A subclass of extinct reptiles, in general characterized by a temporal fenestra that lies below the junction of the postorbital and squamosal bones, a so-called lower temporal opening. In advanced forms, however, the postorbital-squamosal bridge is absent. Mammals arose from this group of reptiles during the Triassic Period. See also Reptilia.
Synapsids first appear in the geological record in the Upper Carboniferous. They flourished during the Late Paleozoic and Early Mesozoic but became extinct at about the end of the Triassic. During this span of time they underwent a broad adaptive radiation on land and made minor invasions of aquatic habitats. There were two major phases of this radiation: one early, by pelycosaurs, and the other later (Late Permian and Triassic), by therapsids. See also Pelycosauria; Reptilia.
