Three-dimensional primate molar enamel thickness
Introduction
Quantitative investigations of primate enamel thickness, spurred by an interest in the taxonomic status of the genus Ramapithecus (now a junior synonym of Sivapithecus), were undertaken by Gantt, 1977, Gantt, 1982, Kay, 1981, and Martin, 1983, Martin, 1985. These studies (1) demonstrated the utility of enamel thickness quantification for distinguishing primate taxa, (2) brought enamel thickness into the fold of dental characters used to interpret paleodiet, and (3) represented a sequence of methodological refinement, culminating in the method of measuring enamel thickness that is the most widely used today (Martin, 1983, Martin, 1985).
In recent years, enamel thickness has been highlighted in diagnoses of fossil taxa (e.g., Conroy et al., 1992, Begun and Kordos, 1993, White et al., 1994, Brunet et al., 1995, Pickford and Ishida, 1998, Asfaw et al., 1999; Haile-Selassie, 2001; Leakey et al., 2001, Senut et al., 2001, Brunet et al., 2002), largely due to the diagnostic dichotomy between relatively thin-enameled African apes and relatively thick-enameled hominins (e.g., Martin, 1985; for more recent evaluations of this dichotomy, see Shellis et al., 1998, Kono, 2004; and Smith et al., 2005). Enamel thickness has also been measured in several previously described fossil hominoid taxa (e.g., Martin and Andrews, 1984, Grine and Martin, 1988, Andrews and Martin, 1991, Beynon et al., 1998, Dean and Schrenk, 2003, Smith et al., 2003, Smith et al., 2004), and enamel thickness has been measured in representatives of all major extant primate radiations (e.g., Shellis et al., 1998, Ulhaas et al., 1999, Schwartz, 2000, Martin et al., 2003, Grine et al., 2005).
Previous studies of enamel thickness were typically performed on cross sections of molars produced via histological methods or by grinding the tooth to the location of the desired plane and polishing that surface (e.g., Martin, 1983, Martin, 1985, Beynon et al., 1998, Shellis et al., 1998, Ulhaas et al., 1999, Grine, 2002, Grine, 2005, Martin et al., 2003, Smith et al., 2003, Smith et al., 2004, Smith et al., 2005, Smith et al., 2006a). Recent advances in microtomographic imaging techniques have advanced methods for measuring enamel thickness in two notable ways. First, these imaging techniques facilitate the nondestructive production of virtual planes of section, preserving valuable museum collections and making available for analysis fossils that could not be physically sectioned (e.g., Kono, 2004, Tafforeau, 2004, Tafforeau et al., 2006, Olejniczak and Grine, 2005, Olejniczak, 2006, Smith et al., 2006b). Second, these techniques facilitate the analysis of three-dimensional, whole-crown enamel thickness measurements, rather than limiting measurements to a single plane of section (e.g., Kono, 2004, Tafforeau, 2004, Olejniczak, 2006).
Recent microtomographic studies of whole-crown primate molar enamel thickness have concentrated on the evolution of anatomically modern humans (e.g., Suwa and Kono, 2005, Smith et al., 2006b), great apes (e.g., Kono, 2004, Tafforeau, 2004), and lesser apes (Olejniczak, 2006). This focus on hominoid primates is warranted, as enamel thickness is among the characters distinguishing hominins from African apes, and the first standardized two-dimensional studies of enamel thickness were performed on hominoid molars (e.g., Martin, 1983). Studies of two-dimensional enamel thickness in other primates, however, have provided important tests of functional and phylogenetic hypotheses regarding enamel thickness. For example, these studies have explored molar adaptations to hard-object feeding (e.g., Dumont, 1995, Martin et al., 2003) and the relationship between body size and cross-sectional tooth dimensions (e.g., Shellis et al., 1998).
As nondestructive techniques for measuring enamel thickness become commonplace, data from a wide range of taxa with varying dietary proclivities will prove useful as a basis for comparison. The goal of this study is to generate, and make widely available, volumetric primate enamel thickness measurements based on microtomographic imaging and a taxonomically broad sample.
Section snippets
Materials and methods
The molars studied here represent 16 primate genera, including representatives of all three extant anthropoid superfamilies and a small sample of strepsirrhines (Table 1). A total of 182 molars were studied, with sample sizes for each taxon ranging from a single molar (e.g., Cebus apella) to 39 molars (Homo sapiens). Each molar was scanned using microCT to produce a high-resolution image stack suitable for imaging and accurately measuring internal and external dental features (such as the
Results
Results of the 3D enamel thickness measurements for each tooth examined in this study appear in Appendix A. Variable definitions and units are as given in the materials and methods section. Summary enamel thickness measures (both absolute and relative) for each genus are given in Table 2.
Results of 3D measurements of great ape and human molar relative enamel thickness are in general agreement with 2D studies. Gorilla molar enamel is relatively the thinnest (mean RET3D = 9.77), and Homo molar
Discussion
The relative enamel thickness data presented here are generally in agreement with measurements from 2D sections (see Appendix B in Martin et al., 2003). Hominoid primates have a wide range of relative enamel thicknesses, with Gorilla and Symphalangus at the thin end of this range, followed in order from relatively thinnest to relatively thickest enamel by Pan, Hylobates, Pongo, and Homo. Cercopithecoids have intermediate relative enamel thickness, which is greater than that of ceboids, on
Acknowledgements
We are grateful to Tanya Smith and Jean-Jacques Hublin for the invitation to present this work at the Max Planck Institute for Evolutionary Anthropology, as well as our fellow workshop participants. Fred Grine provided access to some of the material studied here, for which we are grateful. We also thank Fred Grine, Callum Ross, Stefan Judex, Bill Kimbel, Tanya Smith, Jean-Jacques Hublin, and two anonymous reviewers for critical comments on aspects of this manuscript. The European Synchrotron
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