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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
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 from other tetrapods 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 potential diapsids, the araeoscelidians, first appeared about three hundred million years ago during the Late Carboniferous,^[1] but these may actually be stem-amniotes or otherwise have acquired their diapsid skulls convergently from true diapsids. Non-araeoscelidian diapsids are often united into the clade Neodiapsida; the earliest known members of this group are Orovenator and Maiothisavros from the Early Permian, around 290 million years ago.^[2][3] Early diapsids were ancestrally lizard-like, but non-saurian diapsids include semiaquatic taxa (like Claudiosaurus and some tangasaurids)^[4] the gliding lizard-like Weigeltisauridae,^[5] and possibly the Triassic chameleon-like drepanosaurs.^[6] The position of the highly derived Mesozoic marine reptile groups Thalattosauria, Ichthyosauromorpha and Sauropterygia within Neodiapsida is uncertain, and they may lie within Sauria.^[7]

Modern diapsids are extremely diverse, and include birds and all modern reptile groups, including turtles, which were historically thought to lie outside the group.^[8] 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,^[9] and 7,925 snakes, lizards, tuatara, turtles, and crocodiles.^[10]

The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings (temporal fenestrae) posteriorly above and below the eye. This arrangement allows for the attachment of larger, stronger jaw muscles, and enables the jaw to open more widely. A more obscure ancestral characteristic is a relatively long lower arm bone (the radius) compared to the upper arm bone (humerus).

Diapsids were originally classified as one of four subclasses of the class Reptilia, all of which were based on the number and arrangement of openings in the skull. The other three subclasses were Synapsida (one opening low on the skull, for the "mammal-like reptiles"), Anapsida (no skull opening, including turtles and their relatives), and Euryapsida (one opening high on the skull, including many prehistoric marine reptiles). With the advent of phylogenetic nomenclature, this system of classification was heavily modified. Today, the synapsids are often not considered true reptiles, while Euryapsida were found to be an unnatural assemblage of diapsids that had lost one of their skull openings. Genetic studies and the discovery of the Triassic Pappochelys have shown that this is also the case in turtles, which are actually heavily modified diapsids. In phylogenetic systems, birds (descendants of traditional diapsid reptiles) are also considered to be members of this group.

Some modern studies of reptile relationships have preferred to use the name "diapsid" to refer to the crown group of all modern diapsid reptiles but not their extinct relatives. However, many researchers have also favored a more traditional definition that includes the prehistoric araeoscelidians. In 1991, Michel Laurin gave phylogenetic definitions to Diapsida and Neodiapsida. Diapsida was defined as "the most recent common ancestor of araeoscelidians, lepidosaurs, and archosaurs, and all its descendants".^[11] Neodiapsida was defined as the branch-based clade containing all animals more closely related to "Younginiformes" (later, more specifically, emended to Youngina capensis) than to Petrolacosaurus.^[12]

A cladistic analysis by Laurin and Piñeiro (2017) recovers Parareptilia as part of Diapsida, with pareiasaurs, turtles, millerettids, and procolophinoids recovered as more derived than the basal diapsid Younginia. However, this excludes mesosaurs, who were found to be basal among the sauropsids.^[13] A 2020 study by David P. Ford and Roger B. J. Benson also recovered Parareptilia as deeply nested within Diapsida as the sister group to Neodiapsida.^[14]

Cladogram after Bickelmann et al., 2009^[15] and Reisz et al., 2011:^[16]

The cladogram below is based on the 2017 study by Pritchard and Nesbitt.^[17]

The following cladogram was found by Simões et al. (2022): ^[18]

• Vertebrate paleontology
• Synapsida
• Anapsida
• Euryapsida

Wikimedia Commons has media related to Diapsida.

• Data related to Trilletrollet/sandbox at Wikispecies

Category:Reptile taxonomy Category:Cisuralian first appearances Category:Extant Permian first appearances Category:Taxa named by Henry Fairfield Osborn