Therapsids, also known as therapsida, are a group of synapsids that includes mammals' early evolutionary ancestors, hence their nickname of 'mammal-like reptiles'. Despite this nickname, therapsids were neither mammals nor reptiles, but an intermediate stage between the two classes. Therapsids evolved from a permian pelycosaurs. Their history ranges from the early Permian to the early Cretaceous.
|Wikipedia has a more detailed and comprehensive article on Therapsid!|