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Enamel thickness of human maxillary molars reconsidered.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 92389-200 (1993)
Enamel Thickness of Human Maxillary Molars Reconsidered
GABRIELE A. MACHO AND MARGIT E. BERNER
Hominid Palaeontology Research Group, Department of Human Anatomy
and Cell Biology, The University of Liuerpool, England (G.A.M.),and
Department of Anthropology, Natural History Museum Vienna, Austria
(M.E.B.)
KEY WORDS
Enamel
Maxillary molars
thickness,
Orofacial
biomechanics,
ABSTRACT
Forty-four modern human maxillary molars (M1 = 21,
M2 = 12, and M3 = 11) were sectioned through the mesial cusps in a plane
perpendicular to the cervical margin of the crown. Eight measurements of
enamel thickness as well as bucco-lingual (BL) and mesio-distal (MD) diameters were recorded for each tooth in order to investigate differences in these
dimensions between tooth categories. Uni- and multi-variate analyses revealed first maxillary molars to have generally thinner enamel than second or
third upper molars, especially with regard to the occlusal basin. Furthermore,
the decrease of MD diameters from anterior to posterior is greater than that of
BL diameters. Principal Component Analysis using enamel thickness measurements resulted in complete separation of first molars, while second and
third maxillary molars showed a certain amount of overlap. This finding casts
doubt on using an overall measure of “molar enamel thickness” derived from
mixed samples of molars for taxonomic purposes. There appears to be a
relationship between bite force and enamel thickness such that posterior
molars, where masticatory forces are stronger, have thicker enamel than
anterior teeth. It is suggested that the gradient of enamel thickness between
(and within) teeth in extant and extinct species may thus provide further
information about relative wear resistance as well as the biomechanical constraints of the orofacial skeleton. o 1993 Wiley-Liss, Inc.
Recently, enamel thickness has become
an important character in taxonomic studies
of hominoids. For instance, Homo and Pongo
are usually characterised as thick-enamelled hominoids, whereas Gorilla and Pan
have relatively thinner enamel (Gantt,
1983; Martin 1983,1985;Molnar and Gantt,
1977). The thick enamel in Pongo has been
interpreted as either retention of the primitive condition (Martin, 1985) or as parallelism (Beynon et al., 1991). [In his earlier
work Schwartz (e.g., 1984) argued that thick
enamel in Pongo and Homo could also be a
synapomorphy (but see Schwartz, 199011.
All hominids, however, are thick enamelled.
The functional importance of enamel has
also been noted (Molnar and Gantt, 1977)
and different dietary regimes are associated
with varying enamel thicknesses (Andrews
0 1993 WILEY-LISS, INC
and Martin, 1991; Gantt, 1983; Kay, 1981;
Shellis and Hiiemae, 1986; Simons, 1976). It
has been established that species that habitually eat fruits and hard objects, such as
seeds and nuts, or are adapted to a more
varied diet, have thick enamelled molars
(e.g., Andrews and Martin, 1991; Kay,
1981). By contrast, folivorous taxa are characterised by thin-enamelled molars but
thick-enamelled incisors, in particular lingually (Shellis and Hiiemae, 1986). However, even within an individual tooth differ-
Received September 21,1992; accepted April 29, 1993.
Address correspondence to Dr. Gabriele A. Macho, Hominid
Palaeontology Research Group, Dept. of Human Anatomy and
Cell Biology, University of Liverpool, P.O. Box 147, Liverpool
L69 3BX, England.
190
G.A. MACHO AND M.E. BERNER
ences in enamel thickness are observed and
are explained in terms of differential functional demands placed on the individual
cusps; in maxillary molars enamel is thicker
on lingual cusps than it is on buccal cusps,
whereas the opposite holds for lower molars.
This unequal distribution appears to be
common to all Primates which have been
examined (Molnar and Gantt, 1977; Molnar
and Ward, 1977).
Although it would seem that taxonomic
and functional differences are well documented, inspection of the literature led us to
conclude that most of these assumptions are
based on small sample sizes. The maximum
number of either upper or lower molars
studied for any one species in the published
literature is nine (Grine and Martin, 1988),
which is too small to determine differences
between various molars. While early studies
were broad and innovative and helped resolve questions pertaining to the taxonomic
status of the later Miocene hominoids, the
paucity of data for individual tooth types
leaves open the possibility for further investigation. In order to enlarge sample sizes
used to measure hominoid enamel thicknesses, non-destructive techniques, such as
Computed Tomography (CT), have recently
been employed (Conroy, 1991; Grine, 1991;
Macho and Thackeray, 1992; Spoor et al.,
1993). Owing to inherent limitations of CT,
however, data derived from actual sections
of teeth seem essential to establish the true
range of variation of enamel thickness
among hominoids. This is especially true
where small dimensions are concerned, such
as enamel thickness in the cervical region in
all teeth or in the case of occlusal enamel
thickness in thin-enamelled species (Spoor
et al., 1993).
