Armadillos are the only living mammals that have a protective armor covering made of skin (dermal) bones known as osteoderms. The only extinct mammals with such a boney covering are close relatives of armadillos such as glyptodonts, which were giant, tortoise-like beasts, and pampatheres, which looked something like a cross between a glyptodont and an armadillo. The most simple explanation for this pattern is that osteoderms (protective skin bones) were present in the common ancestor of all these armored mammals and inherited by all of its descendants, eventually evolving into myriad forms that can be found throughout most of the South American fossil record.

Examples of the diversity of fossil armadillo, pampathere, and glyptodont osteoderms. Illustrations not to same scale. Modified from Hoffstetter (1958:figs. 9, 19).

Although the boney shell of armadillos and their extinct relatives (technically known as cingulates) is unique among mammals, cingulates are not the only…