Class Reptilia is made up of the first animals to produce amniotic eggs. In this type of egg, the embryo develops in a fluid filled sac called an amnion. This prevents the egg from drying out, and allowed the reptiles to live only on land. Reptiles have tough skin made of the protein keratin. Since reptiles do not need to breathe through the skin, it is much thicker than the skin of amphibians. Reptiles periodically shed their skin in a process called molting. Another adaptation allowing reptiles to live on land is a well-developed respiratory system with branched bronchial tubes in their lungs. Reptiles have teeth adapted for holding prey rather than chewing it because most species in this class swallow their prey whole. Reptiles have good hearing and vision and a tongue is used for smell as well as taste. The only disadvantage to reptiles is that they are ectothermic, meaning they warm up by absorbing heat rather than generating their own heat, like birds and mammals.

The Mesozoic Age is also known as the age of the reptiles because dinosaurs and other reptiles were the predominant animal during that period. Most of those species became extinct about 65 million years ago after a mass extinction possibly caused by a meteor or increased volcanic activity. However, one lineage of dinosaurs became the birds, and an earlier lineage led to the mammals, who also became more abundant after the age of reptiles. Other reptiles became the current orders, which include Chelonia, the turtles and tortoises, Crocodilia, the alligators and crocodiles, Squamata and Sauria, the snakes, and Rhynchocephalia, assorted other reptiles.