PALEVO 1631-0683 Elsevier S1631-0683(17)30040-4 10.1016/j.crpv.2017.05.002 Research article Human Palaeontology and Prehistory Foreword/Avant-propos Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids? Le rapport d’épaisseur de l’émail des molaires déciduales/permanentes est-il un indicateur taxinomique chez les hominidés actuels et fossiles ? Macchiarelli Roberto Zanolli Clément Zanolli Clément clement.zanolli@gmail.com a Bayle Priscilla b Bondioli Luca c Dean M. Christopher d Le Luyer Mona b e Mazurier Arnaud f Morita Wataru g Macchiarelli Roberto h i UMR 5288 CNRS, Laboratoire AMIS, Université Toulouse-3 Paul-Sabatier, 31062 Toulouse, France UMR 5288 CNRS, Laboratoire AMIS, Université Toulouse-3 Paul-Sabatier 31062 Toulouse France UMR 5199 CNRS, Laboratoire PACEA, Université de Bordeaux, 33615 Bordeaux, France UMR 5199 CNRS, Laboratoire PACEA, Université de Bordeaux 33615 Bordeaux France Sezione di Bioarcheologia, Museo Nazionale Preistorico Etnografico “Luigi Pigorini”, Rome, Italy Sezione di Bioarcheologia, Museo Nazionale Preistorico Etnografico “Luigi Pigorini” Rome Italy Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK Department of Cell and Developmental Biology, University College London London WC1E 6BT UK School of Anthropology & Conservation, University of Kent, Canterbury, UK School of Anthropology & Conservation, University of Kent Canterbury UK UMR 7285 CNRS, Institut de chimie des milieux et matériaux de Poitiers, Université de Poitiers, 86073 Poitiers, France UMR 7285 CNRS, Institut de chimie des milieux et matériaux de Poitiers, Université de Poitiers 86073 Poitiers France Department of Oral Functional Anatomy, Graduate School of Dental Medicine, Hokkaido University, Hokkaido, Japan Department of Oral Functional Anatomy, Graduate School of Dental Medicine, Hokkaido University Hokkaido Japan UMR 7194 CNRS, Laboratoire HNHP, Muséum national d’histoire naturelle, 75116 Paris, France UMR 7194 CNRS, Laboratoire HNHP, Muséum national d’histoire naturelle 75116 Paris France Unité de formation Géosciences, Université de Poitiers, 86073 Poitiers, France Unité de formation Géosciences, Université de Poitiers 86073 Poitiers France 16 5-6 S1631-0683(17)X0005-5 Hominin biomechanics, virtual anatomy and inner structural morphology: From head to toe. A tribute to Laurent Puymerail Hominin biomechanics, virtual anatomy and inner structural morphology: From head to toe. A tribute to Laurent Puymerail 702 714 © 2017 Académie des sciences 2017 Académie des sciences Full (PDF)

In Primates, enamel thickness variation stems from an evolutionary interplay between functional/adaptive constraints (ecology) and the strict control mechanisms of the morphogenetic program. Most studies on primate enamel thickness have primarily considered the permanent teeth, while the extent of covariation in tooth enamel thickness distribution between deciduous and permanent counterparts remains poorly investigated. In this test study on nine extant and fossil hominids we investigated the degree of covariation in enamel proportions between 25 pairs of mandibular dm2 and M1 by a so-called “lateral enamel thickness diphyodontic index”. The results did not provide an unambiguous picture, but rather suggest complex patterns likely resulting from the influence of many interactive factors. Future research should test the congruence of the “diphyodontic signal” between the anterior and the postcanine dentition, as well as between enamel and the enamel-dentine junction topography.

Chez les primates, le patron de variation d’épaisseur de l’émail est issu d’un compromis évolutif entre contraintes fonctionnelles/adaptatives (écologiques) et mécanismes de contrôle morphogénétique. La majorité des études portant sur l’épaisseur de l’émail des primates concerne les dents permanentes, tandis que le degré de covariation de distribution d’épaisseur de l’émail entre les équivalents déciduaux et permanents reste encore méconnu. Dans cette étude préliminaire, nous explorons le degré de covariation des proportions d’émail entre 25 paires de dm2 et M1 mandibulaires de neuf hominidés actuels et fossiles en élaborant un « indice diphyodonte d’épaisseur de l’émail latéral ». Les résultats ne montrent pas un signal évident, mais suggèrent plutôt des modèles complexes résultant probablement de l’influence d’interactions entre des facteurs variés. De futures recherches sur le sujet devraient tester le degré de congruence du « signal diphyodonte » entre les dents antérieures et post-canines, ainsi qu’entre l’émail et la topographie de la jonction émail–dentine.

 Enamel, Dm2, M1, “Diphyodontic index”, Hominids Émail, Dm2, M1, « Indice diphyodonte », Hominidés presented Handled by Roberto Macchiarelli and Clément Zanolii Introduction

Following the pioneering methodological work developed by L.B. Martin for measurement procedure and standardization (Martin, 1985), the bi-three-dimensional assessment of tooth enamel thickness has become routine in taxonomic and adaptive/evolutionary studies of fossil and extant primates (e.g., Alba et al., 2013, Kono, 2004, Kono et al., 2014, Macchiarelli et al., 2013, Macchiarelli et al., 2004, Macchiarelli et al., 2009, Olejniczak et al., 2008a, Olejniczak et al., 2008b, Olejniczak et al., 2008c, Olejniczak et al., 2008d, Pan et al., 2016, Skinner et al., 2015, Smith et al., 2011, Smith et al., 2003, Smith et al., 2005, Smith et al., 2012, Suwa et al., 2009, Zanolli et al., 2016a and Zanolli et al., 2015). Commonly used to infer durophagy and considered as a proxy of the dietary niches exploited by extinct species (e.g., Constantino et al., 2012, Constantino et al., 2011, Lucas et al., 2008, Martin et al., 2003, Schwartz, 2000a, Teaford, 2007, Teaford and Ungar, 2015 and Vogel et al., 2008), occlusal enamel thickness is seen as intimately related to dietary abrasiveness and selectively responsive to lifetime dental wear resistance (Pampush et al., 2013 and Rabenold and Pearson, 2011).

