Diapsids include all the reptiles (except turtles) and birds. They are distinguished by having two holes in the rear upper part of their skulls and two holes behind the eyes.

Diapsids ("two arches") are a group of tetrapod animals that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all birds, crocodiles, lizards, snakes, tuataras, and possibly even turtles. While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their ancestry.