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: rtkono@kahaku.go.jp
2
The University Museum, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo, 113–0033 Japan
E-mail: suwa@um.u-tokyo.ac.jp

Abstract Enamel thickness and distribution of the molar crown are important in the study ofhominoid 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, andphylogenetic 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 comparedextant 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 distribution pattern, greater occlusal than lateral crown average thicknesses, despite the wide range ofabsolute and relative whole-crown thicknesses. We hypothesize that thin occlusal enamel is ashared derived feature of the genus Pan, possibly an adaptation to relatively soft-fruit frugivory.
Key words : Enamel thickness, Hominoid, Molar, Occlusal fovea

Introduction
Average enamel thickness was initially definedin 2-dimensional cross sections of molars as
enamel area divided by enamel-dentine junction(EDJ) length (Martin, 1983). It was devised as aconvenient method of describing and comparing
overall thickness of the enamel cap and has been
the standard method of describing enamel thickness until recently (e.g., Dean and Schrenk,
2003; Martin et al., 2003; Smith et al., 2003,
2004, 2005). We first reported volumetric andsurface area measures of enamel and dentin portions of the crown (Kono et al., 2002), and proceeded to compare modern human and extant
great ape molars by means of the 3-dimensional(3D) version of Martin’s (1983) average enamel
thickness, that is, enamel volume divided by EDJ
surface area (variable AET) (Kono, 2002; Kono,
2004). Subsequent to this work, with the rapid

increase in the application of micro-CT technology, a series of investigations has now been published that report comparable 3D measures of
AET (Smith et al., 2006; Olejniczak et al.,2008a, b, c, d).
In these studies, AET is presented as a more
appropriate measure of overall enamel thickness
than the conventional 2-dimensional version ofMartin (1983), a point that we agree with (e.g.,
Kono, 2004). However, there are some differences between some of our results, such as with
Pongo. We are not sure yet if this stems fromsmall sample biases, different populational orsubspecies representations of our respective
species samples (see also discussion in Olejniczak et al., 2008b), and/or methodological discrepancies such as in defining the cervix formeasurement.
Aside from the 3D-based quantification of
overall enamel thickness, it has been recognized

LM1
LM2
LM3
UM1
UM2
UM3
LM1
LM2
LM3
UM1
UM2
UM3
LM1
LM2
LM3
UM1
UM2
UM3
LM1
LM2
LM3
UM1
UM2
UM3
LM1
LM2
LM3
UM1
UM2
UM3

Homo sapiens

Gorilla gorilla

Pongo pygmaeus

Pan paniscus

Pan troglodytes

Tooth

9
9
3
3
3
3
9
7
1
5
7
1
7
5
—
6
4
—
3
3
—
1
1
—
3
3
1
1
1
1

n

0.889
1.089
1.232
0.880
1.051
1.223

0.897
1.075

0.963
1.061

0.121
0.086

0.145
0.097

0.090
0.085

0.680
0.726

0.763–1.004
1.016–1.183

0.839–1.122
1.004–1.173

0.566–0.786
0.610–0.814

0.627–0.799
0.627–0.991

0.691–1.031
0.713–0.940

0.141
0.091
0.071
0.136

1.065–1.512
1.071–1.507
1.019–1.352
1.345–1.478
1.256–1.604
1.064–1.266
0.709–0.869
0.752–0.894

Range

0.125
0.135
0.172
0.067
0.180
0.114
0.058
0.058

SD

1.314
1.283
1.210
1.405
1.456
1.195
0.787
0.816
0.798
0.851
0.807
0.904
0.729
0.804

Mean

AET

1.015
1.220
1.359
0.990
1.227
1.448

0.993
1.207

1.155
1.251

0.668
0.711

1.531
1.600
1.466
1.627
1.788
1.479
0.738
0.754
0.790
0.810
0.749
0.963
0.714
0.740

Mean

Descriptive statistics of the 3-dimensional average enamel thickness variables.

Species

Table 1.

