Sauropsida

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Sauropsida

Sauropsida ("lizard-face") is a group of amniotes that includes reptiles, dinosaurs, and bird

Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
s. Among amniotes, sauropsida is distinguished from theropsida ("beast-face"), also called synapsids.

term "Sauropsida" ("lizard faces") was first coined in 1916 by E.S. Goodrich

Edwin Stephen Goodrich

Edwin Stephen Goodrich , was an English zoologist, specialising in comparative anatomy, embryology, paleontology, and evolution. He held the Linacre Chair of Zoology in the University of Oxford from 1921 to 1946....
to include lizards, birds, and their relatives and distinguish them from mammal

Mammal

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Sauropsida ("lizard-face") is a group of amniotes that includes reptiles, dinosaurs, and bird

Bird

History of classification

The term "Sauropsida" ("lizard faces") was first coined in 1916 by E.S. Goodrich

Edwin Stephen Goodrich

Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s and their extinct relatives, which he included in the sister group Theropsida (now usually replaced with the name Synapsida). 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

Paleozoic

The Paleozoic or Palaeozoic Era is the earliest of three geology Era of the Phanerozoic Eon . The Paleozoic spanned from roughly , and is subdivided into six period ; from oldest to youngest they are: the Cambrian, Ordovician, Silurian, Devonian period, Carboniferous, and Permian...
amphibian

Amphibian

Amphibians , such as frogs, toads, salamanders, newts and caecilians, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form....
s as well as early "reptile

Reptile

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scale as opposed to hair or feathers....
s" not recognized as true sauropsids.

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. Thus his Sauropsida included Procolophonia

Procolophonia

The Procolophonia are an Order of herbivore reptiles that lived from the Middle Permian till the end of the Triassic period. They were originally included as a suborder of the Cotylosauria but are now considered a clade of Parareptilia....
, Eosuchia

Eosuchia

Eosuchians are an extinct Order of diapsid reptiles. Depending on which taxa are included the order may have ranged from the late Carboniferous to the Eocene but the consensus is that eosuchians are confined to the Permian and Triassic....
, Millerosauria

Millerettid

The milleretids is an extinct group of anapsids that lived in South Africa during the Upper Permian. They were small insectivores and probably resembled modern lizards in appearance and lifestyle....
, Chelonia

Turtle

Turtles are reptiles of the Order Testudines , most of whose body is shielded by a special bone or cartilage animal shell developed from their ribs....
(turtles), Squamata

Squamata

Sphenodontia

Sphenodontia is an order of Lepidosauria reptiles that includes only one living genus, the tuatara . Despite its current lack of diversity, the Sphenodontia at one time included a wide array of genera in several families, and represents a lineage stretching back to the Mesozoic Era....
, Crocodilia

Crocodilia

Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria....
, "thecodont

Thecodont

Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period....
s" (paraphyletic

Paraphyly

In phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor Common descent but does not contain all the descendants of that ancestor....
basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group form an outgroup to the rest of the clade, such as in the following example:...
Archosaur

Archosaur

Archosaurs are a group of diapsid reptiles represented by modern birds and crocodilians. This group also includes extinct non-avian dinosaurs, pterosaurs and relatives of crocodiles....
ia), non-avian dinosaur

Dinosaur

Dinosaurs were the dominant vertebrate animals of Landform ecosystems for over 160 million years, from the late Triassic Period until the end of the Cretaceous Period , when most of them became extinct in the Cretaceous?Tertiary extinction event....
s, pterosaur

Pterosaur

Pterosaurs were flying reptiles of the clade or Order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight....
s, ichthyosaur

Ichthyosaur

Ichthyosaurs were giant marine reptiles that resembled fish and dolphins. Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared approximately 245 million years ago and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct....
s, and sauropyterygians

Sauropterygia

Sauropterygia is a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct. They are united by a radical adaptation of their shoulder, designed to support powerful flipper strokes....
.

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer

Alfred Romer

Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution....
's classic Vertebrate Paleontology
Vertebrate Paleontology (Romer)

Vertebrate Paleontology is an advanced textbook on vertebrate paleontology by Alfred Sherwood Romer, published by the University of Chicago Press....
) into four subclasses according to the positioning of temporal fenestrae
Skull

The skull is a bone structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
, openings in the sides of the skull behind the eyes. Those divisions were:

Anapsid
Anapsid

An anapsid is an amniote whose skull does not have temporal fenestra near the Temple s.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a coherent group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related: scientists still debate the exact relationshi...
a – no fenestrae
Synapsid
Synapsid

Synapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes....
a – one low fenestra (no longer considered true reptiles)
Euryapsida
Euryapsida

Euryapsida is a polyphyletic group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit , under which the post-orbital and squamosal bones articulate....
– one high fenestra (now included within Diapsida)
Diapsid
Diapsid

Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period....
a – two fenestrae

Since the advent of phylogenetic nomenclature

Phylogenetic nomenclature

Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to Biological classification, applying definitions from cladistics ....
, 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 birds. 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.

Rarareptilia

A twist on this scheme is the position of the turtle

Turtle

Turtles are reptiles of the Order Testudines , most of whose body is shielded by a special bone or cartilage animal shell developed from their ribs....
s. Depending on author, turtles or synapsid

Synapsid

Synapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes....
s may be the sister group of the rest of the amniots. In this scheme, the content of the group Sauropsida is somewhat different. The Turtles are classed as Parareptilia

Parareptilia

Parareptilia is a subclass or clade of reptiles which are variously defined as an extinct group of primitive anapsids, or a more cladistically correct alternative to Anapsida....
(usually given the rank of class

Class (biology)

A class is the taxonomic rank in the biological classification of organisms in biology below phylum and above Order .The orders of taxonomy are life, Domain , kingdom , phylum, class , order , family , genus, and species....
), the mammals and the mammal-like reptiles are classed as Synapsida and the Sauropsida is left with the tuatara

Tuatara

The tuatara is a reptile endemism to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia....
, squamata

Squamata

Squamata, or the scaled reptiles, is the largest recent order of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scale or shields....
, crocodiles and birds.

This system leaves the basic question unanswered: In what group do the earliest reptiles go? One solution would be to include them in Sauropsida, which would then leave Sauropsida paraphyletic, just like Reptilia. Another solution would be to leave them out of Sauropsida as "basal amniotes". These early amniots undoubly falling under the term "reptile" as in the old physiological definitions, Sauropsida would then end up not containing all non-synapsid (and possibly non-parareptlian) reptiles, but rather a subset, leaving the old anapsids undefined.

Phylogeny

The cladogram

Cladistics

Cladistics is the hierarchical classification of species based on evolutionary ancestry. Cladistics is distinguished from other taxonomic systems because it focuses on evolution rather than similarities between species, and because it places heavy emphasis on objective, quantitative analysis....
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

Tree of Life Web Project

The Tree of Life Web Project is an ongoing Internet project providing information about the biodiversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world....
.