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Diapsid reptiles Temporal range: Cisuralian–Present, 290–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Possible Pennsylvanian records
Skull of Orovenator, one of the earliest known diapsids
Nile crocodile (Crocodylus niloticus)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Romeriida
Clade:	Diapsida Osborn, 1903
Subgroups
†Araeoscelidia? †Parareptilia? †Varanopidae? †Acallosuchus †Claudiosaurus †Cryptovaranoides †Dolerosaurus †Elachistosuchus †Kenyasaurus †Kudnu †Lanthanolania †Maiothisavros †Orovenator †Palacrodon †Palaeagama †Protoavis (dubious) †Saurosternon †Younginiformes (possibly paraphyletic) †Tangasauridae †Younginidae †Avicephala (possibly non-monophyletic) †Drepanosauromorpha †Weigeltisauridae †Longisquama? †Ichthyosauromorpha †Sauropterygia †Thalattosauria †Choristodera Sauria

Diapsida ("two arches") is a clade of sauropsids (reptiles in the wide sense), distinguished by the presence of two holes, known as temporal fenestrae, in each side of their skulls. Modern reptiles and birds belong to the diapsid subclade Sauria.

The earliest tetrapods with two pairs of temporal fenestrae were the araeoscelidians, which first appeared about three hundred million years ago during the Late Carboniferous.^[1] Araeoscelidians are often regarded as the most basal (early-diverging) diapsid reptiles, but have been found by some phylogenetic analyses to instead be stem-amniotes and therefore not closely related to other diapsids.^[2] Non-araeoscelidian diapsids are often placed in the subclade Neodiapsida; the earliest known members of this group are Orovenator and Maiothisavros from the Early Permian, around 290 million years ago.^[3][4] Two other groups, Parareptilia and Varanopidae, have sometimes been recovered as basal diapsids, but are usually considered to be non-diapsid reptiles and synapsids (stem-mammals) respectively.^[5]

Early diapsids were ancestrally lizard-like, but non-saurian diapsids include semiaquatic taxa (like Claudiosaurus and some tangasaurids),^[6] the gliding lizard-like Weigeltisauridae,^[7] and possibly the Triassic chameleon-like drepanosaurs.^[8] The position of the highly derived Mesozoic marine reptile groups Thalattosauria, Ichthyosauromorpha and Sauropterygia within Diapsida is uncertain, and they may lie within Sauria.^[9]

Modern diapsids are extremely diverse, and include birds and all modern reptile groups, including turtles, which were historically thought to lie outside the group.^[10] Although some diapsids have lost either one hole (lizards), or both holes (snakes and turtles), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. At least 17,084 species of diapsid animals are extant: 9,159 birds,^[11] and 7,925 snakes, lizards, tuatara, turtles, and crocodiles.^[12]