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Sauropsida

(Class)

Overview

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Sauropsida ("lizard faces") is a group of that includes (among other things) all existing reptiles, dinosaurs, and birds. The Sauropsida is distinguished from Theropsida ("beast faces"), more commonly called Synapsida, which includes mammals.

History of Classification

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Huxley and the Fossil Gaps

The term Sauropsida ("lizard faces") has a long history, and hails back to Thomas Henry Huxley, and his opinion that birds had risen from the dinosaurs. He based this chiefly on the fossils of Hesperornis and Archaeopteryx, that were starting to become known at the time.[1] In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrate classes informally into Mammals, Sauroids, and Ichthyoids (the latter containing the anamniotes), based on the gaps in physiological traits and lack of transitional fossils that seem to exist between the three groups. He subsequently proposed the names of Sauropsida and Ichthyopsida for the two latter.[2] It is worth noting that mammal-like reptiles were poorly known and understood at the time Huxley gave his systematic lecture, though the synapsid Dicynodon had been described as a reptile by Richard Owen as early as 1845.

Sauropsids Redefined

By the early 20th century, the fossils of Permian synapsids from South Africa had become well known, allowing palaeontologists to trace synapsid evolution in much greater detail. The term Sauropsida was taken up by E.S. Goodrich in 1916 much like Huxley's, to include lizards, birds and their relatives. He distinguish them from mammals and their extinct relatives, which he included in the sister group Theropsida (now usually replaced with the name Synapsida). Goodrich's classification thus differs somewhat from Huxley's, in which the synapsids (or at least the Pelycosaurs) would quite possibly have fallen under the sauropsids. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ("first lizards"), which included some Paleozoic amphibians as well as early reptiles predating the sauropsid/synapsid split (and thus not true sauropsids).[3]

Mammal-Like Reptiles and Other Reptiles

In 1956 D.M.S. Watson observed that the sauropsids and synapsids diverged from each other very early in their history, and so he divided Goodrich's Protosauria among the two groups. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively, making them paraphyletic, unlike Goodrich's definition. Thus his Sauropsida included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropyterygians.[4]

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology[5]) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Since the advent of phylogenetic nomenclature, the term Reptilia has fallen out of favor with many taxonomists, who have used Sauropsida in its place to include a monophyletic group containing the traditional reptiles and the birds.

Cladistics and the Sauropsida

The class Reptilia has been known to be an evolutionary grade rather than a clade for as long as evolution has been recognised. Reclassifying reptiles has been among the key aims of phylogenetic nomenclature.[6] The term Sauropsida is used to denote all species not on the synapsid side after the synapsid/sauropsid split, a branch-based clade. This group encompasses all now-living reptiles as well as birds, and as such is comparable to Goodrich's classification, the difference being that better resolution of the early amniote tree has split up most of the Goodrich's "Protosauria".[7]

Some taxonomists, such as Benton (2004), have co-opted the term to fit into traditional rank-based classifications, making Sauropsida and Synapsida class-level taxa to replace the traditional Class Reptilia.

Phylogeny

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The cladogram presented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by Laurin and Gauthier (1996), presented as part of the Tree of Life Web Project.[8]

Photos

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Taxonomy

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The Class Sauropsida is a member of the Series Amniota. Here is the complete "parentage" of Sauropsida:

The Class Sauropsida is further organized into finer groupings including:

Orders

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Araeoscelidia

[more]

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]

Captorhinida

[more]

Choristodera

[more]

Cotylosauria

[more]

Crocodylia

Crocodilia (or Crocodylia) is an order of large that appeared about 84 million years ago in the late Cretaceous Period (Campanian stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian stem group, the clade Crurotarsi, appeared about 220 million years ago in the Triassic Period and exhibited a wide diversity of forms during the Mesozoic Era. [more]

Eosuchia

[more]

Ichnites

[more]

Ichthyosauria

Mesosauria

[more]

Neornithischia

[more]

Nothosauroidea

[more]

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

[more]

Plesiosauria

[more]

Prolacertiformes

[more]

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

[more]

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]

Testudines

Turtles are of the order Testudines (the crown group of the superorder Chelonia), characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the discussion below. [more]

Thalattosauria

[more]

Thecodontia

[more]

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.

References

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  1. ^ Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures
  2. ^ Huxley, T.H. (1863): The Structure and Classification of the Mammalia. Hunterian lectures, presented in Medical Times and Gazette, 1863. original text
  3. ^ Goodrich, E.S. (1916). "On the classification of the Reptilia". Proceedings of the Royal Society of London 89B: 261–276. 
  4. ^ Watson, D.M.S. (1957). "On Millerosaurus and the early history of the sauropsid reptiles". Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences 240 (673): 325–400. 
  5. ^ Romer, A.S. (1933). Vertebrate Paleontology. University of Chicago Press. , 3rd ed., 1966.
  6. ^ Gauthier, .A., Kluge, A.G & Rowe, T. (1988). The early evolution of the Am niota. Pages 103–155 in Michael J. Benton (ed.): The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst. Ass. Spec. Vol. 35A. Clarendon Press, Oxford.
  7. ^ Laurin, M. & Gauthier, J.A. (1996). Amniota, Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives. Version 01 January 1996. The Tree of Life Web Project.
  8. ^ Laurin, M. and Gauthier, J.A. (1996). "Amniota. Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives." Version 01 January 1996. http://tolweb.org/Amniota/14990/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/

Sources

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Last Revised: September 22, 2009
2009/09/22 06:37:41