The amphibians have never achieved emancipation from the water even though the major part of the adult life is spent on land. They still require the aquatic environment for external fertilization, for development, and for respiration. The reptiles, however, achieved stupendous success on land and attained heights of magnificence only surpassed by the mammals. Their success as terrestrial animal is attributable to a number of factors which are briefly summarized here.
1. Internal fertilization obviates the need for external water.
2. Cleidoic development renders the embryo independent of aquatic surroundings.
3. With no aquatic larval stage there is never need for gill breathing.
4. The skin is impermeable to water.
5. The lungs form an adequate respiratory surface and are well protected from drying. Breathing movements ensure that exchange of gases in rapid enough.
6. Uricotelic excretion minimize water loss.
7. There is effective resorption of water in the kidneys and rectum.
8. Limbs suitable for running and supporting the body in the less dense medium have been evolved.
9. Loss of the lateral line system and improvement of more necessary sense organs has been achieved.
10. Increased development of the cerebral cortex makes more complex behaviour possible.
11. Large –scale colonization of the land by plants provided constant food for herbivores, and they, in their turn, provided food for carnivores.
Entering upon the terrestrial scene, the reptiles had little competition from other animals. In such conditions, a dominant group soon extends into a variety of specializations. By such extension, the descendants of one original type are able to exploit all the possibilities of the environment. The process is known as adaptive radiation, and while example of it may be found in any of the larger group of animals, it is most obvious in insects, reptiles, birds and mammals.
In their Mesozoic heyday, the reptiles branched out into a great variety of forms. They ranged from giants to dwarfs; some of the dinosaurs were the largest terrestrial animals that have existed; Brachiosaurus weighed about 50 tonne, and Dipplodocus was about 26m long. At the other end of the scale were animals the size of small lizards. There were large carnivores and larger herbivores, swift runners and slow crawlers, burrowers and climbers. The Ichthyopterygia, of fish-like form, became secondarily aquatic and inhabited the seas; the turtles and freshwater tortoises have become aquatic, without such striking adaptation. Flying reptiles, the Pterodactyla, flourished during the Jurassic and Cretaceous periods. The wings were membranous and stretched from very elongated fourth finger to the hind limb. This brief survey by no means exhausts the variety of lines along which the reptiles evolved, but it serves to illustrate the meaning of adaptive radiation.