In this study we report the amount of variation in enamel thickness found among
maxillary molars of a modern human population. Moreover, we discuss our findings
with regard to recent taxonomic studies that
have placed great significance on enamel
thickness. Since a n understanding of the relationship between structure and function is
essential if we aim to comprehend the adaptive significance of variation observed between extant and extinct taxa or to contribute to arguments about evolutionary trends,
emphasis is placed on the functional importance of differences in enamel thickness between molars.
MATERIAL AND METHODS
Thirty-eight unworn modern human maxillary molars (M1 = 16, M2 = 12, M3 = 10)
were used in the present study. I n addition,
five slightly worn M‘s and one slightly worn
M3 were incorporated for reasons given below, but measurements made in these worn
areas were coded a s missing values. The
teeth were dissected from subadult skulls
which came from the Slavic necropolis near
Zwentendorf, Lower Austria (10th century
A.D.). They were excavated by F. Hampl between 1953 and 1958 (Ladenbauer-Orel,
1971), and are now housed in the Department of Anthropology, Natural History Museum Vienna, Austria. A detailed anthropological description of the material is still
pending.
Prior to sectioning, bucco-lingual (BL) and
mesio-distal (MD) diameters were recorded
and Carabelli’s cusps were scored (Scott,
1980) and each tooth was photographed in
occlusal view. Thereafter, teeth were cut
through the tips of the mesial cusps and perpendicular to the base of the crown with a n
ISOMET 11-1180 low speed annular saw
producing a 300 pm wide cut. In cases where
the tips of the dentine horns were not cut
precisely, the tooth was ground down with a
rotating diamond disk to the optimum plane
section. Thereafter the cut faces were polished with aluminum oxide paste.
Each tooth was photographed under a
Wild-Photomakroskop at a magnification of
1% Measurements of enamel thickness
were recorded in millimeters from enlarged
photographs (1:lO) using a Vernier sliding
caliper. Figure 1 lists the eight measurements of enamel thickness taken for each
tooth. Measurements 1 and 8 were taken
perpendicular to the dentino-enamel junction (DEJ) from a point on the DEJ intersecting with a reference plane through the
lowest point of the DEJ between the cusps
and parallel to the crown base (Macho and
Thackeray, 1992). Measurements 3 and 7
correspond to CT(L) and CT(B) (Grine and
Martin, 1988), while variables 4 and 6 are
those defined as “i” and “ h by Martin
ENAMEL THICKNESS DISTRIBUTION BETWEEN MAXILLARY MOLARS
I
L
B
Fig. 1. Cross-section through maxillary molar showing enamel thickness measurements used in this study.
(1983). Variable 2 is the maximum thickness of the latero-lingual enamel measured
a s perpendicular as possible to both the DEJ
and the enamel surface (Spoor et al., 1993).
Finally, enamel thickness a t the intercuspal
fissure was taken from the deepest point a t
the occlusal surface to the lowest point of the
DEJ. The amount of sexual dimorphism in
enamel thickness of M3s was also analysed
for specimens which could be reliably sexed
on the basis of sexually dimorphic features
of the skull and the postcranium.
In order to test for measurement errors
five teeth were repeatedly measured by both
authors on 3 consecutive days. In none of the
instances did the differences exceed 1% of
the dimension.
RESULTS
Table 1 lists the descriptive statistics of
all measurements taken. Decrease of buccolingual and mesio-distal diameters from
anterior to posterior through the series
M1-M2-M3was statistically significant, although reduction of the MD diameter was
more marked than that of the BL diameter.
Analysis of variance (ANOVA) was also
employed to test for differences in enamel
thickness between tooth types. Six out of
eight measurements yielded statistically
significant results a t the 1% probability
level. Only the greatest thickness along the
lingual slope of the protocone (variable 2)
and the thickness at the intercuspal fissure
(variable 5) did not differ between teeth.
Since enamel thickness of molars with reported Carabelli traits were within the
range of values reported for other molars,
191
these teeth were included in all statistical
analyses. These similarities in enamel
thickness were unexpected but may have
been due to the fact that the sections were
almost exclusively posterior to the Carabelli
cusp; this may account for the fact that the
measurements taken were unaffected.
