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Diapsida

(Subclass)

Overview

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Diapsids ("two arches") are a group of that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara. Under modern classification systems, even birds are considered diapsids, since they evolved from diapsid ancestors and are nested within the diapsid clade. While some diapsids have lost either one hole (lizards), or both holes (snakes), or even 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 (over 14,600 when birds are included).

Characteristics

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diagram of the diapsid skull

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

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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 S ynapsida (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. The Synapsids today are often not considered true reptiles, while the 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 dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and many more obscure lineages. The classification of most of the early groups is fluid and subject to change.

Taxonomy

Phylogeny

Diapsida|--Araeoscelida|-?Sphodrosaurus|-?Palacrodon|-?Omphalosaurus`--+--Avicephala    `--Neodiapsida          |--Apsisaurus          `--Eosuchia                 |-?Younginiformes                 `--+-?Claudiosaurus                       |-?Ichthyopterygia                       `--Sauria                              |-?Thalattosauriformes                              |--Lepidosauromorpha                              `--Archosauromorpha

Photos

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Taxonomy

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The Subclass Diapsida is a member of the Class Sauropsida. Here is the complete "parentage" of Diapsida:

The Subclass Diapsida is further organized into finer groupings including:

Orders

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Araeoscelidia

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Avetheropoda

Tetanurae, or "stiff tails", is a that includes most theropod dinosaurs, as well as birds. Tetanurans (or tetanurines) first appear during the early or middle Jurassic Period. [more]

Choristodera

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Eosuchia

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Ichnites

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Ichthyosauria

Neornithischia

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Nothosauroidea

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Ornithischia

Ornithischia or Predentata is an extinct of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos (?????e???) meaning 'of a bird' and ischion (?s????) meaning 'hip joint'. They are known as the 'bird-hipped' dinosaurs because of their bird-like hip structure, even though birds actually descended from the 'lizard-hipped' dinosaurs (the saurischians). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. They were prey animals for the theropods and were smaller than the sauropods. [more]

Placodontia

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Plesiosauria

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Prolacertiformes

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Pterosauria

Pterosaurs " class="IPA">/?t?r?'dækt?l/) were flying of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period (220 to 65.5 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the legs to a dramatically lengthened fourth finger. Early species had long, fully-toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Many sported furry coats made up of hair-like filaments known as pycnofibres, which covered their bodies and parts of their wings. Pterosaurs spanned a wide range of adult sizes, from the very small Nemicolopterus to the largest known flying creatures of all time, including Quetzalcoatlus and Hatzegopteryx. [more]

Rhynchosauria

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Saurischia

Saurischia meaning 'lizard' and ischion (?s????) meaning 'hip joint') is one of the two , or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure. Saurischians ('lizard-hipped') are distinguished from the ornithischians ('bird-hipped') by retaining the ancestral configuration of bones in the hip. [more]

Sphenodontida

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Squamata

Squamata, or the scaled reptiles, is the largest recent of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. They are the most variably-sized order of reptiles, ranging from the 16 mm (0.63 in.) Jaragua Sphaero (Sphaerodactylus ariasae) to the 8 m (26 ft.) Green Anaconda (Eunectes murinus). [more]

Thalattosauria

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Thecodontia

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Trilophosauria

Trilophosaurs were lizard-like diapsid reptiles related to the archosaurs. The best known genus is Trilophosaurus, a herbivore up to 2.5 meters long. It had a short, unusually heavily-built skull, equipped with massive, broad flattened cheek teeth with sharp shearing surfaces for cutting up tough plant material. Teeth are absent from the premaxilla and front of the lower jaw, which in life were probably equipped with a horny beak. [more]

Younginiformes

[more]

More info about the Order Younginiformes may be found here.

Sources

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Last Revised: September 22, 2009
2009/09/22 06:48:58