Nature of laminations and mineralization in rhinoceros enamel using histology and X-ray synchrotron microtomography: Potential implications for palaeoenvironmental isotopic studies

Abstract

Incremental features of rhinoceros molar enamel were studied quantitatively on histological slices. The different kinds of incremental features of enamel were related to one another and counted to determine the rate and duration of crown formation and to clarify the status of the laminations. The mineralization and the microstructure of the enamel were examined in a rhinoceros molar germ using X-ray synchrotron microtomography respectively using absorption and phase contrast mode at different levels of resolution, from 45.71 μm for the largest scale up to 0.28 μm for the finest one. The different microtomographic data were combined to obtain a multiscale integrated overview of the microstructure and mineralization in order to understand better the relationship between these two aspects. Additional human and herbivorous mammal teeth were examined qualitatively using histological sectioning to obtain a better understanding of laminations in mammal tooth enamel. Laminations, regularly spaced features parallel to the Retzius lines, represent regular isochrons of enamel deposition. This study shows that laminations and cross-striations are equivalent daily features in enamel of rhinoceros, humans, and probably of middle sized and large herbivorous mammals in general. Laminations are shown to be three-dimensional alignments of cross-striations. In addition to the fact that cross-striations are visible only in prismatic enamel, observation of histological slices, and high resolution phase contrast X-ray synchrotron microtomography demonstrate three main parameters influencing the dominant aspect of daily features in rhinoceros and human enamel: the angle between prisms and Retzius lines, the enamel deposition rate and the thickness of the histological slices. Given their daily nature, laminations can be used to determine the crown formation time of middle sized and big herbivorous mammals. Quantitative study of the enamel mineralization in the rhinoceros germ fragment using X-ray synchrotron microtomography suggests that the innermost 20 μm of the enamel near the enamel–dentin junction are highly mineralised (77–90% of the final enamel mineralization) during, or just after the deposition of the corresponding enamel matrix. Integrated microtomographic study shows that this area corresponds to the thin aprismatic enamel layer at the EDJ level plus a thin layer of radial enamel directly adjacent. It appears that, in some parts of the enamel, there may be a considerable time lag between matrix deposition and full maturation, which could have important implications for geochemical studies. This study reinforces the previously suggested hypothesis that isotopic measurements derived from the innermost enamel may represent the period most closely related to matrix secretion. By using information derived from incremental features (laminations and Retzius lines) and restricting the isotopic measurements to the innermost part of the enamel, precise temporal calibration of isotopic signals should be possible, and the complication of time averaging due to enamel maturation could be avoided or strongly reduced.