Therapsida is a group of the most advanced reptile-grade synapsids, and the ancestors of mammals. While mammals strictly speaking are surviving therapsids, the name is usually only applied to the non-mammalian evolutionary grade. Many of the traits today seen as unique to mammals had their origin within the Therapsida, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be Tetraceratops insignis (Lower Permian). Therapsids evolved from 'pelycosaurs' (specifically sphenacodonts) 275 million years ago. They replaced the pelycosaurs as the dominant land animal. Mammals appeared 200 million years ago, and all therapsids other than mammals became extinct in the Early Cretaceous period (146 Ma to 100 Ma).
Characteristics
Therapsids'
temporal fenestrae are greater than those of the pelycosaurs. The jaws of therapsids are more complex and powerful, and the
teeth are differentiated into frontal
incisors for nipping, great 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
, 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 bigger
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.
Taxonomy
Classification
, a Biarmosuchian]]
, a Dinocephalian]]
, an Anteosaur]]
, a Gorgonopsid]]
, a Therocephalian]]
, a Cynodont]]
Class Synapsida
ORDER THERAPSIDA *
*?Family †Tetraceratopsidae
* Suborder †Biarmosuchia *
** Family †Biarmosuchidae
** Family †Eotitanosuchidae
* Eutherapsida
** Suborder †Dinocephalia
*** Family †Estemmenosuchidae
***?Infraorder †Anteosauria
**** Family †Anteosauridae
**** Family †Brithopodidae
**** Family †Deuterosauridae
**** Family †Syodontidae
****?Family †Stenocybidae
*** †Tapinocephalia
**** Family †Styracocephalidae
**** Family †Tapinocephalidae
**** Family †Titanosuchidae
** Neotherapsida
*** Suborder †Anomodontia *
**** Superfamily †Venyukoviamorpha
***** Family †Otsheridae
***** Family †Venyukoviidae
**** Infraorder †Dromasauria
***** Family †Galeopidae
**** Infraorder †Dicynodonta
***** Family †Endothiodontidae
***** Family †Eodicynodontidae
***** Family †Kingoriidae
***** (unranked) †Diictodontia
****** Superfamily †Emydopoidea
******* Family †Cistecephalidae
******* Family †Emydopidae
****** Superfamily †Robertoidea
******* Family †Diictodontidae
******* Family †Robertiidae
***** (unranked) †Pristerodontia
****** Family †Aulacocephalodontidae
****** Family †Dicynodontidae
****** Family †Kannemeyeriidae
****** Family †Lystrosauridae
****** Family †Oudenodontidae
****** Family †Pristerodontidae
****** Family †Shanisiodontidae
****** Family †Stahleckeriidae
*** Theriodontia *
**** Suborder †Gorgonopsia
***** Family †Gorgonopsidae
**** Eutheriodontia
***** Suborder †Therocephalia
****** Family †Lycosuchidae
***** (unranked) †Scylacosauria
****** Family †Scylacosauridae
***** Infraorder †Eutherocephalia
****** Family †Hofmeyriidae
****** Family †Moschorhinidae
****** Family †Whaitsiidae
****** Superfamily Bauriodea
******* Family †Bauriidae
******* Family †Ericiolacteridae
******* Family †Ictidosuchidae
******* Family †Ictidosuchopsidae
******* Family †Lycideopsidae
***** Suborder Cynodontia *
****** Family †Dviniidae
****** Family †Procynosuchidae
****** (unranked) Epicynodontia
******* Family †Galesauridae
******* Family †Thrinaxodontidae
******* Infraorder Eucynodontia
******** (unranked) †Cygnognathia
********* Family †Cynognathidae
********* Family †Diademodontidae
********* Family †Traversodontidae
********* Family †Trirachodontidae
********* Family †Tritylodontidae
******** (unranked) Probainognathia
********* Family †Chinquodontidae
********* Family †Probainognathidae
********* (unranked) †Ictidosauria
********** Family †Trithelodontidae
********* (unranked) Mammaliformes
********** Class Mammalia
Phylogeny
|label2=
Eutherapsida
|2=
|label2=
Neotherapsida
|2=
|label2=
Theriodontia
|2=
|label2=
Eutheriodontia
|2= }}
|label2=
Cynodontia
|2=
}} }} }} }} }} }} }} }} }}
See also
Vertebrate paleontology
Evolution of mammals
Timeline of evolution
Notes
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.
External links
"Therapsida: Mammals and extinct relatives" Tree of Life
"Therapsida: overview" Palaeos
Category:Synapsids
Category:Therapsids
