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Therapsids
Fossil range:
Early PermianEarly Cretaceous, 275–100 Ma
Descendant taxon Mammalia survives to present.
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Illustration of Pristerognathus, a therocephalian therapsid
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Synapsida
Order: Therapsida *
Broom, 1905
Clades

Therapsida is a group of synapsids that include mammals and their immediate evolutionary ancestors. Other than the mammals, all lineages of the therapsids are extinct, with the last known non-mammalian therapsids dying out in the Early Cretaceous period (146 Ma to 100 Ma).

Contents

Characteristics

Therapsids' temporal fenestrae are larger than those of the pelycosaurs. The jaws of therapsids are more complex and powerful, and the teeth are differentiated into frontal incisors for nipping, large lateral canines for puncturing and tearing, and molars for shearing and chopping food. Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs.

Evolutionary history

Raranimus, a primitive therapsid

Therapsids evolved from a group of pelycosaurs called sphenacodonts. Therapsids became the dominant land animals in the Middle Permian, displacing the pelycosaurs. Therapsida consists of three major clades, the dinocephalians, the herbivorous anomodonts and the mostly carnivorous theriodonts, with the carnivorous biarmosuchians as a paraphyletic assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian (Guadalupian) but the anomodont dicynodonts and the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first cynodonts.

Like all land animals, the therapsids were seriously affected by the Permian–Triassic extinction event, with the very successful gorgonopsians dying out altogether and the remaining groups, dicynodonts, therocephalians and cynodonts of a few species, each surviving into the Triassic. The dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of Pangea at the end of the Carnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.

Some exceptions were the still further derived eucynodonts. At least three groups of them survived. They all appeared in the Late Triassic period. The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous. An extremely mammal-like family, Trithelodontidae, are unknown later than the Early Jurassic. The third group, Morganucodon and similar animals, were mammaliformes or the "stem-mammals".

Some non-eucynodont cynodonts survived the Permian-Triassic extinction, such as Thrinaxodon but only to become extinct by the Middle Triassic.

The therocephalians, relatives of the cynodonts, managed to survive the Permian-Triassic extinction and continued to diversify through the Early Triassic period. Approaching the end of the period, however, the therocephalians were declining to extinction and eventually became extinct, possibly due to climatic changes and competition from cynodonts and other animals struggling to survive.

Dicynodonts have been thought to have become extinct before the end of the Triassic, but there is evidence that they survived the extinction. Their fossils have been found in Gondwana. Other animals that were common in the Triassic also took refuge here, such as the temnospondyls. This is an example of Lazarus taxon.

Mammals, the only living therapsids, evolved in the Early Jurassic period. They radiated from a group of Mammaliaformes that is related to the symmetrodonts. The Mammaliaformes themselves evolved from probainognathians, a lineage of the eucynodont suborder.

Proposed Paleocene therapsid

Chronoperates is a newly described genus of vertebrate that lived 55 million years ago. Its true identity is still debated and it has even been suggested that it is a symmetrodont. Should it turn out to be a therapsid, the extinction date for this group would be pushed forward almost 45 million years.

Taxonomy

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Classification

Biarmosuchus, a Biarmosuchian
Estemmenosuchus, a Dinocephalian
Anteosaurus, an Anteosaur
Inostrancevia, a Gorgonopsid
Bauria, a Therocephalian
Oligokyphus, a Cynodont

Phylogeny

Synapsida 
 Sphenacodontia 

Sphenacodontidae


 Therapsida 

?Tetraceratops



 Biarmosuchia 

Eotitanosuchidae



?Phthinosuchidae



 Eutherapsida 
 Dinocephalia 

Anteosauria



Tapinocephalia



 Neotherapsida 
 Anomodontia 

Dromasauria



Dicynodontia



 Theriodontia 
 Gorgonopsia 

Lycaenops



Inostrancevia



 Eutheriodontia 
 Therocephalia 
 Eutherocephalia 

Bauria




 Cynodontia 

Mammalia











See also

External links

References

  • Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd
  • Carroll, R.L. (1988). Vertebrate Paleontology & Evolution. W.H. Freeman & Company, NY
  • Kemp, T.S. (2005). The origin and evolution of mammals. Oxford University Press
  • Romer, A.S. (1966). Vertebrate Paleontology. University of Chicago Press, 1933; 3rd ed.

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