Reptile skin is a complex system of scales separated by flexible joints, which are identical to scales, except thinner. In birds, the only living group of dinosaurs, the scales are modified into feathers, except around the feet and beak. So, the hind feet of extinct dinosaurs probably looked like a bird's foot, but what about the rest of their body?
Rare, non-avian dinosaur "mummies" provide intriguing direct evidence. Impressions of dinosaur skin are only preserved under most extraordinary circumstances, for example, in an environment that is extremely dry or that lacks sufficient oxygen to support organisms that quickly decompose soft tissues. One of these is the AMNH Edmontosaurus "mummy, one of the greatest dinosaur fossils ever collected. Impressions of skin are preserved over almost the entire body. The surface is covered by tubercles of varying size. Some are large and diamond-shaped; others are small and rounded. Around the base of the limbs, on the neck, and at other joints, the skin is folded, like that surrounding the joints of an elephant, presumably to allow flexibility during movement. The mummy's remarkable preservation is attributed to the notion that it lay unscavenged on the sandbar of an ancient stream for a short period, during which its skin dried out and its tendons and ligaments contracted, leaving the carcass in a contorted "death pose." Shortly thereafter, a flood quickly buried it.
Another remarkable recent discovery in Patagonia reveals what the skin of some giant dinosaurs was like just before they hatched. Some of the fossilized eggs, about the size of a grapefruit, contained fossilized embryos of sauropods called titanosaurs, which were about 30-36 cm long, and patches of fossilized skin revealed that bumpy, scale-like plates, arranged in rows and rosettes, covered the embryos. The embryos were killed when a flood inundated their nesting site and buried the eggs.