Therapsida

Therapsids Temporal range: Early Permian–Early Cretaceous, 275–100 Ma Descendant taxon Mammalia survives to present. PreЄ Є O S D C P T J K Pg N
Mounted skeleton of Inostrancevia alexandri, a gorgonopsian therapsid
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Synapsida
Node:	Sphenacodontoidea
Order:	Therapsida Broom, 1905
Clades
Suborder †Biarmosuchia * (unranked) Eutherapsida Suborder †Dinocephalia Infraorder †Anteosauria Infraorder †Tapinocephalia (unranked) Neotherapsida Suborder †Anomodontia * Infraorder †Dicynodontia (unranked) Theriodontia * Suborder Cynodontia * Suborder †Gorgonopsia Suborder †Therocephalia

Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, 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 large land animals in the Middle Permian. They remained the dominant fauna until replaced by archosaurs and rhynchosaurs in the Middle Triassic although some therapsids, the kannemeyeriiforms for example, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts and dicynodonts survived the Triassic–Jurassic extinction event. The last of the non-mammalian therapsids, the cynodont tritylodontids, became extinct in the Early Cretaceous, approximately 100 million years ago.

Characteristics

Illustration of Pristerognathus, a cat-sized therocephalian therapsid

While the early therapsids had skulls very similar to those of their pelycosaurian ancestors, they differed in the post-cranial skeleton.

Legs and feet

Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs. The feet were more symmetrical, with the first and last toes short and the middle toes longer, indication the foots axis was placed parallel to that of the animal, not sprawling out sideways. This would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs.

Jaw and teeth

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.

Evolutionary history

See also: Evolution of mammals

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 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.

Lemurosaurus

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. Another extremely mammal-like family, Tritheledontidae, are unknown later than the Early Jurassic. The third group, Morganucodon and similar animals, were mammaliformes or the "proto-mammals" (or very early mammals depending of taxonomic preferences).

Some non-eucynodont cynodonts, such as Thrinaxodon, survived the Permian-Triassic extinction 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.^{[citation needed]}

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

Biarmosuchus, a Biarmosuchian

Estemmenosuchus, a Dinocephalian

Anteosaurus, an Anteosaur

Inostrancevia, a Gorgonopsid

Bauria, a Therocephalian

Oligokyphus, 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 †Tritheledontidae
                  • (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

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