Whether additional stresses were actingon life at that time is not known. What is known, however, is thatregardless of the ultimate cause, life on Earth was irrevocablychanged at the end of the Paleozoic.


CalculatingPercentages Is Earthcurrently in the mid-dle of a mass extinc-tion? If ten millionspecies exist today,and 5.5 species extinc-tions occur every day,calculate how manyyears it would takefor 96 percent of thespecies living today to become extinct.

1. Explain the geologic events that resultedin the formation of Pangaea.

2. What is the relationship betweencyclothems and glaciation?

3. What do scientists hypothesize were themain causes of the Late Permian massextinction?

4. Thinking Critically In the previous section,you learned about the development ofthe seed. In this section, you learnedabout the development of the amnioteegg. Explain the similarities between

these two important developments in thehistory of life on Earth.

SKILL REVIEW

5. Concept Mapping Reorganize the eventsbelow into an events chain that describesthe formation of Pangaea. OuachitaOrogeny occurs; Ancestral Rockiesuplifted; North America joined toGondwana; Pangaea formed; Part ofGondwana collides with eastern Laurasia;Alleghenian Orogeny occurs; Part ofGondwana collides with SE Laurasia.




Symmetry, Shape and Shells

Brachiopods and bivalves have been present in Earth’soceans since the Cambrian. Both have two shells and live

in marine environments. But the similarity ends there. Howcan you tell the two apart? Oysters are bivalves that are knownfor the pearls they secrete inside their shells. Can you distin-guish an oyster from a brachiopod? If you were searching forpearls, you would want to know how!

ProblemDistinguish between brachiopods andbivalves and interpret the environmentwhere a brachiopod lived based on itsshell.

Materialsfossil brachiopods (4), each belonging

to a different speciesfossil bivalves (4), each belonging to a

different speciespaperpencil

ObjectivesIn this GeoLab you will:• Determine if a fossil is a brachiopod

or a bivalve.• Describe the symmetry of fossil

brachiopods and bivalves.• Infer the environment in which differ-

ent fossil brachiopods lived.

Safety Precautions Always wear safety goggles and anapron in the lab.

Preparation

GeoLab 619

1. Design a data table like the oneabove. You may choose to add morecolumns to record additional data.

2. Examine the fossils provided by yourteacher.

3. Determine where the plane of sym-metry is for each specimen. Anorganism that can be divided intotwo nearly identical halves has bilat-eral symmetry.

4. Identify the specimens as brach-iopods or bivalves based on their

symmetry and record this in yourdata table. Brachiopod symmetryruns across both shells. Bivalve symmetry runs between the shells.

5. Divide the brachiopods into twogroups based on whether you thinkthey lived in a deeper water, lowenergy environment, or in a shallowwater, high-energy environment.Record this in your data table.

1. Interpreting Observations Explainhow symmetry is useful in determin-ing whether a fossil is a brachiopodor a bivalve.

2. Applying and Interpreting If you only had one shell, how could you

determine if it was the shell of a bra-chiopod or the shell of a bivalve?

3. Comparing and Contrasting Explainthe similarities and differencesbetween a streamlined auto and a smooth brachiopod, in terms oftheir place in wind or water.

1. What principle did you use to deter-mine the environment in which thefossil brachiopods lived? Explain.

2. Hypothesize about the reasons for the different shell types for brachiopods that live in differentenvironments.

3. All living brachiopods pump waterthrough their shells and filter organicparticles out of that water to feed.Some brachiopod shells close along a straight line, whereas others closealong a zig-zag line. What is the ben-efit of having a zig-zag opening for a filter feeding brachiopod?

Procedure

Analyze

Conclude & Apply

Deeper water, low energy environment or Specimen Brachiopod or Bivalve shallow water, high-energy environment

123

FOSSIL DATA

Triassic Extinction Event cleared the way for newkinds of life, including the dinosaurs. The extinc-tion at the end of the Mesozoic cleared the wayfor mammals to dominate. Without those extinc-tions, humans might not be here.

Many scientists believe that we are currentlyin the midst of another mass extinction. Over athousand species of animals, and probably evenmore species of plants, are in danger of becom-ing extinct within our lifetime. This extinction isnot caused by a regression or climatic changes,but by the impact of human activities.

