Reptile

Reptiles are animals in the (Linnaean) class Reptilia characterized by breathing air, a "cold-blooded" (poikilothermic) metabolism, laying tough-shelled amniotic eggs (or retaining the same membrane system in species with live birth), and skin with scales or scutes. All reptiles are tetrapods, either having four limbs or being descended from four-limbed ancestors, and and most lay amniotic eggs, in which the embryo is surrounded by a membrane called the amnion. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized.

Most reptiles are oviparous (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals; many also provide their young with initial care after birth.

Classification
Classification of reptiles is difficult and has long been a matter of debate. The reptiles were from the outset of classification grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are often found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturae. The terms "reptile" and "amphibian" were largely interchangeable, "reptile" (from Latin repere, "to creep") being preferred by the French. Josephus Nicolaus Laurenti was the first to formally use the term "Reptilia" for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus. Not until the beginning of the 19th century did it become clear that reptiles and amphibians are in fact quite different animals, and Pierre André Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds and mammals.

The British anatomist Thomas Henry Huxley made Latreille's definition popular, and together with Richard Owen expanded Reptilia to include the various fossil “Antediluvian monsters”, including the Mammal-like (synapsid) Dicynodon he helped Describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into Mammals, Sauroids, and Ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of Sauropsida and Ichthyopsida for the two.

Around the end of the 19th century, the class Reptilia had come to include all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes the extant crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, as well as fossil groups like dinosaurs, synapsids and the primitive pareiasaurs. 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 evolved from the original reptile. Colin Tudge writes:

"Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synapomorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian amniotes'."

The terms "Sauropsida" ("lizard faces") and "Theropsida" ("beast faces") were taken up again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on the 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.

Rise of the reptiles
The origin of the reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs such as Proterogyrinus. The oldest known animal that may have been an amniote, i.e. a primitive reptile rather than an advanced amphibian is Casineria. A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years show typical reptilian toes and imprints of scales. The tracks are attributed to Hylonomus, the oldest unquestionable reptile known. It was a small, lizard-like animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote) and Paleothyris, both of similar build and presumably similar habit. One of the best known early reptiles is Mesosaurus, a genus from the early Permian that had returned to water, feeding on fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.

Reptile diversification
The first reptiles were anapsids, having a solid skull with holes for only nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving anapsids, since they 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 (see Parareptilia). Both sides cite strong evidence, and the conflict has yet to be resolved.

Very soon after the first reptiles appeared, they split into two branches. One branch, the Synapsida (including both "mammal-like reptiles" and modern, extant mammals such as humans), had one opening in the skull roof behind each eye; the other branch, the Diapsida, possessed a hole in their skulls behind each eye, along with a second hole located higher on the skull. The function of the holes in both groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite. The diapsids and later anapsids are classed as the "true reptiles", the Sauropsida.