In primates, enamel thickness variation stems from an evolutionary interplay between functional/adaptive constraints (ecology) and strict control mechanisms of the morphogenetic program (Horvath et al., 2014, Kelley and Swanson, 2008, Kono, 2004, Simmer et al., 2010, Smith et al., 2012 and Vogel et al., 2008). It appears to respond relatively quickly in evolutionary time to dietary/ecological changes (Grine and Daegling, 2017, Hlusko et al., 2004 and Le Luyer and Bayle, 2017), thus being prone to homoplasy (Smith et al., 2012; rev. in Macho, 2015).

Most studies on enamel thickness have primarily considered the permanent teeth, especially the molar series, while the extent of covariation in tooth enamel thickness between deciduous and permanent counterparts has been the object of limited quantitative analyses, including in hominids (for a recent synthesis and review of studies on deciduous enamel thickness in humans, see table 1 in Mahoney, 2013; additionally, among other contributions, see Benazzi et al., 2011, Fornai et al., 2016, Fornai et al., 2014, Macchiarelli et al., 2013, Macchiarelli et al., 2006, Peretto et al., 2015, Zanolli, 2015a, Zanolli et al., 2010a, Zanolli et al., 2014 and Zanolli et al., 2012). Accordingly, quantitative support to answer a number of questions remains so far elusive. More specifically: whenever, in a comparative intertaxonomic assessment, we score a permanent hominid tooth as relatively “thinly”- or “thickly-enamelled” and order it accordingly within a series of investigated specimens, does the primary element score similarly and does it (tend to) occupy a comparable position within the same deciduous series? Can we confidently predict an enamel thickness “category” for a hominid deciduous crown based on the measure of the permanent tooth (or vice versa)? Does a predictable deciduous-permanent pattern exist for tooth enamel thickness in hominids? If so, is it taxon-specific?

The second deciduous (dm2) and the first permanent (M1) molars are part of the same developmental molar series (rev. in Bailey et al., 2016 and Bailey et al., 2014; see also Evans et al., 2016), i.e., they are meristic elements with a similar and serially repeated structure within the same organism (Butler, 1956, Butler, 1967 and Kraus and Jordan, 1965). In this study on some extant and fossil hominids, we thus investigate the degree of covariation in enamel proportions between the dm2 and the M1 (for the extant human condition, see Gantt et al., 2001, Grine, 2005, Huszár, 1972, Mahoney, 2010 and Rossi et al., 1999). In order to perform intertaxonomic comparisons, we established a so-called “lateral enamel thickness diphyodontic index” (LETDI; see § Materials and methods) as a measure of the proportions in the amount of non-occlusal enamel (Macchiarelli et al., 2016 and Zanolli, 2015b). Even if the mandibular dm2 and the M1 specifically used in this study are not successional elements, we introduced the wider concept of “diphyodontic index” referring to their usual differential use-life. Given the exploratory nature of this study, whose main goal is to capture a tendency or trend, if any, and not to assess intraspecific variation, or evolutionary trends, or phylogenetic relationships, the number of cases examined for each taxon (ranging from 1 to 5 tooth pairs) is just minimal. By definition, at this stage of the research the underlying assumption is that the signal revealed by each dm2-M1 crown pair used here, all from mandibular dentitions, represents the average condition of its own taxon, i.e., is taxon-representative.

Apart from some intertaxonomic differences in developmental timing and patterning between the dm2 and the M1 (Dean, 2000, Dean, 2006, Dean, 2010 and Dean and Cole, 2013), given that the dm2 is in functional occlusion for a much shorter time and commonly experiences lower functional constraints at least until the weaning process begins (Fleagle, 2013 and Swindler, 2002), we expect that, independently from their relative qualitative “category” (“thinner” vs. “thicker”), the dm2/M1 enamel relative volume ratios are < 1.

 Materials and methods

The hominid taxa considered in this study include the four extant genera Homo (HOM), Pan (PAN), Gorilla (GOR) and Pongo (PON), and representatives of four fossil genera: the Plio-Pleistocene hominins Paranthropus (robustus) (PAR) and Australopithecus (africanus) (AUS), from the South African sites of Swartkrans and Sterkfontein, respectively, and the late Miocene European apes Ouranopithecus (macedoniensis) (OUR), from Macedonia, and Oreopithecus (bambolii) (ORE), from Sardinia. Besides H. sapiens, humans are also represented by the extinct Neanderthals (Nea). The existence of interspecific differences in molar enamel thickness has been ascertained within the australopith clade (e.g., Grine and Daegling, 2017, Grine and Martin, 1988, Olejniczak et al., 2008b, Pan et al., 2016 and Skinner et al., 2015), but their consideration here is far beyond the specific purposes of our present work.

