Mesozoic Mammaliaforms

Early evolution of mammals

http://scienceblogs.com/pharyngula/2008/05/10/monotreme_cladogram.jpg

Synapsid Transition

Basal SynapsidaDimetrodon

Edaphosaurus

Mammals

Monotremes

MarsupialsPlacentals

What is a mammal??

For Linnaeus animals with hair covering the body, mammary glands to feed their offspring, and development of endothermy.

What is a mammal??

For Simpson and Kermack animals with a dentary-squamosal joint; the presence of one bone in the lower jaw, three auditory ossicles in the middle ear

Osteological Diagnosis

Seven to eight bones are present in the lower jaw of synapsids

Three of the bones of the lower jaw present in basal Therapsids continues in mammals

dentarydentary, which is the only bone of the lower jaw

articulararticular, which is incorporated to the middle ear as the hammerhammer

angularangular, also in the middle ear as the tympanictympanic and that support the tympanic membrane

quadratequadrate, from the skull is also incorporated to the middle ear as the anvilanvil

Most of then decrease in size, and only the dentarydentary increase in size

squamosalsquamosal

jaw jaw joint dentary

dentary

dentary

quadratequadrate

jaw jaw jointjoint

PostdentaryPostdentarybonesbones

articular

Pelycosaur

Therapsid

Early Mammal

ChangesChanges

summarysummary

Mesozoic mammals at the end of the ’70s: monophyletic group, that diverged early into Prototheria/Theria, with representatives of the group already in the Triassic

Living PROTOTHERIA: monotremes

Living THERIA: marsupials and placentals

Crompton and Jenkins, 1979

12

Rowe, 1988

Mesozoic mammals at the end of the ’80s and onward: monophyletic group, without the early divergence.

Pr are groups included in prototheria in the previous phylogeny, indicating that prototheria is not monophyletic

Conflict in definition of MammaliaArrow 1, Mammalia based on osteological diagnosis

Arrow 2, Mammalia based on common ancestry of extant groups

Osteologicaldiagnosis

EXTANT MAMMALSMONOTREMATA

High degree of specialization

•Only three species restricted to Australia and New Guinea

•Oviparous

•Aquatic and terrestrial ant-eatersplatypus

short beaked echidna

Long beaked echidna

•sprawling posture

•limb and girdles bones of robust construction

•Lack of teeth, or only a couple of teeth in the juvenile platypus

EXTANT MAMMALS

MONOTREMATA Fossil History in the Mesozoic

Lower jaw fragment with three distinctive teeth remarkably similar to those of the juvenile platypus

Other two species represented by jaw fragment with tooth were discovered in the Lower Cretaceous, demonstrating an early radiation of monotrems

Monotremes and closely related forms only known in Australia, Madagascar and South America form a quite recently proposed group: AUSTRALOSPHENIDA (more about this latter)

Steropodon

The earliest Monotreme is know from the Early Cretaceous of Australia.

EXTANT MAMMALSMARSUPIALIA

Presence of incisors, canines, premolar and molar, with the primitive forms showing 5/4 incisors, the highest among mammals

275 living species within two basic groups: AUSTRALIDELPHIA, mainly in the Australian region, and AMERIDELPHIA in the New World

The most primitive living marsupial is the American DIDELPHIDAE

Variable habits: Ameridelphians are mainly arboreal and omnivorous. Australidelphians are mostly terrestrial, but some species are arboreal or burrower. Carnivorous, herbivorous and omnivorous

Viviparous, with short gestation period and neonates with minimal anatomical development. Most of them with abdominal pouches

Among diagnostic features are the inflected angle of the dentary and replacement of only one tooth (the last premolar)

Opposum

KangarooKoala

The earliest metatheria are recorded in the Early/Late Cretaceous of North America and Asia

Probably at the end of the Late Cretaceous they reach South America

They are represented by mandibular and skull fragments and many isolated teeth

EXTANT MAMMALS

MARSUPIAL Fossil History in the Mesozoic

Metatherian were diverse in the Late Cretaceous of North America, but their diversity drops after the Cretaceous/Tertiary extinction. They became extinct in North America in the tertiary

Alphadon

Didelphodon

EXTANT MAMMALSMARSUPIAL Fossil History in the Mesozoic

It was a small animal (15 cm in body length) and weighed approximately 30 grams

The earliest and most primitive METATHERIA (monophyletic group that includes extant marsupials and fossil forms most closely related with marsupials) is know from the Early Cretaceous in the Liaoning Province of China (125 ma)

Sinodelphis is represented by nearly complete skeleton, surrounded by well-preserved impressions of fur

