Comparative anatomy and histology of xenarthran osteoderms

Reconstruction of soft tissues in fossil vertebrates is an enduring challenge for paleontologists. Because inferences must be based on evidence from hard tissues (typically bones or teeth), even the most complete fossils provide only limited information about certain organ systems. Osteoderms ("dermal armor") are integumentary bones with high fossilization potential that hold information about the anatomy of the skin in many extant and fossil amniotes. Their importance for functional morphology and phylogenetic research has recently been recognized, but studies have focused largely upon reptiles, in which osteoderms are most common. Among mammals, osteoderms occur only in members of the clade Xenarthra, which includes armadillos and their extinct relatives: glyptodonts, pampatheres, and, more distantly, ground sloths. Here, I present new information on the comparative morphology and histology of osteoderms and their associated soft tissues in 11 extant and fossil xenarthrans. Extinct mylodontid sloths possessed simple, isolated ossicles, the presence of which is likely plesiomorphic for Xenarthra. More highly derived osteoderms of glyptodonts, pampatheres, and armadillos feature complex articulations and surface ornamentation. Osteoderms of modern armadillos are physically associated with a variety of soft tissues, including nerve, muscle, gland, and connective tissue. In some cases, similar osteological features may be caused by two or more different tissue types, rendering soft-tissue inferences for fossil osteoderms equivocal. Certain osteological structures, however, are consistently associated with specific soft-tissue complexes and therefore represent a relatively robust foundation upon which to base soft-tissue reconstructions of extinct xenarthrans.