Any of a clade of tetrapod animals that contains the modern mammals, their ancestors, and extinct relatives. Synapsids are characterized by an opening, the temporal fenestra, on each side of the skull behind the eye socket, thought originally to provide a passage for cheek muscles to operate closure of the jaw. Synapsids first appear in the fossil record during the late Carboniferous period, over 300 mya, and had hairless bodies with a reptilian appearance, such as Dimetrodon, which had a prominent spine sail along its back. The Permian saw the emergence of larger herbivorous and carnivorous forms (see therapsid), but diversity was much reduced in the mass extinction of the late Permian. During the Triassic, synapsids became more mammal-like, with hairy bodies and a more upright gait, though they still laid eggs. These gave rise to the first true mammals during the Jurassic period.