Reptile

I

Introduction

Reptile, common name applied to members of the vertebrate class Reptilia (see Animal), which includes snakes, lizards, turtles, crocodilians, the tuatara, and numerous extinct fossil species. Among the existing forms are about 2,500 species of snakes, 3,000 of lizards, nearly 250 of turtles and tortoises, and 21 species of crocodilians. They are distributed throughout the temperate and tropical regions of the world; being cold-blooded (dependent on the environment for warmth; see below), very few reptiles can develop or live in colder regions. Birds, their descendants, are placed in a separate class.

II

Characteristics

The majority of reptiles are primitively oviparous (egg-laying), but many snakes and lizards are viviparous (giving birth to live young). Existing reptiles are characterized by the development of two embryonic membranes: a protective amnion, or egg sac, and a respiratory allantois, or vascular foetal membrane. The amnion, which all reptiles, birds, and mammals have, prevents the egg from drying out, so the early stages of the animal's life cycle do not have to depend on water. In most snakes and some lizards, only one lung is functional; in other reptiles, both lungs are equally developed. The thorax and abdomen are not separated by a diaphragm, and breathing is accomplished by muscles of the body wall.

Being bound by the temperature of their environment, reptiles hibernate in regions where the winter is cold, and some forms aestivate—that is, become inactive—in exceptionally hot and dry regions. Reptiles are commonly referred to as cold-blooded, which is actually misleading, because some reptiles when active maintain their bodies at a higher temperature than most mammals. The important difference in temperature physiology is that reptiles rely on external sources of heat, whereas birds and mammals use internal heat. Reptiles regulate their body temperatures by taking advantage of different sources of outside warmth, such as direct sunlight, warm stones and logs, and the heated earth. By using such heat sources to varying degrees, individual species of reptiles keep their bodies more or less at the constant temperature characteristic of the particular species. Often this body temperature is well above the temperature of the surrounding air. Only when the animal is inactive or dormant is the body temperature approximately the same as that of the environment.

The reptile skeleton is almost completely ossified (non-cartilaginous). The skull is joined to the vertebral column by a single condyle, or joint, as in birds. The ribs of the thorax are attached to the breastbone; and when a sacrum (pelvis-connected portion of the spine) is present, sacral ribs articulate with the pelvic girdle. Two complete pairs of limbs may be present, or they may be reduced or lost, as in snakes and some lizards. The skin is covered with scales, and bony plates may be embedded beneath the skin.

The reptilian heart consists of three chambers, including two auricles and one ventricle. In the crocodilians, however, the ventricle is almost completely divided into two chambers by a septum, or partition. Both of the embryonic aortic arches (certain arterial branches found in vertebrate embryos) persist in reptiles, in contrast to birds and mammals, in which only one develops. Blood in the veins returns to the heart from the tail and hind limbs, through the kidneys, by a renal portal (vein-capillary) system; blood from the abdominal region returns by a portal system passing through the liver. The urinary bladder is present only in turtles, tortoises, and lizards.

III

Classification

The earliest reptiles appeared during the Upper Carboniferous period of the Palaeozoic era. Many forms evolved and flourished in the Mesozoic era, which is known as the Age of Reptiles. Since then the great majority of reptilian species have become extinct; only 5 of the 23 orders of reptiles that have existed now have living representatives. The best-known orders are cited below.

A

Rhynchocephalia: tuataras

This order contains the lizard-like reptiles known as tuataras, which are differentiated from the lizards by osteological (bone and skeleton) characteristics. Rhynchocephalians are well known from the Triassic and Jurassic periods, but all are now extinct except the New Zealand tuatara, of the genus Sphenodon.