0.164
0.073

0.128
0.079

0.074
0.106

0.071
0.177

0.139
0.096

0.144
0.201
0.197
0.136
0.273
0.124
0.110
0.066

SD

OAET

0.856–1.182
1.163–1.303

1.064–1.301
1.161–1.310

0.551–0.746
0.558–0.791

0.616–0.809
0.475–0.951

0.671–0.959
0.586–0.846

1.280–1.805
1.324–1.875
1.283–1.675
1.495–1.767
1.474–1.968
1.350–1.598
0.546–0.863
0.665–0.824

Range

0.840
1.027
1.164
0.840
0.976
1.121

0.860
1.014

0.886
0.978

0.685
0.734

1.239
1.168
1.113
1.326
1.337
1.095
0.809
0.843
0.801
0.870
0.832
0.875
0.736
0.837

Mean

0.102
0.091

0.155
0.115

0.099
0.080

0.074
0.118

0.146
0.107

0.124
0.124
0.163
0.041
0.144
0.102
0.054
0.084

SD

LAET

0.730–0.932
0.951–1.128

0.748–1.053
0.891–1.108

0.567–0.803
0.633–0.824

0.629–0.808
0.707–1.012

0.690–1.065
0.690–0.985

0.986–1.417
0.941–1.359
0.925–1.226
1.290–1.371
1.179–1.460
0.981–1.177
0.710–0.880
0.775–0.995

Range

2
Reiko T. Kono and Gen Suwa

Occlusal versus Lateral Enamel Thickness of Hominoid Molarsthat the 3D distribution pattern of enamel within
the crown would have significant functional, developmental, and phylogenetic information (e.g.,
Kono et al., 2002; Kono, 2004; Suwa and Kono,
2005; Suwa et al., 2007; Olejniczak, 2008a, d).
In contrast to earlier studies in 2-dimensions (e.g.,
Schwartz, 2000), we investigated 2D-based linear
measures of enamel thickness in some detail, andfound that occlusal and some definitions of lateral enamel thicknesses were methodologically unstable, and potentially misleading in characterizing both local and overall regional enamel thicknesses (Suwa and Kono, 2005; Kono and Suwa,
2005). Therefore, 3D-based measures of enamel
thickness, of various molar regions of interest,
are highly sought, which, so far, has been attempted only by the Kono (2004) study. In particular, that study quantified a contrast between
enamel thickness of the occlusal fovea and thelateral crown, with Pan troglodytes and Pongo
pygmaeus molars exhibiting comparatively thinand thick occlusal enamel, respectively. Recently,
occlusal thickness of the enamel has gained attention from a functional standpoint, as a potential adaptation to withstanding occlusal forces induced by mastication of hard and large food objects (e.g., Lucas et al., 2008; Vogel et al., 2008).
In our first attempt to quantify regional enamel
distribution patterns within a molar (Kono, 2002,
2004), overall enamel thickness within the occlusal fovea and the lateral face of each cusp wasrepresented by the mean value of minimum
enamel thickness from each pixel of the outer
enamel surface in the adopted projected orientation of view (for details, see Kono, 2004). Thiswas done as an alternative to the more appropriate volume to surface area ratio, defined separately for occlusal and lateral face enamel. The
reason the latter method was not applied at thattime was because we had not yet developed thesoftware tools then to accurately segment enamelvolume and EDJ surface area into occlusal andnon-occlusal portions. Some attention is needed
regarding how to 3-dimensionally define andconnect the outer enamel surface occlusal fovea
perimeter with the same of the internal EDJ sur-3

