Therocephalia

Therocephalia is an extinct group of therapsids (mammals and their close extinct relatives) from the Permian and Triassic periods. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were carnivores. Like other non-mammalian synapsids, therocephalians were once described as "mammal-like reptiles". Therocephalia is the group most closely related to the cynodonts, which gave rise to the mammals. Indeed, it had been proposed that therocephalians themselves may have given rise to the cynodonts, and therefore that therocephalians as recognised are paraphyletic in relation to cynodonts and so not a clade. Conventionally, however, Therocephalia is regarded as the sister clade of Cynodontia, together forming the clade Eutheriodontia.

The close relationship of Therocephalia to Cynodontia takes evidence in a variety of skeletal features. Most notable is that the skull roof is narrowed between two enlarged temporal fenestra, allowing for expansive jaw musculature. At the same time, derived therocephalians also share a number of mammalian traits with cynodonts that evolved convergently, including a secondary palate, loss of the postorbital bar behind the eye and developing multi-cusped cheek teeth for herbivory. Other therocephalians retained simpler teeth for a carnivorous diet, often with large canines and sometimes a reduction or even total loss of the postcanine teeth. Such forms include genera that have even suggested to have possessed a venomous bite (namely Euchambersia), which would make therocephalians the oldest tetrapods known to have evolved this characteristic.