Squamates, which include lizards and snakes, are named for their scales (squamatus = “scaly”). Their scales, which are part of their skin, are typically overlapping. As squamates grow, they periodically shed (molt) the entire outer layer of skin, in contrast to humans who shed skin in tiny flakes. Snakes rub against rough surfaces to loosen their skin and peel it back off head, limbs, and torso. The skin of squamates tends to be patterned with colors, which may help camouflage (e.g., green tree snakes) or warn predators of toxicity (e.g., coral snakes). Some squamates (e.g., anoles) have surface pigments that allow them to change colors. Scientists have discovered that squamate skin also has intricate textures, visible only through a microscope. Magnification reveals patterns of ridges, pits, or spines. The microscopic, branching hairs on the ridged toes of geckos have been found to help them cling to vertical surfaces, but the functions of many other squamate skin textures are unknown.

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.