face (see below).
In the present study, we report the values of regional AETs separated into occlusal basin andthe lateral crown face portions. Comparisons arealso made with the whole crown AET. The enamel distribution characteristics of the extant hominoid species will be discussed from these perspectives.
Materials
A hominoid sample consisting of permanentmolars of Pan troglodytes (n30), Pan paniscus(n22), Gorilla gorilla (n10), Pongo pygmaeus (n8), and thirty permanent molars ofJapanese Homo sapiens from Jomon (n10),
Yayoi (n1), Kamakura (n1), and Edo periods(n18) were analyzed in this study. Sex is unknown for most of the specimens, since they
were taken from juvenile individuals.
The chimpanzee, gorilla, and orangutan teeth
are mostly from the collection of the Cleveland
Museum of Natural History. The bonobo specimens are from the collection of the Royal Museum of Central Africa, Tervuren. A small part ofthe chimpanzee and orangutan molars are housed
either at the University of California, Berkeley,
The University Museum, The University ofTokyo, or National Museum of Nature and Science, Tokyo. The Jomon Japanese molars weretaken from the skeletal collection housed in The
University Museum, The University of Tokyo.
The Yayoi, Kamakura and Edo period specimens
derive from the skeletal collections housed in the
National Museum of Nature and Science, Tokyo.
Molars were chosen to have no wear, or slightest wear. With the non-human hominoid species,
molars with slight occlusal wear and/or small
chipping of enamel around the cervical region
were included. In some cases where these losses
were noticeable, digital interpolation and/or reconstruction was attempted to virtually approximate the original conditions.

4

Reiko T. Kono and Gen Suwa

Fig. 1. Methods of obtaining occlusal enamelvolume. Top row, surface rendered occlusal
views of OES (left) and EDJ (right); themasked fovea regions are shaded. Bottom tworows, the same 3-dimensional volume data
after separating the occlusal fovea (left) andlateral crown (right) enamel portions, shown in
different orientations.

Methods
Explanations have been given elsewhere regarding our micro-CT based digitization, 3D reconstruction, and metric quantification methods(Kono, 2004; Suwa and Kono, 2005). In the present study, details of two new variables are presented, in addition to the original, whole crownaverage enamel thickness (AET) (Kono, 2004).
OAET is defined as the average enamel thicknessof the occlusal fovea, calculated as the occlusal
enamel volume divided by the EDJ surface areaof the occlusal fovea. Average thickness of thelateral molar crown, LAET, is defined as the mutual complement of the OAET, lateral enamelvolume divided by the lateral crown face EDJ

surface area, with both volumes and surface
areas non-overlapping and summing to wholecrown enamel volume and EDJ surface area.
These variables correspond to the previous
study’s OAMT and LAMT (Kono, 2004), but are
different as described above.
Occlusal enamel volume was measured as follows (Fig. 1). First the entire volume data of the
tooth is rotated so that the projected area of the
EDJ occlusal fovea region (which is well definedby sharp margins) is maximized. The optimal
amount of rotation is calculated iteratively andempirically determined (see Suwa and KonoTakeuchi, 1998; Suwa and Kono, 2005). Oncethe orientation of the entire volume is adjusted,
the topographic boundaries of the EDJ and outer
enamel surface (OES) are determined in 3-dimensional space in order to digitally reconstruct
the enamel cap morphology of the tooth crown.
Next, in standardized projection (EDJ occlusal
fovea maximized), the occlusal foveae of boththe EDJ and the OES are delineated. This is doneby defining the fovea region on the rendered occlusal view surface images with the aid of contour lines drawn at the elevation intervals of
about 100 or 200 microns. The space between
these two surfaces is 3-dimensionally filled, bybridging the gap between the perimeters of thetwo occlusal surfaces with triangle panels. The“shortest diagonal” algorithm (Christiansen andSederberg, 1978) is employed in the triangulation; when adding a new triangle, the shorter oneof the two diagonals between the two contours is
selected as the side of the triangle. This algorithm is applicable when the numbers of points
on the two contour loops are not equal, such as in
the present case (the OES perimeter usually contains more points than the EDJ).
A new volume file is created that consists of
only the voxels inside the occlusal fovea. Occlusal enamel volume is calculated as the numberof voxels multiplied by voxel size. In order to obtain average occlusal enamel thickness (OAET),
this volume is divided by EDJ occlusal fovea surface area (as delineated above).
The volume of the lateral crown face enamel isOcclusal versus Lateral Enamel Thickness of Hominoid Molars

