A striking difference between xenarthrans and other mammals is the complete loss of tooth enamel in all members but the earliest armadillos. However, sloth and armadillo teeth show structured wear facets, which in all other mammals are formed by tooth enamel. How is that possible? Here, I report about an analysis of fossil and recent xenarthran dental hard tissue microstructure. It shows that osteodentine is not exclusive to fossil Cingulata, but also occurs in some recent taxa. Furthermore, I found profound modifications of orthodentine architecture in comparison to other mammals. Remarkable features are (a) a larger proportion of the highly mineralized, collagen-free peritubular dentine, and (b) a modified architecture of the odontoblastic process with frequent interconnections between the extensions and unusually intensive branching of the extensions forming a complex meshwork, penetrating the intertubular dentine matrix. The orthodentine microstructural build-up is unique in Folivora and Cingulata. J. Morphol., 2011. © 2011 Wiley-Liss, Inc.