The statistically significant trends regarding changes in enamel thickness from
M1 to M3 are graphically depicted in Figure
2. Each bar shows the mean value, standard
deviation and 95% of the observations.
While enamel thickness along the tooth
walls (variables 1 and 8) tends to decrease
posteriorly, all other enamel thickness values increase markedly from first maxillary
molars to the more posterior teeth. Overall,
the occlusal basin (variables 3 , 4 , 6 , and 7) is
endowed with thicker enamel in posterior
molars than it is anteriorly (Table 1,Figure
2). Standard deviations and, hence, the coefficients of variance, decrease in a posterior
direction for enamel thicknesses along the
cusp tips (variables 3 and 7); the differences
are statistically significant at the 5% probability level using Cochrans C'test.
In order to test whether these statistically
significant increases can be explained by individual variation or by sexual dimorphism
within the sample, three approaches were
taken. First, we compared measurements
taken from slightly worn Mls with those of
M'S from the same individual (Table 2 ) . Owing to the delayed maturation in humans, all
individuals with fully formed second molar
crowns had first molars which had already
erupted and were slightly worn. However,
differential wear in these specimens was
minimal and, more importantly, did not affect all areas where measurements were
taken. Hence, comparisons were carried out
(Table 2). It should be noted that for all other
analyses measurements taken from these
worn areas were coded as missing values. In
all instances, measurements of the occlusal
basin (variables, 3 , 4 , 6, and 7) and measurement 2 were smaller in first molars, although
all MD and most of the BL diameters became
smaller distally. With regard to enamel thickness at the intercuspal fissure there appears
to be a slight trend towards decrease from anterior to posterior. Similarly, almost all dimensions of the tooth sides (variables 1and 8 )
G.A. MACHO AND M.E. BERNER
192
TABLE 1. Descriptive statistics of enamel thickness measurements (in millimeters) taken, analysis of variance (ANOVA),
and nrobabilitv levels
Measurement
MD
t
Median
SE
SD
Min-max
BL
P
Median
SE
SD
Min-max
No. 1
!
!
X
Median
SE
SD
Min-max
No. 2
8
X
Median
SE
SD
Min-max
No. 3
E
X
Median
SE
SD
Min-rnax
No. 4
E
X
Median
SE
SD
Min-max
No. 5
P
Median
SE
SD
Min-max
N
No. 6
Ti
Median
SE
SD
Min-max
No. 7
2
Median
SE
SD
Min-max
No. 8
E
X
Median
SE
SD
Min-max
N
M’
M2
M3
10.51
10.60
0.15
0.67
9.4-1 1.8
21
11.57
11.70
0.11
0.48
10.5-12.4
21
1.56
1.53
0.06
0.27
1.10-2.15
21
1.80
1.80
0.04
0.20
1.41-2.16
21
1.59
1.65
0.07
0.27
1.07-1.99
16
1.30
1.26
0.04
0.19
0.9G1.66
21
1.18
1.16
0.06
0.25
0.75-1.72
21
1.25
1.24
0.03
0.15
0.9G1.50
21
1.04
1.06
0.07
0.30
0.39-1.64
17
1.34
1.36
0.04
0.17
0.95-1.61
21
9.13
9.30
0.19
0.65
8.0-10.2
12
11.45
11.50
0.19
0.64
10.cL12.8
12
1.41
1.28
0.09
0.31
1.08-2.04
12
1.99
1.92
0.06
0.19
1.77-2.26
12
2.17
2.17
0.10
0.34
1.6G2.74
12
1.62
1.59
0.06
0.19
1.35-1.96
12
1.17
1.24
0.14
0.47
0.31-2.06
12
1.73
1.66
0.07
0.26
1.29-2.20
12
1.62
1.64
0.07
0.24
1.141.97
12
1.01
0.91
0.09
0.30
0.59-1.47
12
8.64
8.40
0.20
0.68
7.7-1 0.0
11
10.45
10.20
0.28
0.92
9.cL11.9
were smaller in M2s than they were in MIS.
This pattern of differences between teeth of
the same individual clearly match the crosssectional data presented above.
