The modern diapsid reptiles (from left to right): Gharial, Alligator, Crocodile, Tuatara, Constrictor Snakes, Venomous Snakes, Lizards, and the Birds.

Diapsids ("two arches") are a subclass of reptiles that developed two holes (temporal fenestra) in each side of their skulls, approximately 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, worm lizards, tuatara and birds. Under modern classification systems, even birds are considered diapsids, since they evolved from diapsid ancestors and are nested within the diapsid clade. While some diapsids have lost either one hole (lizards), or both holes (snakes), or even have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. There are at least 7,925 species of diapsid reptile existing in environments around the world today (over 14,600 when birds are included).

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