Reptiles are characterized by a unique combination of features that is not found in any other animals.
The characteristics of reptiles are numerous and cannot be fully explained in detail in this report. These characteristics include being cold-blooded, having lungs, direct development without larval forms like amphibians, having dry skin with scales, an amniote egg, internal fertilization, a three or four-chambered heart, two aortic arches for blood circulation, a metanephric kidney, twelve pairs of cranial nerves, and various skeletal features such as limbs with usually five clawed fingers or toes. They also have at least two spinal bones associated with the pelvis, a single ball-and-socket connection at the head-neck joint, and an incomplete or complete partition along the roof of their mouth to separate food and air passageways. However, these traditional defining characteristics have been modified recently. It is now believed that extinct flying reptiles called pterosaurs or pterodactyls were warm-blooded and covered with hair. Many experts also now consider dinosaurs to have been warm-blooded.The archaeopteryx, the earliest known bird, is now considered by many as a small dinosaur despite having feathers. The therapsids, or mammal-like reptiles, which are the extinct ancestors of mammals, are also believed to have been warm-blooded and had hair. There have been suggestions to reclassify pterosaurs, dinosaurs, and other groups out of the reptile class to create one or more new classes. The reptile class is divided into 6 to 12 subclasses depending on different authorities, including both living and extinct species. Some of these subclasses are entirely extinct. There are approximately 24 orders within these subclasses, but only four of them still have living representatives.
Among the various orders of reptiles, two originated before the age of reptiles, a time when dinosaurs reigned supreme. Tuataras, belonging to the order Rhynchocephalia, are exclusively found on the islands of New Zealand, while turtles, equally ancient, from the order Chelonia, can be found almost everywhere. The order Crocodilia appeared alongside the dinosaurs. Today, the most abundant species of reptiles are snakes and lizards from the order Squamata.
The tuatara, or sphenodon (Sphenodon punctatus), is the sole surviving member of the Rhynchocephalia order, which is the oldest order of living reptiles. It closely resembles other related forms assigned to the Rhynchocephalia order, dating back to the Lower Triassic Period over 2,000,000,000 years ago. The tuatara possesses two pairs of well-developed limbs, a strong tail, and a scaly crest running down its neck and back. Its scales vary in size and cover the entire body. In addition, the tuatara possesses a bony arch situated below the skull behind the eye that distinguishes it from lizards. Furthermore, unlike other reptiles, its teeth are acrodont, meaning they are attached to the rim of the jaw rather than being inserted in sockets.
The Chelonia order of reptiles is primarily defined by a protective shell that encloses the vital organs and offers some protection for the head and limbs. This shell, which is made of bone covered by horny shields, contributes greatly to the evolutionary success of turtles. The shell is formed by plates of bone that are fused with ribs, vertebrae, and elements of the shoulder and hip girdles. Different families of turtles have various shell variations and modifications, ranging from minor to extreme. In its most advanced form, the shell is not only remarkably robust but also provides complete protection. The lower part of the shell, known as the plastron, fits so tightly against the upper part (carapace) that even a thin knife blade cannot be inserted between them.
A class of reptiles called Crocodilia is the third order. These animals, which are known as crocodiles, are typically large, heavy, amphibious creatures that somewhat resemble lizards. They are carnivorous and possess powerful jaws with conical teeth, as well as short legs with clawed, webbed toes. Crocodiles also have long and massive tails, and thick, plated skin. Their snouts vary in proportions and shape but are generally relatively long. The body of a crocodile is adapted for an amphibious lifestyle, with thick, large horny plates arranged in a regular pattern covering most of their bodies. Their elongated bodies, along with their long, muscular paddletails, are well-suited for 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 though reptile classification is still disputed, there is agreement that the base reptilian stock originated from an amphibian labyrinthodont stock, known as the Cotylosauria. It is also evident that the Cotylosaurs early on diverged into two lines. One of these lines, known as the pelycosaurs, is the ancestral stock of mammals. The other line led to all other reptiles and eventually birds. As a result, most inquiries regarding reptilian evolution and classification focus on their interrelationship rather than their relationships with other animals.
