To What Extent is Primate Second Molar Enamel Occlusal Morphology Shaped by the Enamel-Dentine Junction?

Abstract

The form of two hard tissues of the mammalian tooth, dentine and enamel, is the result of a combination of the phylogenetic inheritance of dental traits and the adaptive selection of these traits during evolution. Recent decades have been significant in unveiling developmental processes controlling tooth morphogenesis, dental variation and the origination of dental novelties. The enamel-dentine junction constitutes a precursor for the morphology of the outer enamel surface through growth of the enamel cap which may go along with the addition of original features. The relative contribution of these two tooth components to morphological variation and their respective response to natural selection is a major issue in paleoanthropology. This study will determine how much enamel morphology relies on the form of the enamel-dentine junction. The outer occlusal enamel surface and the enamel-dentine junction surface of 76 primate second upper molars are represented by polygonal meshes and investigated using tridimensional topometrical analysis. Quantitative criteria (elevation, inclination, orientation, curvature and occlusal patch count) are introduced to show that the enamel-dentine junction significantly constrains the topographical properties of the outer enamel surface. Our results show a significant correlation for elevation, orientation, inclination, curvature and occlusal complexity between the outer enamel surface and the enamel dentine junction for all studied primate taxa with the exception of four modern humans for curvature (p<0.05). Moreover, we show that, for all selected topometrical parameters apart from occlusal patch count, the recorded correlations significantly decrease along with enamel thickening in our sample. While preserving tooth integrity by providing resistance to wear and fractures, the variation of enamel thickness may modify the curvature present at the occlusal enamel surface in relation to enamel-dentine junction, potentially modifying dental functionalities such as blunt versus sharp dental tools. In terms of natural selection, there is a balance between increasing tooth resistance and maintaining efficient dental tools. In this sense the enamel cap acts as a functional buffer for the molar occlusal pattern. In primates, results suggest a primary emergence of dental novelties on the enamel-dentine junction and a secondary transposition of these novelties with no or minor modifications of dental functionalities by the enamel cap. Whereas enamel crenations have been reported by previous studies, our analysis do not support the presence of enamel tubercles without dentine relief nuclei. As is, the enamel cap is, at most, a secondary source of morphological novelty.

Untangling the environmental from the dietary: dust does not matter

“…A recent study [14] confirms that isolated silica phytoliths can produce striations on the enamel surface during mastication [20,21]. These results place the plant defences hypothesis [22][23][24][25][26] at the forefront of the debate as selective pressure towards dental innovations such as enamel thickening, tooth lengthening or hypsodonty [27][28][29]. Apart from food mechanical properties and bio-silica in plant tissues [30], soil and windblown mineral particles deposited on food found in arid habitats can also contribute to tooth wear [12,31].…”

“…foods that are hard, tough and/or rich in silica phytoliths). Hence, food can be assumed to be one of the key factors promoting dental innovations such as molar hypsodonty in ruminants [6,27], molar lengthening in suids [29] or enamel thickening in hominids during the Neogene [28,44]. Among those, robust australopithecines have received a lot of attention over the last few years [13].…”

Morphological comparison of the enamel–dentine junction and outer enamel surface of molars using a micro-computed tomography technique

“…The third pattern was unique in that it was found only in Chiropotes, in which enamel deposition is fully responsible for the highly complicated OES. In addition to the occlusal patch count, the elevation, inclination, orientation, and curvature were used as quantitative criteria [29,30]. The comparative approach for the primate maxillary second molars confirmed that EDJ morphology was significantly correlated with OES morphology and that the disparity between them was attributable to enamel thickness.…”

A comparison of relief estimates used in three‐dimensional dental topography

“…According to this estimate, a tooth with a small area of very high flat tips or a tooth with a large area of low flat polygons could have the same relief index. This variable also shows a strong negative correlation with RFI, but it has never been used for dietary inferences (Guy et al, ; Guy, Lazzari, Gilissen, & Thiery, ).…”

“…This also means that a tooth with a single high cusp and another one with a single deep valley could theoretically have the same height variation and, therefore, the same average slope and the same RFIs. Similarly, height variation is expected to be independent from the number of structures and may be the same between a tooth with a single tall cusp and another tooth with a large number of small structures such as cusplets, folds, wrinkles, and crenations (Bailey, Skinner, & Hublin, ; Butler, ; Guy et al, ).…”