Résumé
Primates are heterodont and diphyodont mammals, thus growing two sets of dental elements during their life, i.e., the primary (deciduous) teeth and the replacing secondary (permanent) dentition. In addition to differences in developmental timing and patterning distinguishing these two sets, deciduous teeth are in functional occlusion for a much shorter period of time and commonly subject to considerably lower functional constraints (at least until weaning). It is by now recognized that the hominid tooth enamel thickness (ET) patterns stem from an evolutionary compromise between functional/adaptive constraints and strict control mechanisms of the morphogenetic program, though its variability is also presumably linked to a number of biological and environmental factors. ET is considered to be a reliable parameter for tracking diet-related structural adaptations and exploring life-history trajectories, phylogenetic relationships and evolutionary trends [1-2], even if bi-dimensional studies of relative ET show marked overlap among fossil and extant hominin taxa [3]. However, the degree of co-variation of this tissue in deciduous and permanent crowns is still poorly understood in extinct and extant hominids and remains to be assessed in an evolutionary morpho-functional perspective. In order to set free from "average" estimates (generally limited to a gradient scale of single values) and to comparatively quantify the local absolute and relative ET distribution patterns, new investigative approaches based on advanced virtual imaging are necessary. We use here two original methods to measure the degree of intra-taxic co-variation of the deciduous and permanent ET molar variation as possible taxon-specific marker in Miocene (Ouranopithecus=1, Oreopithecus=1) and extant apes (Pongo=5, Gorilla=4, Pan=1), in nonhuman Plio-Pleistocene hominins (Australopithecus africanus=1, Paranthropus robustus=2), and in representatives of our own taxon (H. erectus=1, Neanderthals=2, extant humans=12). In total, we used 30 couples of lower dm2 and M1 belonging to the same individual (except for Oreopithecus and H. erectus, where the deciduous and the permanent molars sample different individuals). The first method corresponds to an improvement of the routine originally developed for virtually unrolling and mapping the long bone shaft and the tooth root local morphometric properties [4] adapted to the lateral ET (in order to avoid occlusal wear). The second original method allows the statistical comparisons among the maps by performing a matching using biological features between a reference surface and the occlusal surfaces. At this stage, we applied this method to unworn M1s only, limiting the investigated sample to 29 tooth crowns representing all previously mentioned taxa except H. erectus (too worn occlusal surface). By using PCA and between-group PCA, we observe a distinction among the taxa compatible with a diet-related signal. However, larger samples and a better understanding of the intraspecific variation are necessary to sort the ecological-adaptive from the phylogenetic signal. In this perspective, the dm2 and M1 crown dimensions and proportions were recently shown to be linked by developmental inhibitory cascade mechanisms [5], but more research is needed in order to fully appreciate their relationships in fossil and extant hominids in terms of subtle inner structural organization and tissue proportions.