Bull. Natl. Mus. Nat. Sci., Ser. D, 34 pp. 1–9, December 22, 2008 Enamel Distribution Patterns of Extant Human and Hominoid Molars: Occlusal versus lateral enamel thickness Reiko T. Kono1 and Gen Suwa2 1 Department of Anthropology, National Museum of Nature and Science, Hyakunincho, Shinjuku-ku, Tokyo, 169–0073 Japan E-mail: [email protected] 2 The University Museum, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113–0033 Japan E-mail: [email protected] Abstract Enamel thickness and distribution of the molar crown are important in the study of hominoid and hominid evolution. This is because the 3-dimensional distribution pattern of enamel within the molar crown has been recognized to have significant functional, developmental, and phylogenetic information. In order to analyze aspects of enamel distribution patterns, we quantified average enamel thicknesses separately in the occlusal basin and lateral molar crown, and compared extant great ape and modern human patterns. Interspecific comparisons show that, among modern apes and humans, chimpanzees and bonobos share a unique pattern of molar enamel distribution, in which the occlusal basin enamel is unexpectedly thin. Other taxa share a common enamel distri- bution pattern, greater occlusal than lateral crown average thicknesses, despite the wide range of absolute and relative whole-crown thicknesses. We hypothesize that thin occlusal enamel is a shared derived feature of the genus Pan, possibly an adaptation to relatively soft-fruit frugivory. Key words : Enamel thickness, Hominoid, Molar, Occlusal fovea increase in the application of micro-CT technolo- Introduction gy, a series of investigations has now been pub- Average enamel thickness was initially defined lished that report comparable 3D measures of in 2-dimensional cross sections of molars as AET (Smith et al., 2006; Olejniczak et al., enamel area divided by enamel-dentine junction 2008a, b, c, d). (EDJ) length (Martin, 1983). It was devised as a In these studies, AET is presented as a more convenient method of describing and comparing appropriate measure of overall enamel thickness overall thickness of the enamel cap and has been than the conventional 2-dimensional version of the standard method of describing enamel thick- Martin (1983), a point that we agree with (e.g., ness until recently (e.g., Dean and Schrenk, Kono, 2004). However, there are some differ- 2003; Martin et al., 2003; Smith et al., 2003, ences between some of our results, such as with 2004, 2005). We first reported volumetric and Pongo. We are not sure yet if this stems from surface area measures of enamel and dentin por- small sample biases, different populational or tions of the crown (Kono et al., 2002), and pro- subspecies representations of our respective ceeded to compare modern human and extant species samples (see also discussion in Ole- great ape molars by means of the 3-dimensional jniczak et al., 2008b), and/or methodological dis- (3D) version of Martin’s (1983) average enamel crepancies such as in defining the cervix for thickness, that is, enamel volume divided by EDJ measurement. surface area (variable AET) (Kono, 2002; Kono, Aside from the 3D-based quantification of 2004). Subsequent to this work, with the rapid overall enamel thickness, it has been recognized 2 Table1. Descriptive statistics of the 3-dimensional average enamel thickness variables. AET OAET LAET Species Tooth n Mean SD Range Mean SD Range Mean SD Range Homo sapiens LM1 9 1.314 0.125 1.065–1.512 1.531 0.144 1.280–1.805 1.239 0.124 0.986–1.417 LM2 9 1.283 0.135 1.071–1.507 1.600 0.201 1.324–1.875 1.168 0.124 0.941–1.359 LM3 3 1.210 0.172 1.019–1.352 1.466 0.197 1.283–1.675 1.113 0.163 0.925–1.226 UM1 3 1.405 0.067 1.345–1.478 1.627 0.136 1.495–1.767 1.326 0.041 1.290–1.371 UM2 3 1.456 0.180 1.256–1.604 1.788 0.273 1.474–1.968 1.337 0.144 1.179–1.460 UM3 3 1.195 0.114 1.064–1.266 1.479 0.124 1.350–1.598 1.095 0.102 0.981–1.177 Pan troglodytes LM1 9 0.787 0.058 0.709–0.869 0.738 0.110 0.546–0.863 0.809 0.054 0.710–0.880 andGenSuwa Kono T. Reiko LM2 7 0.816 0.058 0.752–0.894 0.754 0.066 0.665–0.824 0.843 0.084 0.775–0.995 LM3 1 