mesozoic mammaliaforms early evolution of mammals
DESCRIPTION
Mesozoic Mammaliaforms Early evolution of mammals. http://scienceblogs.com/pharyngula/2008/05/10/monotreme_cladogram.jpg. Monotremes. Placentals. Marsupials. Synapsid Transition. Basal Synapsida. Mammals. Dimetrodon. Edaphosaurus. What is a mammal??. - PowerPoint PPT PresentationTRANSCRIPT
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