Reptilia and are characterized by a combination of features, none of whichalone could separate all reptiles from all other animals.
The characteristics of reptiles are numerous, therefore can not beexplained in great detail in this report. In no special order, thecharacteristics of reptiles are: cold-bloodedness; the presence of lungs;direct development, without larval forms as in amphibians; a dry skin withscales but not feathers or hair; an amniote egg; internal fertilization; athree or four-chambered heart; two aortic arches (blood vessels) carryingblood from the heart to the body, unlike mammals and birds that only haveone; a metanephric kidney; twelve pairs of cranial nerves; and skeletalfeatures such as limbs with usually five clawed fingers or toes, at leasttwo spinal bones associated with the pelvis, a single ball-and-socketconnection at the head-neck joint instead of two, as in advanced amphibiansand mammals, and an incomplete or complete partition along the roof of themouth, separating the food and air passageways so that breathing cancontinue while food is being chewed. These and other traditional defining characteristics of reptiles have beensubjected to considerable modification in recent times. The extinct flyingreptiles, called pterosaurs or pterodactyls, are now thought to have beenwarm-blooded and covered with hair. Also, the dinosaurs are also nowconsidered by many authorities to have been warm-blooded. The earliestknown bird, archaeopteryx, is now regarded by many to have been a smalldinosaur, despite its covering of feathers The extinct ancestors of themammals, the therapsids, or mammallike reptiles, are also believed to havebeen warm-blooded and haired. Proposals have been made to reclassify thepterosaurs, dinosaurs, and certain other groups out of the class Reptiliainto one or more classes of their own. The class Reptilia is divided into 6 to 12 subclasses by differentauthorities. This includes living and extinct species. In addition, a numberof these subclasses are completely extinct. The subclasses contain about 24orders, but only 4 of these are still represented by living animals.
Of the living orders of reptiles, two arose earlier than the age ofreptiles, when dinosaurs were dominant. Tuataras, of the orderRhynchocephalia, are found only on New Zealand islands, whereas the equallyancient turtles, order Chelonia, occur nearly worldwide. The orderCrocodilia emerged along with the dinosaurs. Snakes and lizards, orderSquamata, are today the most numerous reptile species.
The Rhynchocephalia constitute the oldest order of living reptiles; theonly surviving representative of the group is the tuatara, or sphenodon(Sphenodon punctatus). Structurally, the tuatara is not much different fromrelated forms, also assigned to the order Rhynchocephalia, that may haveappeared as early as the Lower Triassic Period (over 2 000 000 000 yearsago). The tuatara has two pairs of well-developed limbs, a strong tail, anda scaly crest down the neck and back. The scales, which cover the entireanimal, vary in size. The tuatara also has a bony arch, low on the skullbehind the eye, that is not found in lizards. Finally, the teeth of thetuatara are acrodont – i.e., attached to the rim of the jaw rather thaninserted in sockets.
Chelonia, another ancient order of reptiles, is chiefly characterised by ashell that encloses the vital organs of the body and more or less protectsthe head and limbs. The protective shell, to which the evolutionary successof turtles is largely attributed, is a casing of bone covered by hornyshields. Plates of bone are fused with ribs, vertebrae, and elements ofshoulder and hip girdles. There are many shell variations and modificationsfrom family to family, some of them extreme. At its highest development, theshell is not only surprisingly strong but also completely protective. Thelower shell (plastron) can be closed so snuggly 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 generallylarge, ponderous, amphibious animals, somewhat lizardlike in appearance, andcarnivorous. They have powerful jaws with conical teeth and short legs andclawed, webbed toes. The tail is long and massive and the skin thick andplated. Their snout is relatively long and varies considerably inproportions and shape. The thick, large horny plates that cover most of thebody are generally arranged in a regular pattern. The form of the is adaptedto 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 andlizards are classified in this order, but lizards are separated into theirown suborder, Sauria. Lizards can be distinguished from snakes by thepresence 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 fromsnakes, however, on the basis of certain internal characteristics. Alllizards have at least a vestige of a pectoral girdle (skeletal supports forthe front limbs) and sternum (breastbone). The lizard’s brain is nottotally enclosed in a bony case but has a small region at the front coveredonly by a membranous septum. The lizard’s kidneys are positionedsymmetrically and to the rear; in snakes the kidneys are far forward, withthe right kidney placed farther front than the left. Finally, the lizard’sribs are never forked, as are one or two pairs in the snake. A natural classification of reptiles is more difficult than that of manyanimals because the main evolution of the group was during Mesozoic time (atime of transition in the history of life and in the evolution of theEarth); 13 of 17 recognized orders are extinct. There is still littleagreement on reptile taxonomy among herpetologists and paleontologists.
Even the major categories of reptile classification are still in dispute. Onthe other hand, there is general agreement that the base reptilian stock isthe Cotylosauria, which evolved from an amphibian labyrinthodont stock. Itis also quite clear that the coty losaurs early divided into two lines, oneof which (the pelycosaurs) represented the stock that gave rise to themammals. Another branch led to all of the other reptiles, and later, to thebirds as well. Thus, most of the questions of reptilian evolution andclassification deal with the reptiles’ interrelationship, rather than withtheir relationships with other animals.