A diapsidian is an animal with two openings in its skull. This is a technical description of the evolutionary modification in reptiles that occurred about 300m years ago splitting them from more primitive mammals. Modern diapsidians include snakes, lizards and crocodiles. Turtles are an example of reptiles that are not diapsidians. Some say birds should also be included a Diapsidians because they evolved from reptiles that shared characteristics with lizards.