Details about the composition and origin of the mandibular dm2 and M1 specimens/samples are provided in Table 1. The extant human teeth, all from individuals of European origins, represent both sexes; conversely, no detailed information, including about their geographic provenance (and if from captive or wild individuals), is available to us regarding the extant great ape representatives. All pairs examined here are from single individuals, except for Oreopithecus. Because of the paucity of fossil materials suitable for such kind of analyses and the even more scanty number of currently available/accessible high-resolution records detailing the hominid tooth crown inner structure, in this first study we preferred to maximize the amount of signals, especially at generic level.

We have used the X-ray microtomographic record available to us of specimens which have been previously scanned at: the University of Poitiers, France, by a Viscom X8050-16 system (all extant taxa; original data); the ID 17 beam line of the European Synchrotron Radiation Facility of Grenoble, France (Neanderthals and Oreopithecus; Bayle, 2008, Bayle et al., 2009, Macchiarelli et al., 2006, NESPOS Database, 2017, Zanolli et al., 2016a and Zanolli et al., 2010b); the South African Nuclear Energy Corporation (Necsa), Pelindaba, by a Nikon XTH 225 ST equipment (Paranthropus and Australopithecus; original data); and the analytical platform set at the Bundesanstalt für Materialforschung und -prüfung (BAM) of Berlin, Germany (Ouranopithecus; Macchiarelli et al., 2008 and Macchiarelli et al., 2009).

The data were reconstructed at a voxel size ranging from 21.0 to 83.2 μm, for the extant teeth, and from 21.6 to 50.0 μm, for the fossil specimens. Using Amira v.5.3 (Visualization Sciences Group Inc.) and ImageJ v.1.46 (Schneider et al., 2012), a semiautomatic threshold-based segmentation was carried out following the half-maximum height method (HMH; Spoor et al., 1993) and the region of interest thresholding protocol (ROI-Tb; Fajardo et al., 2002), taking repeated measurements on different slices of the virtual stack (Coleman and Colbert, 2007).

In order to avoid the problem of occlusal wear nearly invariably affecting at least the dm2 in most molar pairs, we uniquely considered lateral enamel. As lateral enamel thickness topography has a profound effect on crown morphology, it is expected to bring a taxon-specific signature, even if likely diluted compared to that provided by total enamel thickness that includes occlusal enamel (e.g., Kono and Suwa, 2008, Macchiarelli et al., 2013 and Suwa et al., 2009). To quantify lateral enamel, the best-fit plane across the cervicoenamel line was firstly set on each crown and the tooth material below this basal plane eliminated (Olejniczak et al., 2008a). Then, a parallel plane to the former, tangent to the lowest enamel point of the occlusal basin, was defined and all material above it was also removed (Macchiarelli et al., 2013 and Toussaint et al., 2010). Only the enamel and dentine portions between these two planes was preserved to estimate tissue proportions.

On the new set of virtually reduced and simplified crowns, five surface and volumetric variables were digitally measured (or calculated): LVe, the lateral volume of enamel (mm3); LVcdp, the lateral volume of coronal dentine, including the lateral coronal aspect of the pulp chamber (mm3); LSEDJ, the enamel-dentine junction (EDJ) lateral surface (mm2); 3D LAET (= LVe/LSEDJ), the three-dimensional lateral average enamel thickness (mm); 3D LRET (= 100*3D LAET/(LVcdp1/3)), the scale-free three-dimensional lateral relative enamel thickness. Intra- and interobserver tests for measurement accuracy run at different times by four observers revealed differences < 4%. For the taxa with N > 1, we have firstly computed the LETDI for each dm2/M1 pair and then calculated the average value.

Pearson correlation tests among the variables listed above show that, in each molar pair, the 3D lateral relative enamel thickness (3D LRET) exhibits the highest correlation (p < 0.01 vs. p < 0.02 for 3D LAET and p < 0.05 for LVE). A “lateral enamel thickness diphyodontic index” (LETDI) has been thus calculated as follows: 3D LRETdm2/3D LRETM1. Statistical analyses were performed with R v.3.2.1 (R Development Core Team, 2017).

To visualize similarities vs. differences in enamel thickness topography within an assemblage of such variably sized and shaped teeth, ad hoc imaging techniques were used to virtually unroll lateral enamel and to project it into standardized morphometric maps (Bayle et al., 2011, Bondioli et al., 2010 and Macchiarelli et al., 2013; for similar imaging techniques, see also Dowdeswell et al., 2017, Morita et al., 2017, Morita et al., 2016, Puymerail, 2011, Puymerail et al., 2012a, Puymerail et al., 2012b and Tsegai et al., 2017). Because each morphometric map (MM) is scaled according to the maximal value of the analysed tooth, the patterns expressed by the dm2s and the M1s are independent from the absolute and relative enamel thickness values. By using a custom routine developed in R v.3.2.1 (R Development Core Team, 2017) with the packages spatstat (Baddeley et al., 2015) and gstat (Pebesma, 2004), enamel thickness values were standardized between 0 and 1 and each morphometric map was set within a grid of 40 columns and 180 rows. We then performed a between-group principal component analysis (bgPCA; Mitteroecker and Bookstein, 2011) based on the standardized morphometric map outputs with the package Morpho v.2.4.1.1 (Schlager, 2017) for R v.3.3.3 (R Development Core Team, 2017).

