Sauropsida

Sauropsida ("lizard faces") is a group of amniotes 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.

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. 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. 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).

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.

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology ) 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. 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".

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. It is worth noting that Benton, like Watson, excludes birds from Class Sauropsida, and as such considers it paraphyletic.

Phylogeny
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.