5Fig. 2. Regional average enamel thicknesses, LAET and OAET, are plotted against the whole crown AET, respectively. The line of equality between the two variables is shown as the diagonal. Open circles, Homo sapiens; open triangles pointing upwards, Pan troglodytes; open triangles pointing downwards, Pan paniscus; diamonds, Pongo pygmaeus; crosses, Gorilla gorilla.

obtained by subtracting the occlusal portion described above from the entire crown enamel volume. The denominator of LAET, lateral crownface EDJ surface area, is obtained in the sameway (subtracting occlusal EDJ surface area fromwhole crown EDJ surface area).
The measurements were obtained by use of the
3D image analysis software (3D- and CT-Rugle,
Medic Engineering Inc., Kyoto), in part developed through our research project. Specifically,
for the present study, a special routine was codedby Toyohisa Tanijiri to generate the above defined occlusal fovea volume file.
Results
The basic statistics of AET, OAET, and LAETof the five species are summarized in Table 1.
When comparing the values of these variables, inmost cases, the whole crown AET values are in
between those of the two partial AETs. In otherwords, OAET is the largest, AET follows this,and LAET is the smallest. This is not surprisingbased on the fact that the lateral crown averageenamel thickness includes the cervical part

where the enamel becomes thinner and finallyterminates. In the case of the two species of Pan,
however, the mean values of LAET surpass thoseof OAET in all tooth types examined except forthe single UM3 of the common chimpanzee.
To depict this pattern more clearly, the twopartial average enamel thickness values wereplotted against the whole-crown AET (Fig. 2). In
the individual molars of Homo, Gorilla, andPongo, OAET and LAET values lie, respectively,
above and beneath the line that represent equality
with the AET. To the contrary, both OAET and
LAET are nearly equal to AET in the molars of
the two Pan species, with OAET being slightly(or substantially, in several specimens) smaller
than AET. This confirms that the molars of both
species of Pan share a unique pattern of unexpectedly thin occlusal fovea enamel.
Discussion
The present results on the distribution patternof enamel within the molar crown correspond to
our previous results and interpretations based ona cruder method (OAMT and LAMT of Kono,

6

Reiko T. Kono and Gen Suwa

2004). The bonobo is investigated for the firsttime, and it is clearly shown that the pattern ofmolar enamel distribution is similar to that of thecommon chimpanzee. This implies that theuniquely thin occlusal enamel is a derived featureof the genus Pan. Both species of Pan are known
to be a frugivore strongly dependent on ripe
fruits (e.g. White, 1992; Tutin and Fernandez,1993).
Vogel et al. (2008) has recently shown that thefruits actually consumed by common chimpanzees at Kibale are indeed softer than those
consumed by the Kalimantan orangutans. Kono
(2004) previously noted that the thin occlusal
enamel of the common chimpanzee molar has
the effect of emphasizing the EDJ occlusal
topography at the OES, with the combined effectof simultaneously forming a capacious occlusal
fovea and retaining prominent cuspal topography.
It was proposed that the capacious occlusal fovea
may be advantageous in ripe fruit frugivory,
while the retention of salient cusps may be effective in some degree of shear (Kono, 2004). Vogelet al. (2008) compared the actual food items andtheir physical properties of P. troglodytes andPongo pygmaeus. They interpreted the broad occlusal basin of Pan molars endowed by thin
enamel as an adaptation to relatively soft andjuicy fruits, but also noted that the common
chimpanzee do in fact rely on leaves as fallback
food items.
However, Vogel et al. (2008) differ from ourinterpretations in considering Pongo molars tohave higher cuspal topography (after Ungar,
2006), and therefore better shearing capabilities
than in Pan. We ourselves view Pongo as in fact
having reduced cusp saliency (Kobayashi, 2000,
and data not shown), perhaps because of hardfruit eating. In a large primate with absolutely
large masticatory forces (Lucas et al., 1994) anddocumented dependency on not only tough but
also hard food objects (Vogel et al., 2008), we
hypothesize that salient cusps would be prone tobreakage. Selection to reduce cuspal topography
might then override selection to retain saliency,
despite the need for shearing tough foodstuff (asFig. 3. Box plot of the index OAET divided by
LAET. Lower and upper molars are pooled. A
single Hylobates specimen is added for reference.

