From Academic Kids
Diapsids ("two arches") are a group of tetrapod animals that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all birds, crocodiles, lizards, snakes, and the tuatara. While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their ancestry. There are 14,600 species of diapsid existing in environments around the world today, including most flying and poisonous vertebrates.
The ancestral openings are above and below the eye, and allow the jaw to open wider, and the attachment of larger, stronger jaw muscles. A more obscure ancestral characteristic is a relatively long lower arm bone (radius), compared to the upper arm bone (humerus). Extinct groups include the dinosaurs (except for birds), pterosaurs, plesiosaurs, mosasaurs, ichthyosaurs, and many more obscure lineages. The classification of most of the early groups is fluid and subject to change. A minority classify turtles as diapsids as well.
- Diapsida (http://tolweb.org/tree?group=Diapsida&contgroup=Amniota). Michel Lauren and Jacques A. Gauthier. Tree of Life Web Project. June 22, 2000.