synapsid

Etymology

From translingual Synapsida, from syn- + Ancient Greek ἁψίς (hapsís, “arch”).

adj

1. 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, Matt Ridley, Genome, Harper Perennial, published 2000, page 26

noun

1. Any animal (including all mammals) of the class Synapsida.