are an order
(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
Therapsids' temporal fenestrae
were larger than those of the pelycosaurs. 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
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 therapsids. 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
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 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 taxon.
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.
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.
- 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.