The armadillo is a mammal, despite its scaly appearance. There are many varieties, most of which live in South America. A single exception, the nine-banded armadillo, lives in the southern United States. These animals are best known for their bony plates. Even its name refers to this unique skeletal structure, meaning "little armoured thing" in Spanish.
Armadillos have a type of bony armour that allows them to resist predators. Some extinct types were heavily armoured and large. Modern armadillos don't have the same level of protection as their ancestors, though their plates serve a similar function. Only the three-banded armadillo can roll itself entirely into a ball for protection. Other species have too many plates and are not flexible enough.
Modern armadillos are all relatively small---about the size of a cat---except for the giant armadillo, which can weigh as much as a small adult human. Ancient armadillos were much larger, though. The glyptodon and panocthus, early ancestors of the modern armadillo, were heavily armoured and about the size of a small car. The smallest modern armadillo is the fairy armadillo, an endangered creature usually only 4 to 5 inches long.
Armadillo shells are made up of true bone, covered in small scales called scutes. Most species also have bony plates or rings to protect the tails. These bony plates reduce the flexibility of most armadillo species. Flexibility is further limited by the xenarthrous process, a small bone spur on the lunbar vertabrae of the spine that stiffens it and helps support the armour.
Armadillo skeletons are optimised for digging and to accommodate their shells. They have a rigid spine, a strong, enlarged pelvis and broad ribs. In some species of armadillos, parts of the ribs normally made of cartilage in other animals are completely made of bone. The legs are sturdily built, the skull is flattened and features a long lower jaw, and the canines and incisors are missing in most species. Armadillos eat a diet that requires only molars for chewing.
In addition to the obvious shell, armadillos have other unusual skeletal characteristics. Their leg structure includes a flattened, bladelike bone at the joint, which projects a significant distance in many species. These bones are thought to increase the area available for small blood vessels, helping to cool the animal more effectively. These flattened bones may also provide larger attachment areas for muscles.