 Results

The scatterplot of the lateral relative enamel thickness of the dm2 (3D LRETdm2) against the M1 values (3D LRETM1) of all individual and composite (Oreopithecus) tooth pairs used in this study is shown in Fig. 1. We observe little variation among the four extant hominid genera, Gorilla and Pan tending to align with the regression line, whereas Pongo and humans scatter slightly more on both side of this line. Globally, the fossil taxa do not deviate much more than the extant hominids, Australopithecus and Paranthropus being as distant from the regression line as the farthest extant human are. In this context, the composite individual representing Oreopithecus behaves like Pan.

For the ten hominid taxa, the 3D LRET values and those of the LETDI “diphyodontic index” are shown in Table 2. For the LRETdm2, Ouranopithecus (12.0), Paranthropus and the Australopithecus from Sterkfontein (both 10.9) show absolutely thick enamel, while Pongo and Gorilla (global range: 4.7–6.3) and Oreopithecus (6.0) are relatively thin-enamelled. A difference was noticeable between the two African apes, Pan having thicker enamel (6.3–8.9), but on average still thinner than measured in extant humans (8.0–9.2). Enamel in Neanderthals is thinner compared to the extant values (6.4–7.1). As a whole, the decreasing order for the lateral relative enamel thickness of the lower dm2 is as follows: Ouranopithecus > Paranthropus = Australopithecus > extant humans ≥ Pan ≈ Neanderthals > Oreopithecus > Gorilla ≈ Pongo, the variation interval covered by 3D LRET being comprised between 12.0 and 4.7.

Three sets are identifiable for the 3D LRETM1: the first distinguishes the absolutely thickly-enamelled Paranthropus (15.6) and Ouranopithecus (13.4), the second assembles the variably intermediate Homo (all taxa), Pan, Australopithecus and Oreopithecus (range: 8.3–11.8), while the third includes the thinly-enamelled Gorilla and Pongo (range: 5.9–8.1). In this context, Pan (8.8–11.8) is indistinguishable from the extant human condition (9.3–11.2). The Neanderthal range (8.3–9.1) fits the value obtained for Oreopithecus (9.2). Here, the decreasing pattern is as follows: Paranthropus > Ouranopithecus > Australopithecus ≥ extant humans ≈ Pan ≥ Oreopithecus ≈ Neanderthals > Gorilla ≈ Pongo, the values globally ranging from 15.6 to 5.9.

The last column of Table 2 presents the values of the LETDI ratio. LETDI ranges in the whole sample from 0.63 in a Pongo individual and 0.65 in Oreopithecus, to 0.98 in Australopithecus and 0.99 in an extant human (Fig. 2). The totality of the ratios are < 1.0. According to this parameter, even if distinct for its greater amount of enamel, Paranthropus (0.70) is closer to Oreopithecus (0.65) than to Ouranopithecus (0.89), with which it otherwise shares thickly-enamelled dm2 and M1. Within our limited set of investigated cases, Pongo and extant humans display larger variation than Neanderthals and the African apes (Fig. 2).

Distinctly for each taxon and for each molar type, the standardized morphometric maps (MM) imaging the virtually unrolled and projected lateral enamel are shown in Fig. 3. For the extant taxa and Neanderthals, they represent consensus maps generated by merging the available individual records into a single dataset and subsequently calculating the interpolation (Puymerail, 2011, Puymerail et al., 2012a and Puymerail et al., 2012b). Because each MM is scaled according to the maximal value of the analysed tooth, the patterns expressed by the dm2s and the M1s are independent from the absolute and relative enamel thickness values.

In all taxa and both molars, enamel decreases cervically. For the dm2, thickening is commonly found buccally; however, thickening in Ouranopithecus is more evenly distributed around most of the subocclusal contour. The extant human pattern is close to that displayed by the Neanderthal deciduous molars. The African apes and, to a lesser extent, Pongo as well, share similar enamel distribution. In this context, the least contrasted map is that of Paranthropus, which is distinct from Australopithecus and, mostly, from Ouranopithecus, but which in turn recalls that of Oreopithecus. In the MMs of the M1s, thickening is not mainly concentrated buccally, as seen for the dm2s, but more commonly spread buccally/mesiobuccally and also lingually/distolingually. However, this is not exactly the case in Ouranopithecus and, to a lesser extent, in Oreopithecus, where thickening is essentially concentrated mesiolingually, in the former, and distolingually, in the latter. All extant and extinct human representatives show a similar pattern resembling that of Australopithecus. Here again, the signatures displayed by the African apes are similar to the pattern revealed by Pongo, which in turn recalls that of Paranthropus. Finally, in terms of intertooth polarity of the signal, the most similar MMs are those of the extant apes (notably Gorilla and Pongo), while distinct topographic differences are appreciable in Paranthropus and Oreopithecus.

The bgPCA based on the MM scores only provides modest discrimination among the taxa along both bgPC axes (PC1: 56.37%, PC2: 31.11%). However, the representatives of all extant and fossil hominins (HOM, Nea, PAR, AUS) tend to regroup on the positive aspect of bgPC1, whereas the extant apes (PAN, GOR and PON) mostly fall in the negative values along this axis (Fig. 4). The two Miocene hominids (OUR and ORE) show distinct signals, Ouranopithecus being intermediate between Pongo and Homo, while Oreopithecus more closely resembles Gorilla. The specimens in the positive space of bgPC1 display evenly spread relatively thicker enamel deposited towards the more occlusal portion of the entire surface, while the specimens in the negative space of bgPC1 have two vertically projected thickened “pillars” on the buccal and lingual aspects, respectively, separated by two large strips of thinner enamel nearly covering the entire mesial and distal crown sides. Along bgPC2, taxonomic discriminations are weak (Fig. 4).

