Fossils indicate the earliest reptiles lived about 310 million years ago and resembled today’s lizards. In ancient times, the predominant reptiles were dinosaurs. The modern survivors of the Age of Reptiles include lizards, snakes, crocodiles, alligators, turtles, and birds (which are included in the reptile clade). Reptiles belong to the class Reptilia.
Reptiles display a number of adaptations that support their life on land. They have a dry, scaly skin that retards water loss. The structural makeup of their limbs provides better support and allows them to move more quickly than any amphibian.
Reproduction in reptiles occurs exclusively on land. The male places sperm into the body of the female, and the embryo develops within an egg, which is laid on dry land. Other reptilian characteristics pertain to the respiratory and circulatory systems. The lungs have a greater surface area than that of the amphibians and permit more air to be inhaled. The circulatory system includes a three-chambered heart that separates oxygen-rich and oxygen-poor blood.