While many kinds of reptiles once dominated the earth, only four orders have living representatives. The largest of these groups is the Squamata, which includes lizards and snakes. Turtles and tortoises belong to the order Chelonia, and crocodiles, alligators, caimans, and gavials to the order Crocodilia. The tuatara, often called a living fossil, is the only extant member of the order Rhynchocephalia. Prominent among extinct reptiles were the Saurischians (Tyrannosaurus and other "reptile-hipped" bipedal carnivores), from which birds arose, and the Ornithischians ("bird-hipped" herbivores such as Stegosaurus and Hypsilophodon). Mammals arose from the Therapsid line.

This diagram illustrates some of the important anatomical structures common to reptiles, a group that includes snakes, lizards, turtles, crocodilians, and the tuatara. The organization of reptilian internal organs reflects the long, narrow body structure typical of most vertebrates. Reptilian anatomy undergoes modification in different reptile groups, adapting to the requirements of various land-based niches. For example, the limbless reptiles-such as some lizards and most snakes-have many more vertebrae to support movement on land.

Microsoft Illustrations, "Evolution of Reptiles & "Generalized Anatomy of a Reptile" Microsoft® Encarta®. Copyright © 1993-2002 Microsoft Corporation.

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