Diapsids are a group of amniote tetrapods that developed two holes in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all crocodiles, lizards, snakes, tuatara, turtles, and birds. Although some diapsids have lost either one hole, or both holes, or have a heavily restructured skull, they are still classified as diapsids based on their ancestry. At least 7,925 species of diapsid reptiles exist in environments around the world today.
Characteristics
The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings 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 compared to the upper arm bone.
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, Anapsida, and Euryapsida. 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 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". A cladistic analysis by Laurin and Piñeiro recovers Parareptilia as part of Diapsida, with pareiasaurs, turtles, millerettids, and procolophinoids recovered as more derived than the stem-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:
