n. A vertebrate distinguished by a skull with one pair of openings in the sidebehind the eyes, e.g., mammals and their close relatives.

(pronounced sin-AP-sid) Synapsids are a group of animals distinguished by having a skull with an extra low opening behind the eyes; this opening gave these animals stronger jaw muscles and jaws (the jaw muscles were anchored to the skull opening). Synapsids include the mammals, and extinct animals such as Dimetrodon. The pelycosaurs were early synapsids; later synapsids were the therapsids, cynodonts and dicynodonts (from the late Permian period), leading to the mammals. With time, the synapsid gait became more upright and tail length decreased. The oldest-known synapsid is Archarothyris, a pelycosaur 300 million years old.

Synapsids ('fused arch') also known as Theropsids ('beast face'), and traditionally described as 'mammal-like reptiles' , are a group of amniotes (the other being the sauropsids) that developed one opening in their skull (temporal fenestra) behind each eye, about 320 million years ago (mya) during the late Carboniferous Period.