Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all crocodiles, lizards, snakes, tuatara, birds and non-avian dinosaurs. Although some diapsids have lost either one hole (lizards), or both holes (snakes), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. At least 7,925 species of diapsid reptiles exist in environments around the world today (nearly 18,000 when birds are included).