Reptiles have been on earth for about 200 million years. All reptiles are ectothermic or cold-blooded, meaning they depend on their environment for body heat. There are almost 6000 species of reptiles in five families. Most lizards are 4-legged, but a few have only 2 legs and others are legless.
Lizards live in diverse environments and are the most geographically widespread of the reptiles.
In size, lizards can range from 2 inches to 10 feet in length. Only two species, Gila Monsters and Beaded Lizards, are venomous. As a group, they lack legs, hearing, and movable eyelids. Having evolved from lizards, some snakes still possess skeletal remnants of legs. Crocodilians range in size from 3 feet to 25 feet.