11
1.18
1.20
0.06
0.17
0.84-1.40
10
1.87
1.93
0.11
0.34
1.11-2.24
10
2.09
2.05
0.04
0.13
1.94-2.30
10
1.59
1.54
0.07
0.22
1.3cL20.2
Anova
F-value
P
34.20
0.0000
11.26
0.0001
7.90
0.0010
2.42
0.1014
19.27
0.0000
13.02
0.0000
0.14
0.8674
30.41
0.0000
26.82
0.0000
7.92
0.0012
11
1.11
1.18
0.12
0.38
0.27-1.69
11
1.78
1.87
0.08
0.27
1.41-2.21
11
1.64
1.66
0.05
0.16
1.3S1.88
11
1.08
1.00
0.10
0.32
0.62-1.48
11
Second, it could be argued that there
might be a strong bias in sex-ratios between
different age categories due to differential
mortality rates. A demographic analysis of
ENAMEL THICKNESS DISTRIBUTION BETWEEN MAXILLARY MOLARS
193
-
Changes in Enamel Thickness M I M3
3.0
No.1
2.5
I
I
I
I
No.3 I
I
I
I
I
No. 4
I
I
I
2.0
1.5
1.o
.5
2.5
2.0
1.5
1.o
.5
Fig. 2. Plot of mean values and standard deviation (stippled) of six enamel thickness measurements,
by tooth type. Light bars represent 95%of the individuals sampled. (Units on the right are millimeters.)
contemporary skeletal series from the same
region, however, does not support this argument. Stloukal and Vyhnanek (1976) found
mortality rates of girls and boys between 6
years of age and adulthood mortality rates
to be approximately the same. Similarly, in
modern human populations, there is only a
slight bias towards increased male mortal-
ity after the age one (Statistisches Handbuch fur die Republik Osterreich, 1971). It is
thus unlikely that one particular age group
studied here is exclusively represented by
one sex only.
Finally, individuals whose M3s were analysed could be sexed on the basis of sexually
dimorphic features of the skull and the post-
G.A. MACHO AND M.E. BERNER
194
TABLE 2. Enamel thickness of teeth of the same individual as well as differences between M's and w s
No.
Tooth
MD
BL
No. 1
No. 2
No. 3
No. 4
No. 5
No. 6
No. 7
NO. 8
M'
M2
10.0
9.3
1 -0.7
10.2
8.9
-1.3
10.6
9.4
-1.2
9.4
8.1
-1.3
9.9
9.6
-0.3
11.7
11.7
1.10
1.24
0.14
1.83
1.50
-0.33
1.54
1.18
-0.36
1.32
1.08
0.24
1.61
1.57
-0.04
2.02
2.07
0.05
1.83
1.92
0.09
2.13
2.26
0.13
1.59
1.80
0.21
1.64
1.90
0.27
11.571
2.34
[0.77]
[1.011
2.12
11.111
L1.661
2.60
10.931
L1.311
1.87
10.561
L0.501
1.81
[1.31]
1.22
1.64
0.42
1.05
1.35
0.30
1.50
1.81
0.31
1.09
1.35
0.26
1.16
1.59
0.43
1.29
1.15
-0.14
0.98
1.17
0.18
1.76
1.34
-0.42
1.31
1.28
-0.03
1.07
0.71
-0.35
1.23
1.78
0.55
1.23
1.56
0.33
1.39
2.20
0.81
1.03
1.29
0.26
1.17
1.56
0.39
0.65
1.70
11.051
L1.161
1.80
L0.641
[l.lOl
1.65
10.551
11.011
1.38
[0.371
L1.641
1.83
[0.191
1.37
0.83
-0.55
1.45
1.13
-0.33
1.59
1.34
-0.25
1.16
0.59
-0.57
1.40
1.24
-0.17
41
~
51
77
156
178
2
.
~
M'
M"
M2-Mi
M'
MZ
M2-M'
M'
M2
M2-M'
M'
M"
M2-M'
o
11.8
11.5
-0.3
12.0
11.5
-0.5
10.5
10.0
-0.5
10.8
11.4
0.6
i J indicates wear.
TABLE 3. Differences in crown diameters and enamel
thickness between sexes
'
Males
Measurement
MD
x
SD
8.85
0.83
4
N
No.2
x
SD
No. 3
No. 4
x
SD
N
-
X
SD
10.65
1.15
4
1.21
0.10
4
1.70
0.41
4
2.01
0.07
3
1.47
0.12
4
1.11
SD
SD
N
0.34
4
1.81
0.30
4
1.60
0.07
4
1.05
0.37
4
Females
8.51
0.61
7
10.33
0.85
7
1.16
0.22
6
1.98
0.27
6
2.12
0.14
7
1.66
0.24
7
1.11
0.43
7
1.76
0.27
7
1.66
0.19
7
1.10
0.31
7
t-value
0.77
P
TABLE 4 . Coefficients of first two Principal Components
(PC) based on a Principal Component Analysis using siz
enamel thicknesses, which were significantly different
between teeth
0.46
Measurement
0.53
0.61
0.38
0.72
-1.33
0.22
-1.23
0.25
-1.43
0.19
-0.03
0.98
0.29
0.78
-0.58
0.58
-0.24
0.82
IT-tests between males and females and probability levels are also
given. Only specimens with M3s could be reliably sexed.
