Therocephalia - Classification

Classification

The therocephalians evolved from an early line of pre-mammalian eutheriodont therapsids, and are the sister group to the cynodonts which include mammals and their ancestors. Therocephalians are at least as ancient as a third large branch of therapsids, the gorgonopsids, which they resemble in many primitive features. For example, many early therocephalians possess long canine teeth similar to those of gorgonopsids. The therocephalians, however, outlasted the gorgonopsids, persisting into the early-Middle Triassic period.

While common ancestry with cynodonts (and, thus, mammals) accounts for many similarities among these groups, some scientists believe that other similarities may be better attributed to convergent evolution, such as the loss of the postorbital bar in some forms, a mammalian phalangeal formula, and some form of a secondary palate in most taxa (see below). Therocephalians and cynodonts both survived the Permian-Triassic mass extinction, but while therocephalians soon went extinct, cynodonts underwent rapid diversification. Therocephalians experienced a decreased rate of cladogenesis, meaning that few new groups appeared after the extinction. Most Triassic therocephalian lineages originated in the Late Permian, and lasted for only a short period of time in the Triassic.