synapsid /sɪˈnapsɪd/adjective 1noun 1forms 1 adjective1 •Pertaining to the class Synapsida of animals which have an opening low in the skull roof behind each eye, leaving a bony arch beneath each.We are synapsid tetrapods, a group of reptiles that almost went extinct 200 million years ago in competition with the better-designed dinosaurs. — 1999 noun1 •Any animal (all mammals) of the class Synapsida. formssynapsids
