Synapsid

Very briefly, synapsids seem to have evolved from proto-'reptiles' during the Upper Carboniferous, (about 300 million years ago). They were the dominant terrestrial animals of the Permian, but were descimated by the extinction events of the Permian-Triassic extinction. More derived representatives belong within Therapsida.

Three orders of therapsids survived into the Triassic. Two of these, the dicynodonts and cynodonts, flourished but then pretty much died out during the Carnian, Upper Triassic.

Some exceptions were the yet still further derived eucynodonts. At least three groups survived. The extremely mammal-like family, Tritylodontidae[?], were still squeaking around during the Lower Cretaceous. Another extremely mammal-like family, Tritheledontidae[?], are unknown beyond the Lower Jurassic. The third group, Morganucodon and similar critters, were stem-mammals[?].