Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs (most notably ankylosaurs and stegosaurus), crocodylians, phytosaurs, aetosaurs, placodonts, and Nanchangosaurus (a marine reptile with possible ichthyosaur affinities). Osteoderms are uncommon in mammals, but do occur in most edentates (glyptodonts, armadillos, pangolins, ground sloths).