The correlation between LETDI and body mass is shown in Fig. 5. Varying from good (notably in extant humans, but also in Neanderthals, Pan and Ouranopithecus) to modest (in Gorilla, Pongo and Paranthropus), a certain linear agreement is detectable in both smaller- and larger body-sized taxa. However, even ignoring the perhaps biased signal from the Oreopithecus composite representative, also Australopithecus behaves here as an outlier, a result which deserves confirmation from the inclusion in the analysis of the signal from additional individuals. Interestingly, extant humans and Gorilla, which display comparable average LETDIs, differ in average body mass and the two largest-sized taxa considered in our analysis, Gorilla and Ouranopithecus, provided distinct LETDI ratios.

 Discussion

A limiting/complicating factor in our analytical approach is the use of non-occlusal enamel compared to the information imprinted occlusally, or even at specific cuspal level (e.g., Grine, 2005, Kono et al., 2002, Macho and Berner, 1993, Mahoney, 2010 and Schwartz, 2000b). However, lateral enamel has its own signals that have never been previously explored as a functional unit independent of occlusal enamel thickness. In this sense, our study attempts to do this for the first time. While occlusal enamel topography is more directly informative in terms of functional activity and adaptive responses (e.g., Guy et al., 2013; Kono, 2004, Kono and Suwa, 2008 and Olejniczak et al., 2008b), lateral enamel thickness is also involved in dissipating occlusally-related stresses (Benazzi et al., 2013a and Benazzi et al., 2013b). Lateral enamel also resists wear, tooth height loss and maintains interproximal tooth-tooth contacts during the late stages of tooth wear after dentine exposure over the occlusal surface. Final loss of lateral enamel marks the breakdown of the dentition (Dean et al., 1992) and is significant in the life history of individuals. Nonetheless, it is also possible that the use of the entirely unrolled lateral crown band introduced inessential, or even somehow noisy information. In fact, while individual morphometric maps clearly reveal site-specific differences among the compartments which relate to occlusal cusp shape and topography (Fig. 3), at this stage we did not yet decompose the band into its mesial, distal, buccal and lingual components, and did not examine and compare their sometimes distinctly heterogeneous signatures. This task we would expect to limit the effects of differences in tooth crown architecture, notably outer surface convexity and intercuspal groove depth and extension. This, will in any case require additional research and the development of ad hoc analytical protocols, also because the outline shape of the dm2s and M1s used in this study tends to differ and, notably in the case of intertaxonomic analyses using variably-shaped tooth crowns, a risk of comparing not exactly homologous spots exists.

The expectation, formulated in a purely functional perspective, of LETDI ratios < 1.0 (the M1 crown being in principle equipped with a thicker coating of enamel for resisting higher and prolonged wear-inducing loads) is not fully satisfied by the present results (for enamel proportions in extant human lower dm1s-dm2s-M1s, see Mahoney, 2010). In two representatives from as many taxa it is close to be falsified: an extant human individual (0.99), and the Australopithecus representative (0.98), even if the large majority of the ratios are around or below 0.8. The two minima for the LETDI correspond to Oreopithecus (0.65, a value obtained from two individuals, thus prone to bias) and Paranthropus (0.70). This is interesting, and may be relevant whenever confirmed by additional data, as it might indicate that a large difference between the dm2 and the M1 in the proportional amount of enamel volume deposited along the crown walls occurs in both absolutely thickly-enamelled and relatively thinly-enamelled hominids. In any case, the results (Table 2, Fig. 2) show also that the opposite can occur, i.e., that the deciduous and permanent molars of both thickly-enamelled hominids (e.g., Ouranopithecus) and representatives of relatively thinly-enamelled taxa (e.g., Gorilla, Pongo) may present comparable values of lateral relative enamel thickness (3D LRET). In sum, even if present results tend to support the evidence that primate “deciduous teeth have thinner enamel than permanent teeth” (Swindler, 2002: 14), including in humans (Mahoney, 2010), the extent of their enamel proportions, at least for non-occlusal enamel, appears rather variable.

By definition, the study assumed that the signal revealed by each dm2-M1 crown pair represents the average condition of their own taxon (including for the composite Oreopithecus representative). However, even if molar enamel thickness does not seem to behave as sexually dimorphic (e.g., Hlusko, 2016, Hlusko et al., 2004 and Rossi et al., 1999), a growing body of evidence indicates a considerable amount of interspecific temporal and geographic variation (e.g., Kato et al., 2014, Smith et al., 2011 and Smith et al., 2012). Conversely, the extent of intraspecific variation ranges in most cases from poorly reported to simply unknown, and even in extant humans enamel thickness chrono-geographic variation is far from being appropriately documented (Le Luyer and Bayle, 2017) and, with very few exceptions (e.g., Feeney et al., 2010 and Grine, 2005), most currently available information is limited to European or European-derived population samples (rev. in Le Luyer, 2016; see Zanolli et al., 2017). At any rate, the present evidence based on the limited number of African apes represented here suggests variation in lateral enamel thickness may be similarly large in both deciduous and permanent molars (Table 2).

To interpret the “lateral enamel thickness diphyodontic index” more comprehensively – or indeed any other kind of “enamel thickness diphyodontic index” (ETDI) suitable for appropriately assessing the deciduous-permanent tooth enamel volume proportions (and its distribution pattern as well) – a number of biological, behavioural and ecological factors should be taken into account.