was also probably the case in Australopithecus).
Our results on enamel distribution of thebonobo molars are instructive in this regard. The
Pan paniscus molars show the same occlusal
fovea pattern as seen in those of the common
chimpanzee, but has actually a more enhanced
cusp saliency (Yamaguchi, 2008, and data notshown). Kinzey (1984) had previously noted
qualitative difference in the details of molar
cresting pattern and interpreted the bonobo condition to be better suited for shearing. It is known
that the bonobo relies more heavily on THV for
their non-fruit part of the diet (e.g. Malenky andWrangham, 1994). This suggests that cusp
saliency in Pan is in fact related to their partial
reliance on fibrous food items.
The other three genera lack the characteristic
enamel distribution pattern seen in Pan. This wasmost clearly seen when comparing the relationship of occlusal versus lateral enamel thicknesses(Fig. 3). Gorilla is known to have the thinnest
molar enamel among the extant great apes in relative enamel thickness, while modern humans
have the thickest, and orangutans have intermediate enamel thicknesses. It is striking that the oc-Occlusal versus Lateral Enamel Thickness of Hominoid Molars

7Fig. 4. Enamel thickness color maps of the upper second molar. Thickness is measured as the minimum distance from a given point on the OES to the EDJ surface. Color scale is adjusted so that the maximum thickness of the specimen approximately corresponds to the maximum value of the scale. Size is also adjusted forcomparison. Top row, from left to right, Pan troglodytes, Pan paniscus, Hylobates lar; bottom row, from left
to right, Gorilla gorilla, Pongo pygmaeus, Homo sapiens.

clusal versus lateral enamel distribution pattern iscommon to the three genera, despite the large
differences among the three in both absolute andrelative (size adjusted) AET values (Martin,
1983; Kono, 2004; Olejniczak et al., 2008b).
Furthermore, 3D-data of Australopithecus (datanot shown) and Miocene apes such as Gigantopithecus (Olejniczak et al., 2008a), Chororapithecus (Suwa et al., 2007), Dryopithecus, Lufengpithecus, Ouranopithecus (data not shown) as well
as 2D-cross section imagery of a wide range ofMiocene apes (e.g. Andrews and Martin, 1991;
Beynon et al., 1998; Smith et al., 2003, 2004;
Chaimanee et al., 2003, 2006; Pickford and Kunimatsu, 2005; Kunimatsu et al., 2007) strongly
suggest that the Pan condition is the rarer and derived condition.
Among the hominoids we have examined sofar, only the hylobatids share the derived condition with chimpanzees and bonobos (Fig. 3 and
4). Since hylobatids are phylogenetically an outgroup of all the taxa examined in this study andmost if not all of the above cited Miocene ape
genera, the most parsimonious hypothesis would
be that Hylobates evolved the derived Pan-likepattern independently, perhaps in response tosimilar dietary and masticatory needs. According
to our interpretations above, this would involve
relatively soft-fruit frugivory and some amountof fibrous leaves. Hylobatid species more dependent on leaves should tend to have more salient
cusps. Clearly this should be evaluated from larger samples of hylobatid species with divergent dietary dependencies.

8

Reiko T. Kono and Gen Suwa

Acknowledgements
We would like to thank following individuals
and institutions for access to their valuable specimens; Tim D. White, University of California at
Berkeley; Bruce Latimer, Cleveland Museum of
Natural History, Wim Van Neer and Wim Wendelen, Royal Museum of Central Africa, Tervuren. Special thanks to the Japan Monkey Center for allowing access to the gibbon specimens.
We thank T. Tanijiri for his long-term collaboration in developing the Rugle software series. This
study was partly supported by the Sasakawa Scientific Research Grant from the Japan Science
Society, and the Grant-in-Aid for Scientific Research from the Japan Society for the Promotionof Science.
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