Osteoderm

Ampelosaurus, an armored titanosaur

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). Osteoderms have clearly evolved independently in many different lineages, and these varied structures should be thought of anatomical analogues, not homologues, and do not necessarily indicate monophyly. In many cases osteoderms may function as defensive armor.