It was insectivorous and arboreal

EXTANT MAMMALSPLACENTALS

4162 living species in 17 groups. Cosmopolitan

Viviparous with a long gestation period. Neonate may be born with minimal development (many rodents), or in a more developed stage

All kind of habits and life styles

The most primitive living placental in phylogenies based in morphological characters are the American Edentates (e.g. armadillos, sloths and anteaters)

Placental oldest member is from the Early Paleocene

The most primitive living placental in phylogeny based in molecular characters are Afrotheria

EXTANT MAMMALSTwo recent phylogenies give different perspectives of interrelationships and origin of placental mammals

Bininda Emmons et al. 2007 (Supertree) Wible et al. 2007 (Morphological)

EXTANT MAMMALS

EXTANT MAMMALS

PLACENTAL Fossil History in the Mesozoic

The earliest and most primitive EUTHERIA (monophyletic group that includes extant placentals and fossil forms most closely related with placentals) is know from the Early Cretaceous in the Liaoning Province of China (125 ma)

Eomaia is represented by a nearly complete skeleton, surrounded by well-preserved impressions of fur

It was insectivorous and a climber. It could grasp branches and move very fast in trees and on the ground. It was nearly 15 cm long and weighed about 25 grams In the same Early Cretaceous are at least three other younger representatives of the eutherians, indicating an early radiation of the group. At this time they are arboreal as well as terrestrial

TOOTH HISTORY IN MESOZOIC TIMES

Something like 10 to 20 years ago the changes in the molar pattern was an easy sequence to follow:

First came the triconodont pattern: three main cusps in a straight line, with or without outer cingulum and accessory cusps

Then in holotherians the central cusp moved outward or inward to create a triangular pattern, known as trigon/trigonid

Finally the true tribosphenic molar developed:

A new cusp (protocone) is added to the trigon, which occludes against a basin added in the lower teeth (talonid)

The apex of the triangle of the upper tooth points inwards, while that of the lower points outwards. This relationships is known as reversed triangle

The earliest record of a Tribosphenida is from the Middle Jurassic of Madagascar

Triconodont

Holotherians

Tribosphenida

TOOTH HISTORY IN MESOZOIC TIMES

Coming back to the ’70s picture:

The Triconodont pattern of teeth was typical of Prototherian, a group that included extant monotremes

While reversed triangles and tribosphenic molars were found in the Theria lineage, including marsupials and placentals

But in the ’80s, a close relationship between monotremes, marsupials and placentals was favoured This hypothesis gained support with the discovery in Australia of the Cretaceous monotreme Steropodon

This fossil showed that monotremes had ancestors with triangular pattern of the molars,

TOOTH HISTORY IN MESOZOIC TIMESUntil recently the earliest records of tribosphenic molars were restricted to northern continents (e.g. Asia, North America)

But in the last five years fossils with tribosphenic molars begin to appears in faunas from Australia, Madagascar and quite recently Argentina

This began to change ideas about the place and time of origin of the tribosphenic molar

The development of the tribosphenic molar is interpreted as key in the evolutionary success of mammals, and as having evolved once in the northern continents

A recent hypothesis suggests that tribosphenic molars evolved independently in two ancient groups during the Jurassic /Early Cretaceous

One in Laurasia: Boreosphenida, that gave rise to marsupials and placentals, and the other in Gondwanan: Australosphenida, including monotremes

Mesozoic diversity of Mammaliaformes

Stem Mammaliaformes

Triconodonta

Gobiconodonta

Docodonta

Symmetrodonta

Dyolestidae

Multituberculata

Eupantotheria

Monotremata

Metatheria

Eutheria

Many different groups of Mammaliaforms were represented during the Mesozoic:

Two major periods of Mammaliaform diversification:

1. From Early Jurassic to Early Cretaceous: radiation of forms that in the main did not survive past the Mesozoic

2. From the Early Cretaceous: first records of forms closely related to the three extant mammals, and the beginning of a great diversification of multituberculates

Mesozoic Mammaliaformes are widely distributed around the world, Europe, Asia, North and South America, Australia and Africa

In Africa are known in South Africa, Tanzania, northwestern Africa and Madagascar

Stem MammaliaformesThese is a paraphyletic group of basal mammaliaformes, not included in a family and with conflicting phylogenetic positions. Many of them were previously included in TriconodontaAdelobasileus, the earliest and most basal Mammaliaform, is known from the Upper Triassic of Texas (230 million years ago)

Megazostrodon is the stem Mammaliaform known from the Early Jurassic of South Africa and LesothoMorganucodon from the Early Jurassic of U.K. and China is the best known stem Mammaliaform

