Therapsids

Evolutionary history

Therapsids are members of the clade Sphenacodontia, with their closest relatives within the primitive "pelycosaur" synapsids being Sphenacodontidae, including the famous Dimetrodon. When they split off from Sphenacodontidae has been debated by experts. A number of studies have suggested that therapsids split off from sphenacontids during the Gzhelian stage of Late Carboniferous, more recent suggested a later split between the Asselian and Sakmarian stage of the Early Permian, though both of these analyses ignore Cryptovenator, generally accepted as a sphenacodontid, which dates to the Carboniferous-Permian boundary c. 300 Ma. The early evolutionary history of the group is obscure, with no confirmed records from the Early Permian (though Tetraceratops has been controversially suggested by some authors to be more closely related to therapsids than to sphenacodontids) leaving a substantial ghost lineage. Therapsids became the dominant land animals in the Middle Permian, displacing the pelycosaurs, though the nature of this transition is obscure due to a poor fossil record at the Early-Middle Permian transition ("Olson's Gap"). Therapsida consists of four major clades: the dinocephalians, the herbivorous anomodonts, the carnivorous biarmosuchians, and the mostly carnivorous theriodonts. After a brief burst of evolutionary diversity, the dinocephalians, while dominant during the Middle Permian, died out at the end of the Middle Permian as part of the end-Captianian mass extinction event, but the anomodont dicynodonts as well as the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first of the cynodonts.