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, Laurin defined Diapsida as a
clade, "the most recent common ancestor of araeoscelidians,
lepidosaurs, and
archosaurs, and all its descendants". The clade Neodiapsida was given a
phylogenetic definition by Laurin in 1991. He defined it as the branch-based
clade containing all animals more closely related to "
Younginiformes" (later, more specifically, emended to
Youngina capensis) than to
Petrolacosaurus (representing
Araeoscelidia). The earliest known neodiapsids like
Orovenator are known from the Early
Permian, around 290 million years ago. All
genetic studies have supported the hypothesis that turtles are cladistically diapsid reptiles despite being morphologically anapsid, most placing them as more closely related to living archosaurs (including crocodiles and birds) than to lepidosaurs (lizards, snakes, etc). Modern reptiles and birds are placed within the neodiapsid subclade
Sauria, 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). A cladistic analysis by Laurin and Piñeiro (2017) recovers
Parareptilia as part of Diapsida, with pareiasaurs, turtles, millerettids, and procolophonoids recovered as more derived than the basal diapsid
Younginia. 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. They united this relationship between Parareptilia and Neodiapsida in the new clade
Neoreptilia, defining it as the last common ancestor and all descendants of
Procolophon trigoniceps and
Youngina capensis. However, this excludes
mesosaurs, who were found to be basal among the sauropsids. Other recent studies have found the more traditional arrangement of parareptiles being outside of Diapsida. In the 2022 and 2023 studies, Araeoscelidia was found to have no close relationship with Neodiapsida and was not even part of Amniota in one study's topology. and Reisz et al., 2011: |label1=
Sauropsida}} The cladogram of Lee (2013) below used a combination of genetic (molecular) and fossil (morphological) data. This second cladogram is based on the 2017 study by Pritchard and Nesbitt. The following cladogram was found by Simões et al. (2022): Traditional
parareptiles are highlighted in orange: |3= }} }} }} }} }} }} }} }} }}|label1=
Sauropsida|sublabel1=(=
Diapsida)}} ==See also==