Definition of synapsid
: any of a subclass (Synapsida) of extinct chiefly Permian and Triassic reptiles (as the pelycosaurs) that resembled mammals, had a single pair of lateral temporal skull openings, and are usually held to be ancestral to mammals
Origin and Etymology of synapsid
New Latin Synapsida, from Greek syn- + apsid-, apsis arch, vault — more at apsis
First Known Use: 1956
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