cranium. There were no statistically significant differences between the sexes in any of
the measurements studied when t-tests
were employed (Table 3). Although male
teeth were on average larger with regard to
BL and MD diameters, there were no systematic trends in enamel thicknesses in ei-
~
Principal Component Analysis
_.
PC
I1
PC I _
~
_
_
No.8
0.23
-0.48
-0.46
-0.49
-0.48
0.20
0.64
0.21
0.31
0.08
-0.00
0.66
B variance
55.10
26.91
No. 1
No.3
No. 4
No.6
No. 7
_
ther sex (although females have an average
thicker enamel). On the grounds of these
findings any bias towards a particular sex
within a specific age category seems unlikely.
Principal Component Analysis was then
employed using the six enamel thickness
measurements which were significantly different between molars (Table 1,Fig. 2). This
multivariate technique was chosen in order
to explore the relationships between a number of variables and to analyse the patterning of enamel thickness between teeth. Table 4 lists the coefficients of the first two
components, which account for 82% of the
variance. Interestingly, PCI discriminates
on shape, while only PCII can be regarded as
a size component. This interpretation is
based on the fact that the loadings of the
coefficients of the first variate have different signs whereas those ofthe second component are all positive (Bilsborough, 1984).
(There is one negative coefficient in PCII,
.
~
ENAMEL THICKNESS DISTRIBUTION BETWEEN MAXILLARY MOLARS
195
Principal Component Analysis
+4
::I
+1
2
2
2
.
:
:
1
1
1
1
33
1
I:
1
1 ’ 1 1
.............
-3-
I
Fig. 3. Plot of first two Principal Components
(M’ = 1; M2 = 2; M3 = 3).
however, which equals a value of zero.) PCI
contrasts enamel thickness along the tooth
walls (variables 1 and 8) with that of the
occlusal basin (variables 3, 4, 6, and 7). As
can be seen in Figure 3 this component entirely separates first maxillary molars from
the remaining molars.
Of the sexed individuals neither males
nor females had exclusively positive or negative coefficients. Thus, the possibility of
PCII being a sex component can be discarded.
DISCUSSION
Previous studies stressed the importance
of hominoid enamel thickness for taxonomic
purposes, although the range of variation in
extant taxa had not yet been adequately established. The present study set out to provide such a yardstick for maxillary molars of
Homo supiens. Interestingly, enamel thickness not only displays a great amount of
variation but, more importantly, it differs
systematically between molars. Figure 4
shows the typical distribution of enamel
among upper molars. The marked difference
occurs in enamel thickness at the occlusal
basin of first maxillary molars as compared
with those of second and third molars. By
and large, enamel is thinnest in M’s. Fur-
Fig. 4. Ground sections through mesial cusps of M’,
M2. and M3.
thermore, separation is complete when more
than one variable is studied simultaneously
by Principal Component Analysis (Fig. 3).
This complete distinction between molars
within a single human population raises
questions pertaining to the usefulness of calculating a measure of overall “molar enamel
thickness” for taxonomic purposes. At the
protocone (variable 3) enamel increases by
36%from anterior to posterior, whereas the
increase at the paracone (variable 7) is even
more substantial, with a value of some 56%
(Table 1,Fig. 2). It is interesting to note that
Aiello et al. (1991) also suggest that relative
enamel thickness tends to increase distally
in extant hominoids with M,s displaying the
thickest enamel. Similarly, in their study on
Plio-Pleistocenehominids Beynon and Wood
196
G.A. MACHO AND M.E. BERNER
TABLE 5 . Total crown area (mrn’), BL, and MD
diameters of one first and one second maxillary molar
used in this studv‘
Formulae usually employed to calculate
“relative”or “average” enamel thickness are
those devised by Martin (1983, 1985). MarM’
MZ
(M2/M1) * 100
tin maintained that the total area of enamel
Total crown area
101.5
81.3
80.1%
divided by the length of the DEJ, measured
11.6
BL
11.9
97.5%
from
a section of a tooth through the dentine
MD
11.1
8.8
79.3%
horns and perpendicular to the crown base,
Relatcuelaverage enamel thickness
Area of enamel cap “c”
26.4
31.9
120.8%
approximates the ideal measure of enamel
Area of dentine “b”
47.3
48.4
97.7%
volume
over the surface area of the DEJ. In
22.2
20.7
Length of DEJ “e”
93.210
ele
1.2
125.0%
1.5
the present study various “relative”and “av111.9%
(6
* 100)
109.5
122.5
erage” enamel thicknesses (sensu Grine and
131.010
17.1
L(c/e)/m * 100
22.4
Martin, 1988) were calculated for two teeth
2.92
143.8%
[(var3 + v a r 7 ) / 2 1 / 6
2.03
145.8%
2.05
I(var4 + v a r 6 ) 1 2 1 / 6
2.99
(Table 5).
