Crocodilians have heavily armored bodies. Parts of their skin are hardened into individual hard scales made of fingernail material (keratin) and usually reinforced with bony plates. What they do provide is a tough but flexible body covering. Flexibility is essential for a predator that relies on quick maneuvering. The position of crocodilian eyes and nostrils on top of their heads allow them to lie camouflaged just under the water surface. When prey is near enough to ambush, crocodilians lunge out of the water and clamp down with powerful jaws. While a crocodilian is a great lunger, it cannot sustain long-distance movements. Huge tail and leg muscles help the crocodilian lunge quickly, then twist and pull heavy prey underwater. These characteristics of crocodilians are ancient, as they have retained nearly the same body form during 200 million years on Earth.

Reptiles first appeared on Earth about 300 million years ago (during the Pennsylvanian). Before then, all back-boned animals (vertebrates) were tied to the water, living in or near swamps. Their eggs were laid underwater. A set of gradual adaptations, such as thicker, scaly skin, allowed life on land to emerge. The biggest change was the development of an egg containing layers of membranes (amniotic membranes) to retain water, supply food, and store waste for an embryo. Two further innovations – the eggshell and internal fertilization – marked the first reptiles. Fertilization inside the female and shelled, amniotic eggs freed reptiles from aquatic life. They diversified into dinosaurs, crocodiles, lizards, snakes, turtles, tuataras, and birds as they colonized a variety of land habitats. For more than 100 million years, reptiles dominated the Earth. The largest reptiles were wiped out in the extinction event at the end of the Mesozoic, but their smaller descendants live on today.