Reptile skin is a complex system of scales separated by flexible joints. In birds, the only living group of dinosaurs, the scales are modified into feathers, except around the feet and beak. 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. 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. 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.