Mammal-like Reptiles Or Synapsids also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. 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 life forms 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, they included the prehistoric ancestors of mammals and in this sense, synapsids are still very much a living class of vertebrates.