The four extant and four extinct hominid genera represented in our analysis are known for exploiting, or are reported to have exploited, respectively, a wide range of food resources in a variety of diverse environments (Fleagle, 2013, Guatelli-Steinberg, 2016, Hartwig, 2002, Merceron et al., 2005, Nelson and Rook, 2016, Scott et al., 2005, Sponheimer and Lee-Thorp, 2015, Ungar, 2007 and Ungar and Sponheimer, 2013). Depending on the taxon-specific feeding habits, the time spent feeding may be considered as another variable which, together with food abrasiveness, likely plays a role in the selection of enamel thickness because of dental wear resistance, i.e., adaptation is not only resistance to fracture, but also to prolonged periods of wear to which enamel thickness can be related (Grine and Daegling, 2017 and Pampush et al., 2013). The investigative tool used here – the LETDI – did not reveal any immediately obvious link with known dietary and/or ecological niche; for example, relative medium-low (< 0.80) values are shared by Neanderthals, Pan, Gorilla, Pongo, Paranthropus and Oreopithecus, while extant humans, Australopithecus and Ouranopithecus provided medium-high values (> 0.80). We note, anyhow, that in the bgPCA of the morphometric maps (Fig. 4): the more folivorous taxa (Pan, Gorilla, and perhaps Oreopithecus) tend to be in the negative space of bgPC1; Pongo, a slightly more diversified folivorous/frugivorous feeder, is found in the positive space of bgPC1; the omnivorous humans are mostly scattered across the positive space of bgPC1 and the positive space of bgPC2; Paranthropus and Australopithecus, which likely also with Ouranopithecus relied on diverse diets but shared the inclusion of hard/gritty food items, are found in the positive part of bgPC1, but scattered along bgPC2 (Fig. 4); finally, Ouranopithecus sets in the negative space of bgPC2, the dm2 and M1 being spread along the bgPC1 axis. In sum, even if we agree the reliability of enamel thickness as a dietary indicator breaks down in some cases where phylogenetically closely-related species that consume different amounts of hard items are considered (Grine and Daegling, 2017), at a first glance, differences in “dental ecology” (sensu Cuozzo et al., 2012) seem to play a role in affecting the polarity of the dm2/M1 ratio used in the present study. If so, additional research – using any kind of ad hoc ETDI – should be performed on the anterior teeth.

The taxa investigated here are also diverse in body mass (Fleagle, 2013 and Hemmer, 2015), a variable that in extant primates is correlated to a number of life history attributes (e.g., weaning age, age at maturity, age at first breeding in females), as well as to tooth size (e.g., molar crown area) (rev. in Hemmer, 2015). Even if our current analyses comparing LETDI with body size only shows limited linear correlation between these variables, it still represents a promising research track for the future.

Our “diphyodontic index” seems to be poorly or not related to the age at eruption of the first lower permanent molar, another key life history trait which in hominins marks the end of infancy (Kelley and Bolter, 2013). In fact, while also a strong genetic contribution to variation in timing of primary tooth emergence is well documented in humans (Chan et al., 2012), and likely also in hominids (Swindler, 2002), the LETDIs of Pan and of the Australopithecus representative used here that, for example, show comparable ages at LM1 eruption (Hemmer, 2015: table 15), differ markedly.

 Concluding remarks

In a previous study, we noted that “some evidence suggests deciduous versus permanent molar enamel thickness distribution and relative proportions vary among extant and fossil hominid taxa… Inner signatures extracted from the primary and secondary dentition, respectively, may or may not provide similar/comparable pictures of time-related intrataxic evolutionary changes in tooth tissue proportions” (Macchiarelli et al., 2013: 259). The results collected from the present exploratory test using a newly developed analytical tool – the “lateral enamel thickness diphyodontic index” (LETDI) – did not, however, provide an unambiguous and immediately readable picture, as might otherwise have been predictable on the basis of some ontogenetic and morphological studies using sequential teeth (e.g., Bailey et al., 2016, Bailey et al., 2014 and Evans et al., 2016). Rather, our results suggest complex patterns that likely result from the influence of a number of interactive factors. Increasing evidence exists for lifetime-related enamel thickness and dietary wear association in extant primates (e.g., Pampush et al., 2013) and positive selection for adaptation in human evolution has been shown for the genes coding for the enamel matrix proteins (e.g., Daubert et al., 2016 and Horvath et al., 2014). However, given also the high phenotypic plasticity of enamel thickness (e.g., Hlusko, 2016, Kato et al., 2014 and Smith et al., 2012), it is possible that a fraction of the signal provided by any kind of tooth enamel “diphyodontic index” is non-adaptive, or that the degree of adaptability and functional significance of this trait varies topographically across the dentition. With this respect, together with some methodological advancement in the identification of the most reliable parameters and tooth crown areas to be considered for intertaxonomic investigations, a fruitful area of research would be to test the congruence of the “diphyodontic signal” between the anterior and the postcanine dentition, as well as between enamel and the enamel-dentine junction topography.

 Acknowledgements

The present study contributes to the Palevol thematic issue “Hominin biomechanics, virtual anatomy and inner structural morphology: From head to toe. A tribute to Laurent Puymerail”, promoted by C.Z. and R.M. Our friend and colleague Laurent was among the developers of the “unrolling” routine used in this study, and also provided relevant conceptual inputs. With his innovative and thoughtful work, only developed along a terribly short time, Laurent marked the field of “virtual palaeoanthropology”. We acknowledge also his elegant style and sharp intellect. We really miss him.

