Any of a group of synapsid animals that includes the immediate ancestors of modern mammals. The therapsids flourished during the Permian period, first appearing in the fossil record around 280 million years ago (mya). Although they still resembled reptiles in some respects, mammal-like features were increasingly apparent, such as teeth specialized for different functions. They diversified in form, with both carnivores and herbivores occupying a wide range of habitats. For example, the plant-eating dicynodonts had horny beaklike jaws with teeth much reduced except for a pair of prominent tusklike upper canines. However, many therapsids disappeared during the end-Permian mass extinction, and it was their successors, the cynodonts, that dominated the Triassic and from which the earliest true mammals evolved about 200 mya.