Histology of ‘uncommon’ osteoderms of Glyptodon reticulatus Owen, 1845 (Mammalia, Xenarthra) from the late Pleistocene of Argentina and its systematic and developmental implications

Abstract The osteoderms of Glyptodon reticulatus that do not belong to the cephalic shield, dorsal cuirass, or caudal tube are very variable morphologically. Their histological structure is also variable. Some lack a cancellous bone core, unlike those from the dorsal cuirass. One presents a few large cavities, whereas another is finely cancellous. Some present moderate remodeling and thick collagen fiber bundles. The moderate to high remodeling and the presence of fiber bundles perpendicular to the inner surface in some resemble many dorsal cuirass osteoderms of Glyptodon, but differ from previously described cuirass osteoderms of Glyptotherium. At least the perpendicular bundles may be useful to distinguish those glyptodontine genera. However, these characters do not allow distinction from Panochthus and Doedicurus. Histological distinction from sloth osteoderms is not possible because of the variability in the latter in remodeling degree, presence of large resorption spaces, and of an external fundamental system with fine fiber bundles perpendicular to the bone surface. An osteoderm presents evidence of circumferential lamellar accretion, as in dorsal cuirass osteoderms of Glyptodon. The lack of evidence of woven bone and of circumferential lamellar accretion, non-osteonal lamellar or parallel-fibered bone in another osteoderm suggests primary ossification was mostly metaplastic. This difference from the condition in Dasypus may depend of the time of appearance or body region.