Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. 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 that are not therapsids, a monophyletic, more advanced, mammal-like group. The non-mammalian synapsids were described as mammal-like reptiles in classical systematics, but this misleading terminology is no longer in use as synapsids as a whole are no longer considered reptiles. They are now more correctly referred to as stem mammals or proto-mammals. Synapsids evolved from basal amniotes and are one of the two major groups of amniotes, the other being the sauropsids, the group that includes reptiles and birds. The distinctive temporal fenestra developed in the ancestral synapsid about 312 million years ago, during the Late Carboniferous period.
Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago, although some of the larger pareiasaurs at the end of the Permian matched them in size. Their numbers and variety were severely reduced by the Permian–Triassic extinction. By the time of the extinction at the end of the Permian, all the older forms of synapsids (known as pelycosaurs) were already gone, having been replaced by the more advanced therapsids. Although the dicynodonts and eutheriodonts, the latter consisting of the Eutherocephalia (Therocephalia) and Epicynodontia (Cynodontia), continued into the Triassic period as the only known surviving therapsids, archosaurs became the largest and most numerous land vertebrates in the course of this period. There were still some large forms: Lisowicia bojani, a discovery first reported in 2008, was the size of an elephant. The cynodont group Probainognathia, which includes Mammaliaformes, were the only synapsids to survive beyond the Triassic. After the Cretaceous–Paleogene extinction event, the mammalian synapsids diversified again to become the largest land and marine animals on Earth.