‘For both teeth various “relative” and “average” enamel thicknesses
The values obtained render some of the
were calculated (Grine and Martin, 1988) and the percentage differassumptions on which these formulae are
ences between both teeth were computed.
based questionable. Since teeth can rarely
be regarded as square or circular objects the
length of the DEJ in one sectioned plane
(1986) indicate that there might be a slight cannot provide information about its length
trend towards enamel thickness increase in another. As a case in point, the first and
from anterior to posterior, i.e., between pre- the second molars in Table 5 have almost
molars and molars. However, they refrained equal BL diameters as well as lengths of the
from commenting on differences between DEJs in the sectioned plane, but they differ
markedly in MD diameters and total crown
molars due to small sample sizes.
Inspection of Martin’s (1983) data re- area (determined by planimetry). As a convealed enamel thickness at the cusp tips to sequence, although all measures of relative
be smallest in M’s in Pan troglodytes, Go- enamel thickness are greater in the second
rilla gorilla, Pongo pygmaeus, and Homo sa- maxillary molar, the relative volume over
piens; in Pongo both values are greatest in the surface area of the DEJ of the second
M3s, whereas in all other taxa either M2 or molar would still be underestimated, priM3 yielded the greatest figure, but the dif- marily because the equation has not taken
ferences between these posterior teeth are into account the shorter outline of the DEJ
not marked. Overall, the percentage in- (about 20%) in MD direction. Similar probcrease in enamel thickness calculated for lems would also arise when teeth of different
these species is comparable, or even greater, shapes, such as upper and lower molars, are
to that found in this investigation. Simi- compared. It should also be borne in mind
larly, the distribution of enamel along the that within a tooth posterior cusps tend to be
occlusal basin (i.e., variables 4 and 6) from lower than anterior cusps both at the crown
anterior to posterior in Pan and Pongo par- (e.g., Kanazawa et al., 1983, 1988; Mayhall
allel the findings for Homo of the present and Kanazawa, 1989) and the DEJ (see Corstudy. All three taxa exhibit smallest values ruccini and Holt, 1989; Korenhof, 1960,
in M’s. However, Grine and Martin (1988) 1982; Sakai and Hanamura, 1971, 1973).
using various measures of “relative enamel Furthermore, even when the value of “relathickness” observed only “. . . a slight ten- tive” enamel thickness is regarded as repredency for relative enamel thickness to in- senting only one particular section rather
crease from M1 to M3 . . .” (p. 12) within the than the enamel volume of the tooth crown,
same set of data. This could imply that the the derived values may not provide sensitive
overall amount of enamel over the tooth enough measures to equalize within-species
crown may be similar between molars differences. Percentage differences in “relawithin a species, but that the patterning of tive” and “average” enamel thickness found
enamel over the tooth crown changes from between M1 and M2 range from 112 to 145%
anterior to posterior. This assumption was (Table 5 ) . These figures are substantial and
are similar to those obtained for linear meatested (Table 5 ) .
ENAMEL THICKNESS DISTRIBUTION BETWEEN MAXILLARY MOLARS
surements in this study. Therefore, while
measures derived from these formulae tend
to obscure the patterning of enamel thickness, which may yield valuable information
(Macho and Thackeray, 1992; Macho and
Berner, in press), they do not appear to
provide a good estimate of “relative” andlor
“average” enamel thickness within species.’
Hence, only linear measurements were considered in the present study.
