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Diapsids

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Diapsids

Diapsid reptiles
Temporal range: Late Carboniferous-Recent, 302–0Ma
Omeisaurus tianfuensis
Subgroups

Araeoscelidia
Neodiapsida

Diapsids ("two arches") are a group of tetrapods that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period.[1] Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, tuataras, and birds. Although some diapsids have lost either one hole (lizards), or both holes (snakes), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. There are at least 7,925 species of diapsid reptile existing in environments around the world today (nearly 18,000 when birds are included).

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).

Systematics

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 was found to be an unnatural assemblage of diapsids that had lost one of their skull openings. Some studies have suggested that this is the case in turtles as well, and that turtles are actually heavily modified diapsids, which would leave only some prehistoric forms in the Anapsida. In phylogenetic systems, birds (descendants of traditional diapsid reptiles) are also considered to be members of this group.

Well known extinct diapsid groups include the pterosaurs, plesiosaurs, and mosasaurs; there were many more obscure lineages. The classification of most of the early groups is fluid and subject to change.

Taxonomy

Diapsids of uncertain placement (incertae sedis)

Phylogeny

Below is a cladogram showing the relations of the major groups of diapsids.

Cladogram after Bickelmann et al., 2009[2] and Reisz et al., 2011:[3]

Diapsida

Araeoscelidia


Neodiapsida

Orovenator




Lanthanolania




Tangasauridae




Younginidae




Claudiosaurus





Palaeagama



Saurosternon





Coelurosauravus





Thalattosauria




Hupehsuchia



Ichthyopterygia





Sauria











See also

References

External links

  • Diapsida. Michel Laurin and Jacques A. Gauthier. Tree of Life Web Project. June 22, 2000.
  • Mikko's Phylogeny Archive

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