For having granted or facilitated the access to the materials used in this study, we are very grateful to J.-J. Cleyet-Merle (Les Eyzies-de-Tayac), L. de Bonis (Poitiers), D. Grimaud-Hervé (Paris), G. Koufos (Thessaloniki), S. Potze (Pretoria), L. Rook (Florence), F. Sémah (Paris), F. Thackeray (Johannesburg), J.-F. Tournepiche (Angoulême), L. Trebini (Sassari), H. Widianto (Jakarta), M.D. Wandhammer (Strasbourg). The microtomographic records of the specimens detailed outside the platform set at the Univ. of Poitiers have been realized thanks to the support provided by A. Bravin (Grenoble), F. de Beer (Pelindaba-Johannesburg), J. Hoffman (Pelindaba-Johannesburg), B. Illerhaus (Berlin), C. Nemoz (Grenoble), P. Tafforeau (Grenoble). For discussion, we acknowledge D.M. Alba (Barcelona), J. Braga (Toulouse), F.E. Grine (Stony Brook), J. Kelley (Tempe), L. Rook (Florence).

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Scatterplot of the 3D lateral relative enamel thickness values of the dm2 (3D LRETdm2) against the M1 (3D LRETM1) comparatively assessed in four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE). The red line represents the regression line of the 3D LRETdm2 against 3D LRETM1. AUS: Australopithecus (africanus); GOR: Gorilla (sp.); HOM: extant humans; Nea: Neanderthals; ORE: Oreopithecus (bambolii); OUR: Ouranopithecus (macedoniensis); PAN: Pan (sp.); PAR: Paranthropus (robustus); PON: Pongo (sp.).

Graphique représentant l’indice tridimensionnel d’épaisseur relative de l’émail latéral des dm2s (3D LRETdm2) en fonction des M1s (3D LRETM1) pour quatre genres hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE). La ligne rouge représente la droite de régression de 3D LRETdm2 en fonction de 3D LRETM1. AUS : Australopithecus (africanus) ; GOR : Gorilla (sp.) ; HOM : humains actuels ; Nea : Néandertaliens ; ORE : Oreopithecus (bambolii) ; OUR : Ouranopithecus (macedoniensis) ; PAN : Pan (sp.) ; PAR : Paranthropus (robustus) ; PON : Pongo (sp.).

Boxplots of the “lateral enamel thickness diphyodontic index” (LETDI) comparatively assessed in four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE). AUS: Australopithecus (africanus); GOR: Gorilla (sp.); HOM: extant humans; Nea: Neanderthals; ORE: Oreopithecus (bambolii); OUR: Ouranopithecus (macedoniensis); PAN: Pan (sp.); PAR: Paranthropus (robustus); PON: Pongo (sp.). The boxplot shows the median, the range (lower and upper whisker), the first quartile (lower hinge) and the last quartile (upper hinge).

Graphique illustrant le degré de variation de « l’indice diphyodonte d’épaisseur de l’émail latéral » (LETDI) estimé chez quatre genres d’hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE). AUS : Australopithecus (africanus) ; GOR : Gorilla (sp.) ; HOM : humains actuels ; Nea : Néandertaliens ; ORE : Oreopithecus (bambolii) ; OUR : Ouranopithecus (macedoniensis) ; PAN : Pan (sp.) ; PAR : Paranthropus (robustus) ; PON : Pongo (sp.). Les boîtes à moustaches montrent la médiane, les limites de variation (moustaches inférieure et supérieure), le premier quartile (partie inférieure de la boite) et le dernier quartile (partie supérieure de la boîte).

Standardized morphometric maps of the lateral enamel thickness of the dm2 (left column) and M1 (right) in four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE) rendered by a chromatic scale ranging from thin (blue) to thick (red). For the extant taxa and Neanderthals, the consensus maps representing the “average” condition are shown. AUS: Australopithecus (africanus); GOR: Gorilla (sp.); HOM: extant humans; Nea: Neanderthals; ORE: Oreopithecus (bambolii); OUR: Ouranopithecus (macedoniensis); PAN: Pan (sp.); PAR: Paranthropus (robustus); PON: Pongo (sp.); m: mesial; d: distal.

Cartographies morphométriques standardisées de l’épaisseur de l’émail latéral des dm2 (colonne de gauche) et des M1 (colonne de droite) chez quatre genres d'hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE) représentées par une échelle chromatique allant d’un émail fin (bleu) à épais (rouge). Pour les taxons actuels et les Néandertaliens, les cartes consensus représentant la condition « moyenne » sont illustrées. AUS : Australopithecus (africanus) ; GOR : Gorilla (sp.) ; HOM : humains actuels ; Nea : Néandertaliens ; ORE : Oreopithecus (bambolii) ; OUR : Ouranopithecus (macedoniensis) ; PAN : Pan (sp.) ; PAR : Paranthropus (robustus) ; PON : Pongo (sp.) ; m : mésial ; d : distal.

bgPCA plot based on the standardized morphometric maps (MM) of the dm2 and M1 of four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE). The MMs below the plot show the extreme conditions along bgPC1 and bgPC2. AUS: Australopithecus (africanus); GOR: Gorilla (sp.); HOM: extant humans; Nea: Neanderthals; ORE: Oreopithecus (bambolii); OUR: Ouranopithecus (macedoniensis); PAN: Pan (sp.); PAR: Paranthropus (robustus); PON: Pongo (sp.); m: mesial; d: distal.