Studies on extant and extinct species
placed emphasis on developmental processes in order to explain the differences
found, since identical developmental pathways would constitute strong evidence for
homology (Grine and Martin, 1988). Molar
crown formation times are broadly similar
in pongids (Dean and Wood, 1981; but see
Anemone et al., 1991), hominids (Beynon
and Wood, 1987), and modern humans
(Shellis, 1984; and references therein). Although Shellis’ (1984) data on crown formation times are slightly different from those
presented by other investigators, it is significant that he gives the smallest value for
second molars. Bearing in mind the similarities in crown formation times the marked
differences in enamel thickness between
taxa are striking, as are those between
human maxillary molars. Differences in
enamel thickness between genera have been
explained by varying enamel extension and
enamel secretion rates (Beynon and Dean,
1988; Beynon and Wood, 1986, 1987). In
modern humans deciduous and permanent
teeth follow a different pattern of enamel
formation as well, whereby the former exhibit a faster enamel extension and secretion rate than the latter. With regard to our
data the question arises whether differences
in enamel thicknesses between maxillary
molars can be explained by these processes
as well.
A slowing of enamel formation during the
cervical stage of crown formation and a proportional increase of appositional enamel in
posterior teeth is common to all hominoids
(Beynon and Dean, 1988; Dean, 1987). Re-
‘The scaling factor CORl used by Beynon and Wood (1986) is
thus a more useful measure since it takes the tooth crown area of
each tooth category into account.
197
cently, evidence has been provided that
ameloblasts of posterior maxillary molars
may exhibit slightly reduced secretion rates
in humans (Beynon et al., 1991). Based on
these studies we may infer that the thicker
enamel of posterior molars may be attained
through an increased extension rate during
the cuspal stage and/or through a prolonged
ameloblast secretory activity in the cuspal
region. Both of these mechanisms could occur without prolonging total crown formation. Either explanation would also explain
the thickened enamel along the buccal wall
of the protocone (variable 4) and the lingual
wall of the paracone (variable 6), and would
account for relatively thin enamel approaching the cervix or the intercuspal fissure
(variable 5).
Investigation of enamel thickness, results
of the Principal Component Analysis, and
the coefficients of variation all indicate that
thickening of enamel in second and third
maxillary molars is systematic rather than
random. On the other hand, BL and MD diameters decrease from anterior to posterior
in this population, while the coefficients of
variation of these measurements increase
(Table 1).Dental anthropologists usually interpret this phenomenon of greater variation of crown diameters posteriorly as support of the field concept (Butler, 1939;
Dahlberg, 1945). According to this theory
the genetic influence diminishes with increasing distance from the most stable tooth
within the morphogenetic field. An alternative hypothesis is that the impact of environmental factors increases resulting in a reduction and greater variability of posterior
teeth. It is worthy of note, therefore, that in
the present study the variation for enamel
thickness at the cusp tips of the protocone
and the paracone are both statistically
smallest in M3s (Table 1, Fig. 2). In light of
the expectations raised by the field theory
this finding is surprising and we surmise
here that functional constraints most probably account for this observation.
The successive eruption of molars with a
spacing of about 6 years in modern humans
would logically imply that first molars have
the thickest enamel, since they are in functional occlusion for a longer period of time
than all other molars. On the other hand,
198
G.A. MACHO AND M.E. BERNER
second and, possibly, third maxillary molars
may be subjected to stronger masticatory
forces than M’s (Benfer and Edwards, 1991;
Koolstra et al., 1988; Mansour and Reynick,
1975a, 1975b; Molnar and Ward, 1977;
Ward and Molnar, 1980). Molnar and Ward
(1977)reported that axial forces tended to be
greatest in distal maxillary molars, although mesial cusps were always more
loaded than distal ones within a tooth (Ward
and Molnar, 1980). Benfer and Edwards
(1991) suggested that the greater wear of
second molars, as judged by crown height,
could be explained by a change towards
mastication of coarser foods in adults as well
as by a shift of mastication to the posterior,
less worn and, hence, more efficient molars.
Although this seems to be a likely interpretation, another explanation comes to mind.
During ontogeny the mandible experiences a major restructuring, especially after
the eruption of first molars (Enlow, 1975a;
Ricketts, 1975). The ramus of the mandible
becomes increased in vertical dimensions,
thereby accommodating the marked downward enlargement of the nasomaxillary
complex (Enlow, 1975b).These developmental changes have an effect on the position of
the temporo-mandibular joint with relation
to the occlusal plane and, moreover, result
in a marked shift of the position of the masseter-pterygoid complex with regard t o the
dental arcade. Based on analogy with extant
mammals one is led to infer that ontogenetic
changes of the masticatory complex in humans lead to an efficient system adapted to
prolonged, forceful grinding (DuBrul, 1974;
Hannam and Wood, 1989; Hylander et al.,
1987; Osborn, 1987; Smith, 19781, although
this is only expressed to the full after eruption of second molars. The morphology of
posterior molars indicates that they are well
adapted to sustain enhanced and prolonged
masticatory loads, an interpretation which
is in keeping with previous findings on
changes of cusp areas within complete upper
tooth rows (Macho and Moggi Cecchi, 1992).
