Therapsids, previously known as the "mammal-like reptiles", are an order of synapsids. Traditionally, synapsids were referred to as reptiles. However, when the term is used cladistically, the taxon also includes the mammals, which are descended from the cynodont therapsids.

A group of pelycosaurs, the Sphenacodontia - a group that included the famous Dimetrodon, were ancestral to therapsids, according to their anatomical features such as the skull, and the vertebrae. Therapsids began to appear in the late Early Permian Period and became the dominant land animals by the Middle Permian, replacing the pelycosaurs. Therapsid temporal fenestrae were larger than those of the pelycosaurs. Endothermy (warm-bloodedness) in therapsids probably evolved by the Middle or Late Permian (Dinocephalians and anomodonts were probably warm-blooded). Therapsids probably had naked skin, like that of mammals, rather than scales as in reptiles and pelycosaurs. Early therapsids did not have fur, which developed in the Middle or Late Permian, in the theriodonts. 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 end Permian extinction event, with the very successful gorgonopsians dying out altogether and the remaining groups being represented by only one or two families of a few species, each surviving into the Triassic. Of these, 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.

1. The extremely mammal-like family, Tritylodontidae, survived into the Early Cretaceous.
2. An extremely mammal-like family, Tritheledontidae, are unknown later than the Early Jurassic.
3. The third group, Morganucodon and similar animals, were stem-mammals.

Taxonomy and Phylogeny

• 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
            • '''Superorder Mammaliformes
              • Class Mammalia

See also

• Mammal-like reptile

External links

• "Therapsida: Mammals and extinct relatives". Tree of Life.
• "Therapsida: overview". Palaeos.

References

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

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