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 Triassic times and today are represented by about 50 genera. These reptiles have a worldwide 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 (nine 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 porosus, 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. The marine iguana of the Galapagos islands, Amblyrhynchus cristatus, spends much of its time feeding on the algae in the sea. The snakes also have a wide distribution and one family, the Hydrophiidae (15 genera), lives principally in the sea. Terrestrial snakes and lizards live in habitats ranging from tropical rainforests to dry desert regions. The remaining major group is the Rhynchocephalia, a relict branch of the reptiles with one surviving species, Sphenodon (the tuatara), which is now confined to a wet temperate environment on a few small islands situated off the coast of New Zealand.