Biology:Diapsid

Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeoscelidians, appeared about three hundred million years ago during the late Carboniferous period. All diapsids other than the most primitive ones in the clade Araeoscelidia are often placed into the clade Neodiapsida. The diapsids are extremely diverse, and include birds and all modern reptile groups, including turtles, which were historically thought to lie outside the group. All modern reptiles and birds are placed within the neodiapsid subclade Sauria. 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, and 7,925 snakes, lizards, tuatara, turtles, and crocodiles.

Characteristics

Diagram of the diapsid skull with temporal openings, unlike in anapsids

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).

Basal non-saurian neodiapsids were ancestrally lizard-like, but basal non-saurian neodiapsids include aquatic/amphibious taxa (Claudiosaurus and some tangasaurids) the gliding lizard-like Weigeltisauridae, as well as the Triassic chameleon-like drepanosaurs.

Classification

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.

The position of the highly derived Mesozoic marine reptile groups Thalattosauria, Ichthyosauromorpha and Sauropterygia within Neodiapsida is uncertain, and they may lie within Sauria.

In the 2022 and 2023 studies, Araeoscelidia was found to have no close relationship with Neodiapsida and was not even part of Sauropsida.

Relationships

Below are cladograms showing the relations of the major groups of diapsids.

Cladogram after Bickelmann et al., 2009 and Reisz et al., 2011:

Sauropsida †Parareptilia     †Millerettidae 50px           †Eunotosaurus     †Hallucicrania   †Lanthanosuchidae 50px   †Procolophonia   †Procolophonoidea 50px     †Pareiasauromorpha 60px           Eureptilia     †Captorhinidae 50px     Romeriida     †Paleothyris     Diapsida   †Araeoscelidia   Neodiapsida   †Orovenator       †Lanthanolania       †Tangasauridae       †Younginidae 80px         †Claudiosaurus             †Palaeagama     †Saurosternon             †Coelurosauravus           †Thalattosauria       †Hupehsuchia     †Ichthyopterygia       Sauria   Lepidosauromorpha     Archosauromorpha

The cladogram of Lee (2013) below used a combination of genetic (molecular) and fossil (morphological) data.

Diapsida

†Araeoscelidia 50px   Neodiapsida   †Claudiosaurus80px       †Younginiformes80px   Sauria Lepidosauromorpha   †Kuehneosauridae80px   Lepidosauria   Rhynchocephalia (tuatara and their extinct relatives)80px     Squamata (lizards and snakes)80px       Archosauromorpha     †Choristodera80px       †Prolacertiformes80px         †Rhynchosauria80px     †Trilophosaurus80px       Archosauriformes (crocodiles, birds, and their extinct relatives) 80px

This second cladogram is based on the 2017 study by Pritchard and Nesbitt.

Neodiapsida

†Orovenator80px       †Drepanosauromorpha80px       †Weigeltisauridae80px       †Claudiosaurus80px       †Tangasauridae80px (Hovasaurus, Thadeosaurus, Acerosodontosaurus)     †Younginidae (Youngina+unnamed species) 80px   Sauria   Lepidosauria80px   Archosauromorpha   †Protorosaurus80px       †Tanystropheidae80px       †Rhynchosauria80px       †Allokotosauria80px       †Prolacerta80px     Archosauriformes80px

The following cladogram was found by Simões et al. (2022):

Template:Vert-label

†Procolophonomorpha80px   Template:Vert-label   †Younginiformes80px       †Eunotosaurus       †Weigeltisauridae80px   Template:Vert-label Lepidosauromorpha   Rhynchocephalia (tuatara and their extinct relatives)80px     Squamata (lizards and snakes)80px     Archelosauria   Pantestudines (turtles and extinct relatives)50px   Template:Vert-label     †Ichthyosauromorpha80px       †Sauropterygia 80px     †Thalattosauria 80px         †Protorosauria   †Protorosauridae80px       †Tanystropheidae80px       †Drepanosauromorpha80px     †Kuehneosauridae80px               †Allokotosauria80px     †Rhynchosauria80px       Archosauriformes (crocodiles, birds, and their extinct relatives) 80px

The following cladogram was found by Jenkins et al. (2025). Traditional parareptiles are highlighted in orange:

Diapsida

†Araeoscelidia 80px       †Bolosauridae 80px   Neoreptilia †Ankyramorpha   †Acleistorhinidae 80px       †Pareiasauromorpha 80px     †Procolophonoidea 80px           †Mesosauridae 80px         †Cabarzia       †Ascendonanus     †Orovenator 80px       Parapleurota   †Millerettidae 80px   Neodiapsida   †Younginidae 80px       †Tangasauridae 80px       †Weigeltisauridae 80px       †Claudiosaurus 80px     Sauria 80px

See also

• Biology:Anapsid – Subclass of reptiles
• Biology:Euryapsida

External links

• Data related to Diapsid at Wikispecies
• Diapsida. Michel Laurin and Jacques A. Gauthier. Tree of Life Web Project. June 22, 2000.

Template:Eureptilia Template:Reptiles Wikidata ☰ Q134688 entry