The Reptiles

Abstract

Reptiles are considered phyletically to represent the first truly terrestrial vertebrates. They originated in the early Mesozoic period from an amphibian-like ancestor. In those times they became the predominant tetrapod vertebrates, living not only on the dry land, but also in the fresh water of lakes and rivers, and in the sea. Four principal groups of reptiles have persisted to the present day. The Chelonia (turtles and tortoises) have changed little since their origin in early Trias-sic times and today are represented by about fifty genera. These reptiles have a world wide distribution; they are usually aquatic (or more strictly amphibious) in their habits. Five species live in the sea although they must return to dry land in order to lay their eggs, A number of Chelonians have adopted a life in arid desert regions and these include the North American desert tortoise, Gopherus agassizii, and the Mediterranean tortoise, Testudo graeca. The Crocodilia (9 genera) have existed in a relatively unchanged form since they first appeared in the late Triassic period. They are mostly aquatic, living in the vicinity of fresh water, but at least one species, Crocodilus porosuss, ventures into the sea for periods of uncertain duration. The Squamata, numerically the principal contemporary reptiles, consist of two main groups; the Lacertilia (lizards) and Ophidia (snakes), which originated in the Jurassic and Cretaceous periods respectively. Today they are each represented by about 300 genera. The lizards have the widest geographic distribution of the reptiles and are even found on many oceanic islands.