Drugs to fight everything from AIDS to cancerhave been found in organisms ranging fromsponges to trees. Plants might be discoveredthat can be used to feed the world’s hungry peo-ple. If a species becomes extinct, we lose thepossibility of discovering any potential uses itmight have had.


The extinction of the dinosaurs and manyother forms of life at the end of the CretaceousPeriod has received much attention. For sheernumbers, though, these extinctions can’t comparewith the extinctions that ended the Paleozoic Era.

The Big Extinction EventAt the end of the Mesozoic, 65 to 75 percent

of all species on Earth became extinct. When thePaleozoic ended, as many as 95 percent of allspecies became extinct. This event is called thePermo-Triassic Extinction Event.

Some marine organisms that became extinctduring this Extinction Event were fusilinid fora-minifera, rugose corals, and trilobites. On land,therapsids or mammal-like reptiles, becameextinct, and insects suffered their only majorextinction in their 390 million year history.

Paleontologists once thought that the Permo-Triassic extinctions took place over millions ofyears. However, recent research indicates that theextinctions were much more rapid. The causesare not well understood, but the extinctionsappear to be the result of a fatal combination ofchanges in sea level, reduced oxygen levels, mas-sive volcanic eruptions and global warming.

Opportunities Lost and GainedAlthough the biosphere took millions of years

to recover from each mass extinction, suchextinctions are not entirely negative. The Permo-

Mass ExtinctionsMost organisms that have ever lived on Earth are now extinct. For shallow-water, ocean-dwelling species, the average lifetime of a speciesis about 4 million years. That works out to an average of two or threespecies becoming extinct every year. The fossil record also indicatesthat, at certain times, many species became extinct over a short time.

Go to earthgeu.com to find links to moreinformation about the use of drugs derivedfrom plants or animals to fight disease.Summarize where the drug is found,whether the plant or animal is in danger ofbecoming extinct, and how it is used tofight disease in humans.

Activity

Trilobite

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Summary

VocabularyBurgess Shale

(p. 604)paleogeography

(p. 601)passive margin

(p. 602)regression (p. 603)transgression

(p. 603)

VocabularyAcadian Orogeny

(p. 608)Antler Orogeny

(p. 608)Caledonian Orogeny

(p. 608)mass extinction

(p. 611)Paleozoic fauna

(p. 609)Taconic Orogeny

(p. 606)vascular plant

(p. 609)

VocabularyAlleghenian Orogeny

(p. 614)amniote egg (p. 616)Ancestral Rockies

(p. 614)cyclothem (p. 613)Gondwana (p. 612)Ouachita Orogeny

(p. 614)

Main Ideas• The ancient North American continent of Laurentia was located

near the equator and surrounded by ocean during the CambrianPeriod. A shallow sea covered most of Laurentia.

• Laurentia was completely surrounded by passive marginsthroughout the entire Cambrian Period.

• Many new organisms developed during the Cambrian explosion.Fossils of trilobites and articulate brachiopods are particularlycommon in Cambrian rocks.

• When environments change position laterally due to changes in sea level, adjacent depositional facies overlie each other invertical succession.

Main Ideas• High evaporation rates in lagoon settings cause the water to

become oversaturated with calcium and sodium. These elementscombine with other elements or compounds in the water andprecipitate out of solution as the evaporite minerals.

• Clastic wedges provide evidence for orogenic events.• The seas were dominated by articulate brachiopods, corals, mol-

lusks, bryozoans, crinoids, graptolites and conodonts. Fisheswere the top predators of the seas during the Devonian. Ferns,sphenopsids, and lycopods covered the landscape by the LateDevonian.

• Two mass extinctions occurred. A mass extinction occurs whenan unusually large number of organisms become extinct over arelatively short period of geologic time.

Main Ideas• Pangaea formed as Laurasia (North America + Europe) collided

with Gondwana (South America, Africa, India, Australia andAntarctica).

• Cyclothems consist of transgressive and regressive rocksequences stacked one on top of another. They represent cyclesof glacial-interglacial periods.

• Seeds evolved and allowed plants to colonize dry land. Theamniote egg evolved and allowed reptiles to colonize dry land.

