Therapsida

Taxobox
name = Therapsids
fossil_range = fossil range|300|100Early Permian - Early Cretaceous (non-mammalian)

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image_width = 240 px
image_caption = Illustration of "Pristerognathus", a therocephalian therapsid
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
superclassis = Tetrapoda
classis = Synapsida
ordo = Therapsida *
ordo_authority = Broom, 1905
subdivision_ranks = Clades
subdivision =
* Suborder Biarmosuchia *
* Suborder Dinocephalia
* Suborder Anomodontia *
** Infraorder Dicynodontia
* (unranked) Theriodontia *
** Suborder Gorgonopsia
** Suborder Therocephalia
** Suborder Cynodontia *

Therapsids are an order of synapsids (Class Synapsida). Traditionally, synapsids were referred to as reptiles and were known as the "mammal-like reptiles". However, they are now classified as a sister-group to the reptiles, and are phylogenetically closer to the mammals. Indeed, when the term is used cladistically, the taxon also includes the mammals, which are descended from the cynodont therapsids. All other lineages of the therapsids are extinct; the last known survivors of the non-mammalian therapsids lived in the Early Cretaceous period.

Characteristics

Therapsids' temporal fenestrae were larger than those of the pelycosaursFact|date=August 2007. The jaws of therapsids were more complex and powerful and the teeth were differentiated into frontal incisors for nipping, large lateral canines for puncturing and tearing, and molars for shearing and chopping food. Therapsids' legs were positioned more vertically beneath their bodies than were the sprawling legs of reptiles and pelycosaurs.

Evolutionary history

The therapsids' evolutionary track began in the Early Permian, when a group of pelycosaurs, the Sphenacodontia, a lineage that included "Dimetrodon" and its relatives, gave rise to therapsidsFact|date=August 2007. Evidence was their anatomical features such as the skull, and the vertebrae. Therapsids became the dominant land animals in the Middle Permian, replacing the pelycosaurs who were becoming rare as Permian period progressed. 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 cynodontsFact|date=August 2007.

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 TriassicFact|date=August 2007. 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 epoch.
# The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous.
# An extremely mammal-like family, Tritheledontidae, 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 epoch, 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 are 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 taxonFact|date=August 2007.

Mammals, the only living therapsids, evolved in the Early Jurassic epoch. 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.

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

Classification

* Class Synapsida
* ORDER THERAPSIDA *
** Suborder Biarmosuchia *
*** Family Biarmosuchidae
**** "Biarmosuchus"
*** Family Eotitanosuchidae
**** "Eotitanosuchus"
** Eutherapsida
*** Suborder Dinocephalia
**** Family Estemmenosuchidae
***** "Estemmenosuchus"
**** Anteosauria
***** Family Syodontidae
***** Family Brithopodidae
***** Family Anteosauridae
**** Tapinocephalia
***** Family Titanosuchidae
***** Family Tapinocephalidae
*** Neotherapsida
****Suborder Anomodontia *
***** Superfamily Venyukoviamorpha
****** Family Venyukoviidae
******* "Suminia
***** "'Infraorder Dicynodonta
**** Theriodontia *
***** Suborder Gorgonopsia
****** Family Gorgonopsidae
***** Eutheriodontia
****** Suborder Therocephalia
****** Suborder Cynodontia *
******* (unranked) Mammaliformes *
* Class Mammalia

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

* Vertebrate paleontology
* Evolution of mammals
* Timeline of evolution

External links

* " [http://tolweb.org/tree?group=Therapsida Therapsida: Mammals and extinct relatives] " "Tree of Life"
* " [http://www.palaeos.com/Vertebrates/Units/400Therapsida/400.000.html Therapsida: overview] " "Palaeos"
* " [http://www3.telus.net/therapsid/ Therapsids in Detail] ". Stephen Priestley - Illustrator

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.