Reptile is a member of the class of animals which includes the snakes, lizards, turtles, crocodilians, and the tuatara. The crocodilians include alligators, crocodiles, the gavial of
, and the caymans of tropical America. The tuatara, a large reptile, looks like a lizard and lives on off the coast of New Zealand. It is the last survivor of a whole group of ancient reptiles. About 6,000 different kinds of reptiles live throughout the world. They are of many different sizes shapes, and habits, because the reptiles have gone through many changes since their ancestors lived millions of years ago. The reptiles seen today resemble the twigs of a great tree of reptile life. They are not new branches, but descent directly from ancient animals. The four large separate limbs are the turtles, crocodilians, tuatara, and the snakes and lizards together. the huge dead limbs include such reptiles as the dinosaurs It is not fully known why one reptile branch died out while another survived. The lizards and snakes together form the only large branch today. This branch includes about 95 percent of all the different kinds of living reptiles. Reptiles are vertebrate, or backboned animals constituting the class Reptilia and are characterized by a combination of features, none of which alone could separate all reptiles from all other animals. The characteristics of reptiles are: cold-bloodedness; the presence of lungs; direct development, without larval forms as in amphibians; a dry skin with scales but not feathers or hair; an amniote egg; internal fertilization; a three or four-chambered heart; two aortic arches (blood vessels) carrying blood from the heart to the body, unlike mammals and birds that only have one; a metanephric kidney; twelve pairs of cranial nerves; and skeletal features such as limbs with usually five clawed fingers or toes, at least two spinal bones associated with the pelvis, a single ball-and-socket connection at the head-neck joint instead of two, as in advanced amphibians and mammals, and an incomplete or complete partition along the roof of the mouth, separating the food and air passageways so that breathing can continue while food is being chewed. These and other traditional defining characteristics of reptiles have been subjected to considerable modification in recent times. The extinct flying reptiles, called pterosaurs or pterodactyls, are now thought to have been warm-blooded and covered with hair. Also, the dinosaurs are also now considered by many authorities to have been warm-blooded. The earliest known bird, archaeopteryx, is now regarded by many to have been a small dinosaur, despite its covering of feathers The extinct ancestors of the mammals, the therapsids, or mammallike reptiles, are also believed to have been warm-blooded and haired. Proposals have been made to reclassify the pterosaurs, dinosaurs, and certain other groups out of the class Reptilia into one or more classes of their own. The class Reptilia is divided into 6 to 12 subclasses by different authorities. This includes living and extinct species. In addition, a number of these subclasses are completely extinct. The subclasses contain about 24 orders, but only 4 of these are still represented by living animals. Of the living orders of reptiles, two arose earlier than the age of reptiles, when dinosaurs were dominant. Tuataras, of the order Rhynchocephalia, are found only on New Zealand islands, whereas the equally ancient turtles, order Chelonia, occur nearly worldwide. The order Crocodilia emerged along with the dinosaurs. Snakes and lizards, order Squamata, are today the most numerous reptile species. The Rhynchocephalia constitute the oldest order of living reptiles; the only surviving representative of the group is the tuatara, or sphenodon (Sphenodon punctatus). Structurally, the tuatara is not much different from related forms, also assigned to the order Rhynchocephalia, that may have appeared as early as the Lower Triassic Period (over 2 000 000 000 years ago). The tuatara has two pairs of well-developed limbs, a strong tail, and a scaly crest down the neck and back. The scales, which cover the entire animal, vary in size. The tuatara also has a bony arch, low on the skull behind the eye, that is not found in lizards. Finally, the teeth of the tuatara are acrodont - i.e., attached to the rim of the jaw rather than inserted in sockets. Chelonia, another ancient order of reptiles, is chiefly characterized by a shell that encloses the vital organs of the body and more or less protects the head and limbs. The protective shell, to which the evolutionary success of turtles is largely attributed, is a casing of bone covered by horny shields. Plates of bone are fused with ribs, vertebrae, and elements of shoulder and hip girdles. There are many shell variations and modifications from family to family, some of them extreme. At its highest development, the shell is not only surprisingly strong but also completely protective. The lower shell (plastron) can be closed so snugly against the upper (carapace) that a thin knife blade could not be inserted between them. A third order of the class Reptilia is Crocodilia. Crocodiles are generally large, ponderous, amphibious animals, somewhat lizardlike in appearance, and carnivorous. They have powerful jaws with conical teeth and short legs and clawed, webbed toes. The tail is long and massive and the skin thick and plated. Their snout is relatively long and varies considerably in proportions and shape. The thick, large horny plates that cover most of the body are generally arranged in a regular pattern. The form of the is adapted to its amphibious way of life. Finally, the elongated body with its long, muscular paddletail is well suited to rapid swimming. The final living order of the class Reptilia is Squamata. Both snakes and lizards are classified in this order, but lizards are separated into their own suborder, Sauria. Lizards can be distinguished from snakes by the presence of two pairs of legs, external ear openings, and movable eyelids, but these convenient external diagnostic features, while absent in snakes, are also absent in some lizards. Lizards can be precisely separated from snakes, however, on the basis of certain internal characteristics. All lizards have at least a vestige of a pectoral girdle (skeletal supports for the front limbs) and sternum (breastbone). The lizard's brain is not totally enclosed in a bony case but has a small region at the front covered only by a membranous septum. The lizard's kidneys are positioned symmetrically and to the rear; in snakes the kidneys are far forward, with the right kidney placed farther front than the left. Finally, the lizard's ribs are never forked, as are one or two pairs in the snake. A natural classification of reptiles is more difficult than that of many animals because the main evolution of the group was during Mesozoic time (a time of transition in the history of life and in the evolution of the Earth); 13 of 17 recognized orders are extinct. There is still little agreement on reptile taxonomy among herpetologists and paleontologists. Even the major categories of reptile classification are still in dispute. On the other hand, there is general agreement that the base reptilian stock is the Cotylosauria, which evolved from an amphibian labyrinthodont stock. It is also quite clear that the coty losaurs early divided into two lines, one of which (the pelycosaurs) represented the stock that gave rise to the mammals. Another branch led to all of the other reptiles, and later, to the birds as well. Thus, most of the questions of reptilian evolution and classification deal with the reptiles' interrelationship, rather than with their relationships with other animals.