0.798 0.790 0.801 UM1 5 0.851 0.141 0.691–1.031 0.810 0.139 0.671–0.959 0.870 0.146 0.690–1.065 UM2 7 0.807 0.091 0.713–0.940 0.749 0.096 0.586–0.846 0.832 0.107 0.690–0.985 UM3 1 0.904 0.963 0.875 Pan paniscus LM1 7 0.729 0.071 0.627–0.799 0.714 0.071 0.616–0.809 0.736 0.074 0.629–0.808 LM2 5 0.804 0.136 0.627–0.991 0.740 0.177 0.475–0.951 0.837 0.118 0.707–1.012 LM3 — UM1 6 0.680 0.090 0.566–0.786 0.668 0.074 0.551–0.746 0.685 0.099 0.567–0.803 UM2 4 0.726 0.085 0.610–0.814 0.711 0.106 0.558–0.791 0.734 0.080 0.633–0.824 UM3 — Pongo pygmaeus LM1 3 0.963 0.145 0.839–1.122 1.155 0.128 1.064–1.301 0.886 0.155 0.748–1.053 LM2 3 1.061 0.097 1.004–1.173 1.251 0.079 1.161–1.310 0.978 0.115 0.891–1.108 LM3 — UM1 1 0.897 0.993 0.860 UM2 1 1.075 1.207 1.014 UM3 — Gorilla gorilla LM1 3 0.889 0.121 0.763–1.004 1.015 0.164 0.856–1.182 0.840 0.102 0.730–0.932 LM2 3 1.089 0.086 1.016–1.183 1.220 0.073 1.163–1.303 1.027 0.091 0.951–1.128 LM3 1 1.232 1.359 1.164 UM1 1 0.880 0.990 0.840 UM2 1 1.051 1.227 0.976 UM3 1 1.223 1.448 1.121 Occlusal versus Lateral Enamel Thickness of Hominoid Molars 3 that the 3D distribution pattern of enamel within face (see below). the crown would have significant functional, de- In the present study, we report the values of re- velopmental, and phylogenetic information (e.g., gional AETs separated into occlusal basin and Kono et al., 2002; Kono, 2004; Suwa and Kono, the lateral crown face portions. Comparisons are 2005; Suwa et al., 2007; Olejniczak, 2008a, d). also made with the whole crown AET. The enam- In contrast to earlier studies in 2-dimensions (e.g., el distribution characteristics of the extant homi- Schwartz, 2000), we investigated 2D-based linear noid species will be discussed from these per- measures of enamel thickness in some detail, and spectives. found that occlusal and some definitions of later- al enamel thicknesses were methodologically un- Materials stable, and potentially misleading in characteriz- ing both local and overall regional enamel thick- A hominoid sample consisting of permanent nesses (Suwa and Kono, 2005; Kono and Suwa, molars of Pan troglodytes (nϭ30), Pan paniscus 2005). Therefore, 3D-based measures of enamel (nϭ22), Gorilla gorilla (nϭ10), Pongo pyg- thickness, of various molar regions of interest, maeus (nϭ8), and thirty permanent molars of are highly sought, which, so far, has been at- Japanese Homo sapiens from Jomon (nϭ10), tempted only by the Kono (2004) study. In partic- Yayoi (nϭ1), Kamakura (nϭ1), and Edo periods ular, that study quantified a contrast between (nϭ18) were analyzed in this study. Sex is un- enamel thickness of the occlusal fovea and the known for most of the specimens, since they lateral crown, with Pan troglodytes and Pongo were taken from juvenile individuals. pygmaeus molars exhibiting comparatively thin The chimpanzee, gorilla, and orangutan teeth and thick occlusal enamel, respectively. Recently, are mostly from the collection of the Cleveland occlusal thickness of the enamel has gained at- Museum of Natural History. The bonobo speci- tention from a functional standpoint, as a poten- mens are from the collection of the Royal Muse- tial adaptation to withstanding occlusal forces in- um of Central Africa, Tervuren. A small part of duced by mastication of hard and large food ob- the chimpanzee and orangutan molars are housed jects (e.g., Lucas et al., 2008; Vogel et al., 2008). either at the University of California, Berkeley, In our first attempt to quantify regional enamel The University Museum, The University of distribution patterns within a molar (Kono, 2002, Tokyo, or National Museum of Nature and Sci- 2004), overall enamel thickness within the oc- ence, Tokyo. The Jomon Japanese molars were clusal fovea and the lateral face of each cusp was taken from the skeletal collection housed in The represented by the mean value of minimum University Museum, The University of Tokyo. enamel thickness from each pixel of the outer The Yayoi, Kamakura and Edo period specimens enamel surface in the adopted projected orienta- derive from the skeletal collections housed in the tion of view (for details, see Kono, 2004).