Diapsids ("two arches") are a clade ofsauropsids, distinguished from more primitiveeureptiles by the presence of two holes, known astemporal fenestrae, in each side of theirskulls. The earliest traditionally identified diapsids, thearaeoscelidians, appeared about three hundredmillion years ago during the lateCarboniferous period.[1] All diapsids other than the most primitive ones in the clade Araeoscelidia are often placed into the cladeNeodiapsida. The diapsids are extremely diverse, and include birds and all modern reptile groups, includingturtles, which were historically thought to lie outside the group.[2] All modern reptiles and birds are placed within the neodiapsid subcladeSauria. 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 stillclassified as diapsids based on their ancestry. At least 17,084species of diapsid animals are extant: 9,159 birds,[3] and 7,925 snakes, lizards,tuatara, turtles, and crocodiles.[4]
Diagram of the diapsid skull with temporal openings, unlike inanapsids
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, strongerjaw muscles, and enables the jaw to open more widely. A more obscure ancestral characteristic is a relatively long lower arm bone (theradius) compared to the upper arm bone (humerus).
Diapsids were originally classified as one of four subclasses of the classReptilia, all of which were based on the number and arrangement of openings in the skull. The other three subclasses wereSynapsida (one opening low on the skull, for the "mammal-like reptiles"),Anapsida (no skull opening, including turtles and their relatives), andEuryapsida (one opening high on the skull, including many prehistoric marine reptiles). With the advent ofphylogenetic 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 TriassicPappochelys 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 prehistoricaraeoscelidians. In 1991, Laurin defined Diapsida as aclade, "the most recent common ancestor of araeoscelidians,lepidosaurs, andarchosaurs, and all its descendants".[8]
The clade Neodiapsida was given aphylogenetic definition by Laurin in 1991. He defined it as the branch-basedclade containing all animals more closely related to "Younginiformes" (later, more specifically, emended toYoungina capensis) than toPetrolacosaurus (representingAraeoscelidia).[9] The earliest known neodiapsids likeOrovenator are known from the Early Permian, around 290 million years ago.[10]
Allgenetic studies have supported the hypothesis that turtles are diapsid reptiles; some have placed turtles within archosauromorpha,[11][12] or, more commonly, as a sister group to extant archosaurs.[13][14][15][16]
Modern reptiles and birds are placed within the neodiapsid subcladeSauria, defined as the last common ancestor of Lepidosauria (which includes lizards, snakes and the tuatara), and Archosauria (which includes crocodilians and dinosaurs, including birds, among others).[17]
A cladistic analysis by Laurin and Piñeiro (2017) recoversParareptilia as part of Diapsida, with pareiasaurs, turtles, millerettids, and procolophonoids recovered as more derived than the basal diapsidYounginia.[18] A 2020 study by David P. Ford and Roger B. J. Benson also recovered Parareptilia as deeply nested within Diapsida as the sister group toNeodiapsida. They united this relationship between Parareptilia and Neodiapsida in the new cladeNeoreptilia, defining it as the last common ancestor and all descendants ofProcolophon trigoniceps andYoungina capensis.[19] However, this excludesmesosaurs, who were found to be basal among the sauropsids.[19] Other recent studies have found the more traditional arrangement of parareptiles being outside of Diapsida.[17]
^Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). "Constraints on the timescale of animal evolutionary history."Palaeontologia Electronica, 18.1.1FC; 1-106; palaeo-electronica.org/content/fc-1
^Katsu, Y.; Braun, E. L.; Guillette, L. J. Jr.; Iguchi, T. (2010-03-17). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes".Cytogenetic and Genome Research.127 (2–4):79–93.doi:10.1159/000297715.PMID20234127.S2CID12116018.
^Bickelmann, Constanze; Müller, Johannes; and Reisz, Robert R. (2009). "The enigmatic diapsidAcerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly ofyounginiform reptiles".Canadian Journal of Earth Sciences.49 (9):651–661.Bibcode:2009CaJES..46..651S.doi:10.1139/E09-038.
Data related toDiapsid at Wikispecies
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