Definition of synapsid
: any of a subclass (Synapsida) of extinct chiefly Permian and Triassic reptiles (such as the pelycosaurs) that resembled mammals, had a single pair of lateral temporal skull openings, and are usually held to be ancestral to mammals
Recent Examples of synapsid from the Web
Four-limbed vertebrates were becoming more diverse and among them were primitive amphibians, early reptiles and synapsids, the group that would one day include mammals.
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