Reptilia

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scales as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and amniotes, whose embryos are surrounded by an amniotic membrane. Modern reptiles inhabit every continent with the exception of Antarctica.

The majority of reptile species are oviparous (egg-laying) although certain species of squamates are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (offspring born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals with some providing initial care for their hatchlings.

History of classification
From the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term. However, in recent years, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile.

The terms "Sauropsida" ("Lizard Faces") and "Theropsida" ("Beast Faces") were coined in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. 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.

In 1956 D.M.S. Watson observed that the first two groups diverged very early in reptilian history, and so he divided Goodrich's Protosauria among them. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively. 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. Those divisions were:
 * Anapsida – no fenestrae
 * Synapsida – one low fenestra (no longer considered true reptiles)
 * Euryapsida – one high fenestra (now included within Diapsida)
 * Diapsida – two fenestrae

All of the above but Synapsida fall under Sauropsida.

Taxonomy
Classification to order level, after Benton, 2004.
 * Series Amniota
 * Class Synapsida
 * Order Pelycosauria*
 * Order Therapsida
 * Class Mammalia
 * Class Sauropsida
 * Subclass Anapsida
 * Order Testudines (turtles)
 * Subclass Diapsida
 * Order Araeoscelidia
 * Order Younginiformes
 * Infraclass Ichthyosauria
 * Infraclass Lepidosauromorpha
 * Superorder Sauropterygia
 * Order Placodontia
 * Order Nothosauroidea
 * Order Plesiosauria
 * Superorder Lepidosauria
 * Order Sphenodontia (tuatara)
 * Order Squamata (lizards & snakes)
 * Infraclass Archosauromorpha
 * Order Prolacertiformes
 * Division Archosauria
 * Subdivision Crurotarsi
 * Superorder Crocodylomorpha
 * Order Crocodilia
 * Order Phytosauria
 * Order Rauisuchia
 * Order Rynchosauria
 * Subdivision Avemetatarsalia
 * Infradivision Ornithodira
 * Order Pterosauria
 * Superorder Dinosauria
 * Order Saurischia
 * Class Aves
 * Order Ornithischia

Evolution
Hylonomus is the oldest-known reptile, and was about 8 to 12 inches (20 to 30 cm) long. Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes. Petrolacosaurus and Mesosaurus are other examples. The earliest reptiles were found in the swamp forests of the Carboniferous, but were largely overshadowed by bigger labyrinthodont amphibians such as Proterogynrius. It was only after the small ice age at the end of the Carboniferous that the reptiles grew to big sizes, producing species such as Edaphosaurus and Dimetrodon. The first true "reptiles" (Sauropsids) are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure, but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.

Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards, and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic) and the archosaurs (today represented by only crocodilians and birds under dinosaurs, but also containing pterosaurs and non-avian dinosaurs).

The earliest, solid-skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals, and are often referred to as mammal-like reptiles, though they are not true members of Sauropsida (a preferable term is "stem-mammals").