Diapsid

Diapsids ("two arches") are a group of amniote tetrapods that developed two holes ( temporal fenestra) in each side of their

skull

s about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all

crocodilia

ns,

lizard

s,

snake

s,

tuatara

,

turtles

, and

birds

. 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

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

).

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".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

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''.

Relationships

Below is a

cladogram

showing the relations of the major groups of diapsids.

Cladogram

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

See also

* Vertebrate paleontology
*

Synapsid

a
*

Anapsid

a
* Euryapsida

References

External links

*

Diapsida
Michel Laurin and Jacques A. Gauthier. ''Tree of Life Web Project''. June 22, 2000.

{{Taxonbar, from=Q134688

01
Reptile taxonomy

Reptiles of the United States
Extant Pennsylvanian first appearances
Taxa named by Henry Fairfield Osborn