Main cusp, one accessory anterior two accessory posterior and cingulum

Retention of postdentary bones

Dentary squamosal craniomandibular joint

Presence of the promontorium

Docodonta

Small group of mouse-to-rat sized animals, restricted to Early Jurassic/Early Cretaceous faunas of northern continents

The best known docodontid is Haldanodon a small insectivorous from the Upper Jurassic of Portugal

Lower jaw with many bones Very complex bucco-lingually

expanded molars, associated with a sophisticated occlusion

DocodontaCastorocauda, Middle Jurassic lake beds of China; more than 6 cm in skull length and at least 42.5 cm in body length

beaver-like tail, strong arms for digging, and sharp teeth specialized for aquatic feeding, similar to the modern river otter

earliest-known swimming mammal with teeth for eating fish

“the most primitive-known mammal to be preserved with hairs”

TriconodontaConsidered by most authors as a paraphyletic group. This group originally included representatives of stem Mammaliaformes (Sinoconodon, Morganucodon)

But they include the monophyletic GOBICONODONTIDS, group known from the Early Cretaceous of northern continents

This is the only group of mammals in which replacement of molar teeth is known The angular process of the mandible is absent

Extremely derived morphology in the antemolar dentition (incisors looks like canine, canine is very reduced), but the molars present a generalized triconodontid pattern

Gobiconodon

Fruitafossor or “Popeye”15 cm long and weighed about 30 grams. Late Jurassic (150 Ma), Colorado U. S.

Single- and open-rooted tubular molars, suggesting continuous growth in life, and similar to the teeth of armadillos and aardvarks

The first known mammal adapted for feeding on ants or termites and for digging and burrowing

Extremely robust humerus and claws adapted for digging; vertebrae with extra joints

MultituberculataThe most successful group of mammals to have ever lived: from late Jurassic to the late Eocene. They lived over 100 million years and survived the K/T extinction. Mostly restricted to northern continents but with some conflicting record in South America Over 200 species are known, some as small as a tiny mice, the largest the size of beavers. They were terrestrial, arboreal and burrowing

They had large incisors, looking somewhat like modern rodents, and had long multi-cusped molars. Remarkable feature in the skull is a tiny jugal bone in the zygoma

Arboreal multituberculates with tarsal features similar to mammals that descend tree headfirst and have prehensile tails

Ptilodus

Multituberculata

Conflict in the phylogenetic placement of Multituberculata:

Are Multituberculata more closely related to theria (placentals and marsupials) than monotremes (and thus included in mammals)? Are Monotremata more closely related to theria than multituberculates (and thus the last group is outside of mammals)?

“Symmetrodonta”/SpalacotheroideaMany authors consider this group as paraphyletic. They are represented largely by isolated teeth and mandibular fragments. Recently was found a new species from the Early Cretaceous of China with a nearly complete skeleton and a partial skull and lower jaw

Symmetrodonta are characterized by postcanine teeth with an imperfectly symmetrical, triangular arrangement of the principal cusps

The front legs are interpreted as splayed out, similar to the condition of reptiles

“Pretribosphenid Therians”

Vincelestes, from the Early Cretaceous of Argentina, is one of the best represented “pretribosphenid therians”, a paraphyletic group of mammals

Vincelestes skulls is about 7cm long and the body length 30 cm, without considering the long tail

Upper molars in Vincelestes show a small protocone and lower molars a short talonid with only one cusp

Mesozoic Mammaliaforms Epilogue

Even when not commonly dominant in Mesozoic faunas, Mammaliaforms were notably diverse

Mesozoic Mammaliaforms were extraordinarily small. The largest size (in gobiconodontids and metatherians) were similar to the Opossum (half a metre in body length)

They are distributed all around the world, except in Antarctica. Outcrops with their remains are notably more common in northern continents

Most of their record is represented by isolated teeth and fragments of mandibles with teeth. In the last ten years many complete specimens of different groups have been found in China

Of the living mammals, Monotremes have representatives in the Early Cretaceous of Australia, Marsupials in the Late Cretaceous of North America and Placentals in the Early PaleoceneBut the earliest records of both Metatheria (group that includes marsupials) and Eutheria (group that includes placentals) occurs in the Early Cretaceous of Liaoning Province, China approximately 125 My ago The most successful group of mammals (both extant or extinct) are Multituberculates, that lived for a period of 100 My, surviving the K/T extinction

Much disagreement among researchers remain regarding phylogenetic placement of Mammaliaform groups. This is particularly evident in the relationships of many Stem Mammaliaformes and in the placement of Multituberculates. There is also discussion about the monophyly of Triconodontids