Synapsid

Synapsids Greek: 'fused arch'; synonymous with theropsids –not to be confused with therapsids, which are a subordinate group to synapsids– are a group of animals that includes mammals and every animal more closely related to mammals than to the other members, reptiles and birds, included in the amniotes clade. They are easily separated from other amniotes by having a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs or pelycosaur-grade synapsids. This informal term consists of all synapsids which are not therapsids, a monophyletic more advanced mammal-like group. The non-mammalian synapsids are described as mammal-like reptiles in classical systematics; they can also be called stem mammals or proto-mammals. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other being the sauropsids, a group that includes modern reptiles and birds. The distinctive temporal fenestra developed in the ancestral synapsid about 312 million years ago, during the Late Carboniferous period.