Physical properties, geochemistry, and diagenesis of xenarthran teeth: Prospects for interpreting the paleoecology of extinct species

Xenarthrans (also called edentates) include modern armadillos, sloths, and numerous extinct clades (e.g., glyptodonts and ground sloths) that lack enamel on their teeth. In contrast to mammals with tooth enamel, which oftentimes preserves geochemical proxy data, paleoecological interpretations of xenarthrans based on geochemical analyses (e.g. stable isotopes) have been limited. Xenarthrans have evolved a distinctive outer dentine layer that seems to be the functional analog of enamel. Here we evaluate the physical and chemical properties of xenarthran outer dentine to understand its potential use for investigating geochemical proxies. The mean hardness (H) of xenarthran outer dentine (3.8) is significantly less than that of enamel (5.7) and there is no difference in H between the two main groups, i.e., armadillos (cingulates) and pilosans (sloths). Although the mean H does not change significantly during diagenesis, its range increases, indicating the replacement of secondary minerals (e.g., carbonate representing the lower range) within the original hydroxylapatite mineral lattice. Results of FT-IR analyses indicate that xenarthran outer dentine has a high degree of organics and H2O, similar to normal dentine, and its crystallinity index and carbonate content are affected in varying degrees during diagenesis. Thus, although xenarthran outer dentine functions like enamel, it retains compositional similarities to dentine.The uptake of rare earth elements (REE) was analyzed from three late Neogene localities in Florida as a proxy for understanding diagenesis in xenarthrans as compared to other mammals. Relative to modern specimens, the fossils have about two orders of magnitude higher concentrations of REE. Uptake rates vary significantly among different dental tissues, i.e., bone > dentine (both inner and outer) > enamel. REE concentrations of fossil vertebrate specimens vary significantly between different localities. The REE Index (ratio of outer dentine/bone or enamel/bone for a given specimen) is proposed here to evaluate relative diagenesis in xenarthrans and other mammals, respectively. Other (non-xenarthran) mammals tend to have a lower REE Index, interpreted to represent less diagenesis. Nevertheless, many xenarthrans likewise have a low REE Index relative to other mammals at the same locality, indicating a similar degree of diagenesis. While the REE Index will likely be a reliable indicator of relative diagenesis in xenarthrans, the outer dentine provides challenges to the ultimate goal of interpreting proxy geochemical (e.g., stable isotope) data from these extinct mammals.