• One possible cause of the Permo-Triassic Extinction Event wasregression. Marine habitats around Pangaea shrank. Feedbackfrom this event caused global warming, which affected organ-isms on land as well as in the sea.

SECTION 23.1

The EarlyPaleozoic

SECTION 23.2

The MiddlePaleozoic

SECTION 23.3

The LatePaleozoic

Study Guide 621earthgeu.com/vocabulary_puzzlemaker

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1. Where was Laurentia located during thePaleozoic?a. over the south pole c. over the equatorb. over the north pole d. where it is today

2. During what period was Laurentia completely surrounded by passive margins?a. the Cambrian c. the Devonianb. the Ordovician d. the Permian

3. What term describes a drop in sea level and theresulting seaward movement of the shoreline?a. regression c. explosionb. excursion d. transgression

4. What are the most common fossils of theCambrian fauna?a. sharks c. crinoidsb. trilobites d. rugose corals

5. What is the calm area behind a reef called?a. a clastic wedge c. a lagoonb. a passive margin d. a continental shelf

6. Which of the following is not an evaporite mineral?a. anhydrite c. haliteb. quartz d. gypsum

7. What evaporite mineral is used to make plasterand drywall?a. gypsum c. haliteb. aragonite d. quartz

8. What is a triangular-shaped sedimentary rockdeposit that was deposited adjacent to anuplifted area called?a. a passive margin c. a clastic wedgeb. an active margin d. an orogeny

Understanding Main Ideas 9. What landmass collided with Laurentia andcaused the Acadian Orogeny?a. Gondwana c. Balticab. Avalonia d. Siberia

10. What group of fishes were able to walk on land?a. ray-finned fishes c. lobe-finned fishesb. sharks d. Dunkleosteus

11. What group of fishes gave rise to the amphibians?a. acanthodians c. lobe-finned fishesb. sharks d. Dunkleosteus

12. What is special about the fossils of the BurgessShale?

Use the figure below to answer question 13.

13. What change(s) in sea level is/are represented bythe sequence of rocks?

Applying Main Ideas

DRAWINGS AND DIAGRAMS WITH TEXTIf a written description is included with a draw-ing or diagram, read it carefully. Sometimes thewritten description can clarify or change themeaning of the graphic.

Test-Taking Tip

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Assessment 623

1. What is it called when the sea level rises andshorelines move inland?a. regression c. transgressionb. passive Margin d. laurentia

2. Which tectonic event was probably caused by a small microcontinent or island arc that collided with present-day western North America?a. Taconic Orogeny c. Acadian Orogenyb. Caledonian Orogeny d. Antler Orogeny

INTREPRETING SCIENTIFIC ILLUSTRATIONSUse the figure to answer question 3.

14. Describe the differences between the Cambrianand Paleozoic fauna.

15. What kind of clues do geologists use to deter-mine whether an area was the site of an orogenyin the geologic past?

16. How did paleontologists use rugose corals todetermine that Earth rotated more rapidly in thegeologic past than it does now?

17. How did the Pennsylvanian cyclothems form?

18. Explain why many paleontologists consider thedevelopment of the seed and the amniote egg tobe two of the most important events in Earth’shistory.

19. How do the many coral reef deposits in the GreatLakes area of North America support the hypothe-sis that Laurentia was positioned across theequator in the Late Paleozoic?

20. Explain how identical clastic wedges formed oneither side of the mountains that were formed bythe Acadian Orogeny.

21. Distribution of plants occurs with the help of ani-mals that eat them. Explain how the evolution ofthe seed ensured the success of this type of plantdistribution.

Thinking Critically

Standardized Test Practice

3. During the last period of the Paleozoic Era,much of what is now western North Americawas covered with dry land. What was thisperiod called?a. Pennsylvanian c. Permian b. Devonian d. Ordovician

4. What was formed in North America whenGondwana and Laurasia collided?a. Ancestral Rocky Mountainsb. Appalachian Mountainsc. Ouachita Mountainsd. Great Permian Reef

290 M.Y.B.P.

362 M.Y.B.P.

408 M.Y.B.P.

439 M.Y.B.P.

500 M.Y.B.P.

540 M.Y.B.P.

Cambrian

Silurian

Mississippian andPennsylvanian

?

?

?

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Documents

Chapter 23: The Paleozoic Era