Phthinosuchus, an early therapsid
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.
Therapsids' evolutionary track began in the Early Permian, when a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to Dimetrodon and its family, gave rise to therapsids. The evidence was their anatomical features such as the skull, and the vertebrae. Therapsids 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. 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.
- 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 stem-mammals.
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 taxon.
Taxonomy and Phylogeny
- Class Synapsida
- ORDER THERAPSIDA *
- Suborder Biarmosuchia *
- Suborder Dinocephalia
- "Therapsida: Mammals and extinct relatives". Tree of Life.
- "Therapsida: overview". Palaeos.
- "Therapsids in Detail". Stephen Priestley - Illustrator
- 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.