Synapsids

Synapsids ('fused arch') are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name. Primitive synapsids are usually called pelycosaurs; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals" under cladistic terminology. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other major group being the sauropsids (reptiles and birds). They are distinguished from other amniotes by having a single opening (temporal fenestra) in their skull behind each eye, which developed in the ancestral synapsid about 324 million years ago (mya) during the late Carboniferous Period.

Synapsids were the dominant terrestrial animals in the middle to late Permian period. As with almost all groups then extant, their numbers and variety were severely reduced by the Permian extinction. Some species survived into the Triassic period, but archosaurs quickly became the dominant animals and few of the non-mammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit they included the mammal descendants, and in this sense synapsids are still very much a living group of vertebrates. In the form of mammals, Synapsids (most recently and notably humans) again became the dominant land animals after they outcompeted birds following the K-T extinction event.