In this volume we begin to discuss the various members of the class Reptilia. There are some 6,500 species of living reptiles. What are they like? Who were their ancestors? When and why did they develop?
Evolution. About 330 million years ago, in the Carboniferous period of geological time, there occurred an event of great significance in the history of vertebrate evelution: the development of the amniote egg. This is the special kind of egg laid by birds and reptiles. Inside it, wrapped in a membrane (the amnion), is everything needed to nourish and preserve the embryo until it hatches. The particular importance of the amniote egg was that it made the animals that possessed it independent of the water - they could lay and hatch their eggs on land. The reptiles, the first animals to develop the amniote egg, were thus the first that could live and multiply on land, opening and entire new environment for their colonization.
The oldest known reptile egg was found in Texas and dates from about 250 million years ago. Reptiles, however, are probably older than that; some reptile bones have been found that date form 285 million years ago. From the time they first evolved, however, reptiles quickly expanded their numbers. They dominated the earth for 200 million years. Then, for various reasons, they began to die off. Today, only four orders of the class Reptilia remain.
Classification. Zoologist divide the class Reptilia into six subclasses. These are based on the anatomy of the skulls, especially of the cheek region. Depending on whether there are one, two, or no openings in the skull behind the eye and on the position of these openings, a reptile is placed in one of these subclasses: anapsida (includes living and extinct turtles), Euryapsida (plesiosaurs, etc.), Ichthyopterygia (ichtyosaurs, etc.), Synapsida (mammal-like reptiles), Archosauria (dinosaurs and crocodiles), and Lepodosauria (lizards, snakes, and tuataras).
Each subclass consists of on or more orders. Only four of these orders contain living species. These are the chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia (tuatara), and Crocodilia (crocodilians).
30 feet, and there are still a few giant tortoises that can attain a length of 6 feet. There is only one giant lizard, the Komodo dragon, a monitor that may be as long as 10 feet.
General Characteristics. Reptiles are scaly-skinned, cold-blooded vertebrates. They breathe by means of a pair of lungs, and their hearts have three or four separate chambers. Reptiles are generally considered to be four-lengged animals. Snakes have simply lost their limbs in the course of evolution.
Being cold-blooded, reptiles must adjust their body temperature to their environment. this does not mean that their bodies work equally well at any temperature, however. An animal reaches its ideal temperature only when it gets enough, but not too much, heat form its surroundings. Therefore reptiles are often seen basking in the sun, soaking up its heat. They are quite dependent on the climate in the areas where they live.
The ideal temperature for a reptile varies from species to species. It ranges from 50oF
for animals that live in cool areas to about 104oF. for those animals that live in hot deserts. Of course, no place on earth offers the ideal temperature for any reptile all the time. For this reason reptiles have to be active at times when the temperature is right, and at rest when it is not. In their rest periods they go into a state similar to hibernation. Even during their active times they vary their periods of exposure to the sun with periods of rest in the shade or underground where they can cool off.
A reptile’s scaly skin absorbs heat from the sun. it also prevent the animal from drying out. The skin lacks the insulating qualities of fur or feathers. Thus allowing the reptile to lose heat rapidly. The color of the skin also affects its heating-up qualities. Dark-skinned reptiles warm up quickly in the sun but also lose their heat quickly in the shade.
Besides their skin differences, most reptiles differ from mammals and birds in that their hearts have only three chambers, the ventricle being incompletely divided. The left auricle receives oxygenated blood form the lungs, while the right gets oxygen-depleted blood form the rest of the body. Largely de-oxygenated blood is pumped to the lungs, whereas the blood going out through the arteries is mixed to some extent. Crocodilians have the ventricle completely divided, and the bloods does not mix.
Like most lower vertebrates, reptiles grow throughout their lives, although thir fastest rate of growth is early in life. As they grow, they sed the outer layers of skin.
Reptiles differ from mammals I that different bones make up the articulation between upper and lower jaws; they have the same kind of jaw articulation as birds.
They differ from amphibians in the way their skulls attach to their backbones. In reptiles the back of the skull ha a single knob, or condyle, where it attaches to the first vertebra of the backbone. Amphibians have a double condyle. There is also a difference in the breathing of reptiles and amphibians. Throughout their reptiles breathe through their lives reptiles breathe through their lungs. Amphibians, on the other hand, breathe through gills when they are larvae. Most of them develop lungs later on, during the period when they change from larval to adult life.
Both the immature and the adult forms of the first reptiles were totally adapted to a life on land. This fact had a tremendous effect on the future evolution of vertebrates. From this point on they were freed from a watery environment, and could begin their campaign to conquer the challenging environments of land and air.
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