Graphique bgPCA basé sur les cartes morphométriques standardisées (MM) des dm2 et M1 chez quatre genres d'hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE). Les MM sous le graphique montrent les conformations extrêmes le long de bgPC1 et bgPC2. AUS : Australopithecus (africanus) ; GOR : Gorilla (sp.) ; HOM : humains actuels ; Nea : Néandertaliens ; ORE : Oreopithecus (bambolii) ; OUR : Ouranopithecus (macedoniensis) ; PAN : Pan (sp.) ; PAR : Paranthropus (robustus) ; PON : Pongo (sp.) ; m : mésial ; d : distal.

Values of the “lateral enamel thickness diphyodontic index” (LETDI) compared with body mass estimates (average values, from Fleagle, 2013) in four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE). The red line represents the regression line of the body mass against the LETDI. AUS: Australopithecus (africanus); GOR: Gorilla (sp.); HOM: extant humans; Nea: Neanderthals; ORE: Oreopithecus (bambolii); OUR: Ouranopithecus (macedoniensis); PAN: Pan (sp.); PAR: Paranthropus (robustus); PON: Pongo (sp.).

Valeurs de « l’indice diphyodonte d’épaisseur de l’émail latéral » (LETDI) comparées aux estimations de masse corporelle (valeurs moyennes, d’après Fleagle, 2013) pour quatre genres d'hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE). La ligne rouge représente la droite de régression de la taille corporelle en fonctions de l’indice LETDI. AUS : Australopithecus (africanus) ; GOR : Gorilla (sp.) ; HOM : humains actuels ; Nea : Néandertaliens ; ORE : Oreopithecus (bambolii) ; OUR : Ouranopithecus (macedoniensis) ; PAN : Pan (sp.) ; PAR : Paranthropus (robustus) ; PON : Pongo (sp.).

List of the extant and fossil hominid taxa considered in the present study with their lateral enamel proportions of the second deciduous (dm2) and first permanent (M1) lower molars.

Liste des taxons hominidés actuels et fossiles considérés dans la présente étude avec les proportions d’émail latéral des secondes molaires déciduales (dm2) et des premières molaires permanentes (M1) mandibulaires.

Table 1 Label N crowns (specimens) Collection/site References Extant taxa  Homo HOM 5 dm2s – 5 M1s MNPELP; PBC Original data  Pan PAN 4 dm2s – 4 M1s Univ. Poitiers & Univ. Toulouse, France Original data  Gorilla GOR 5 dm2s – 5 M1s Univ. Poitiers & Univ. Toulouse, France Original data  Pongo PON 4 dm2s – 4 M1s MZS; Univ. Toulouse, France Original data Fossil taxa  Neanderthals Nea 3 dm2s (RdM, S14–5, S42)3 M1s (RdM, S14–7, S49) Roc de Marsal (RdM) and Abri Suard (S), France Bayle, 2008, Bayle et al., 2009, Macchiarelli et al., 2006 and NESPOS Database, 2017  Paranthropus robustus PAR 1 dm2 – 1 M1 (SK 63) Swartkrans, South Africa Original data  Australopithecus africanus AUS 1 dm2 – 1 M1 (STS 24) Sterkfontein, South Africa Original data  Ouranopithecus macedoniensis OUR 1 dm2 – 1 M1 (RPL–83) Ravin de la Pluie, Macedonia, Greece Macchiarelli et al., 2009  Oreopithecus bambolii ORE 1 dm2 (FS1995#0009)1 M1 (FS1996#Fi98) Fiume Santo, Sardinia, Italy Zanolli et al., 2010a, Zanolli et al., 2016b, Zanolli et al., 2016a and Zanolli et al., 2010b

Three-dimensional lateral relative enamel thickness (3D LRET) of the second deciduous (dm2) and the first permanent (M1) lower molars and “lateral enamel thickness diphyodontic index” (LETDI: 3D LRETdm2/3D LRETM1) comparatively assessed in four extant (HOM, PAN, GOR, PON) and five fossil hominid taxa (Nea, PAR, AUS, OUR, ORE). In parentheses, the number of examined molar pairs.

Indice tridimensionnel d’épaisseur relative de l’émail latéral (3D LRET) des deuxièmes molaires déciduales (dm2) et des premières molaires permanentes (M1) inférieures et « indice diphyodonte d’épaisseur de l’émail latéral » (LETDI : 3D LRETdm2/3D LRETM1) comparés chez quatre genres hominidés actuels (HOM, PAN, GOR, PON) et cinq taxons fossiles (Nea, PAR, AUS, OUR, ORE). Le nombre de paires de molaires examinées est indiqué entre parenthèses.

Table 2 Taxon 3D LRETdm2 3D LRETM1 LETDI HOM Mean 8.4 10.4 0.81 Range (5) 8.0–9.2 9.3–11.2 0.74–0.99 Nea Mean 6.7 8.8 0.76 Range (3) 6.4–7.1 8.3–9.1 0.70–0.85 PAN Mean 7.5 10.1 0.74 Range (4) 6.3–8.9 8.8–11.8 0.67–0.77 GOR Mean 5.4 6.9 0.79 Range (5) 4.7–6.3 5.9–8.1 0.73–0.86 PON Mean 5.0 7.2 0.72 Range (4) 4.8–5.3 5.9–8.0 0.63–0.90 PAR 10.9 15.6 0.70 AUS 10.9 11.1 0.98 OUR 12.0 13.4 0.89 ORE 6.0 9.2 0.65