The synapsids are a group of extinct vertebrate animals. The name is derived from the fact, that their skull only has one opening on each side behind the eyes. (cf. Diapsid). 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 period, but were decimated by the Permian-Triassic extinction event. 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 of them survived.

  1. The extremely mammal-like family, Tritylodontidae, were still squeaking around during the Lower Cretaceous.
  2. An extremely mammal-like family, Tritheledontidae, are unknown beyond the Lower Jurassic.
  3. The third group, Morganucodon and similar critters, were stem-mammals.