What is the X in X-mas?
Any of various reptiles having a skull with two pairs of temporal openings. Diapsids evolved in the Permian Period and grew longer and better-developed hindlimbs than forelimbs (unlike therapsids). Diapsids evolved into the archosaurs (including the dinosaurs and their descendants the birds) and all modern reptiles except turtles. Compare anapsid, synapsid, therapsid.