Anthropologists have investigated the
causal relationships between trophic adaptations and facial topographies, and they
have also attempted to relate diet with
enamel thickness. Yet the association between orofacial morphology and enamel
thickness remains open to investigation. For
instance, among hominid species Australopithecus (Paranthropus) robustus and A.(P.)
boisei have relatively orthognathic faces
with a retracted, albeit long, palate, a forward position of the anterior root of the zygoma, and molarized premolars when compared to A. africanus and Homo (see, for
instance, Rak, 1983; Kimbel et al., 1984);
“robust” australopithecine dentognathic
morphology is generally thought t o be indicative of a trophic adaptation. On the basis of
the present study one would predict that the
antero-posterior gradient of enamel thickness in “robust”australopithecines would be
different (less) than that inA. africanus and
Homo, in spite of differences in absolute values. This would occur because more of the
molar teeth are slung under the masseterpterygoid complex in A.(P.) robustus and
A.(P.) boisei. Further work on enamel thickness employing Computed Tomography may
shed light on this hypothesis of differential
gradients of enamel thickness among PlioPleistocene hominids (Macho and Thackeray, in preparation).
In conclusion, the present results as well
as inspection of published data indicate that
differences in distribution of enamel thickness between maxillary molars may be similar in all hominoids, although absolute values differ. This finding casts doubt on the
usefulness of calculating “overall molar
enamel thickness” for taxonomic purposes.
Furthermore, the patterning of enamel distribution between maxillary molars implies
that in the course of evolution enamel thickness has responded to prolonged masticatory loads and increased bite force magnitudes in posterior teeth as reported by
biomechanical studies. Although there is a
clear relationship between orofacial morphology and functional adaptation, more detailed analyses are needed to elucidate the
relationship between enamel thickness and
orofacial morphology. While enamel thickness per se may be indicative of dietary
adaptations, the gradient of enamel distribution between teeth could provide information about biomechanical constraints on the
masticatory apparatus. We contend that an
understanding of the functional adaptation
of enamel thickness within and between
ENAMEL THICKNESS DISTRIBUTION BETWEEN MAXILLARY MOLARS
hominid taxa will substantially contribute
to our understanding of the evolutionary
history and functional adaptations of Homo
sapiens.
ACKNOWLEDGMENTS
We are grateful to J. Szilvassy for permission to section the material and to H.
Kritscher for advice. G. Kurat and H. Schonmann allowed us to use the technical instruments in their care. Special thanks are due
to G. Sverak for his invaluable help with the
sections and to G. Franzke for his help with
the figures. The constructive criticism and
of M.C. Dean on an earlier draft of the
manuscript is greatly appreciated. We
thank C . Wood and two anonymous referees
for their valuable comments.
LITERATURE CITED
Aiello LC, Montgomery C, and Dean C (1991) The natural history of deciduous tooth attrition in hominoids.
J. Hum. Evol. 21:397412.
Andrews P, and Martin L (1991) Hominoid dietary evoLond. B 334t199-209.
lution. Phil. Trans. R. SOC.
Anemone RL, Watts ES, and Swindler DR (1991)Dental
development of known-age chimpanzees, Pun troglodytes (Primates, Pongidae). Am. J. Phys. Anthrop.
86t229-241.
Benfer RA, and Edwards DS (1991) The principal axis
method for measuring rate and amount of dental attrition: Estimating juvenile or adult tooth wear from
unaged adult teeth. In MA Kelley and CS Larsen
(eds.): Advances in Dental Anthropology. New York:
Wiley-Liss, pp. 325-340.
Beynon AD, and Dean MC (1988) Distinct dental development patterns in early fossil hominids. Nature
335t509-514
Beynon AD, and Wood BA (1986) Variations in enamel
thickness and structure in East African hominids.
Am. J. Phys. Anthrop. 70r177-193.
Beynon AD, and Wood BA (1987) Patterns and rates of
enamel growth in the molar teeth of early hominids.
Nature 326:493-496.
Beynon AD, Dean MC, and Reid DJ (1991) On thick and
thin enamel in hominoids. Am. J. Phys. Anthrop.
86t295-309.
Bilsborough A (1984) Multivariate analysis and cranial
diversity in Plio-Pleistocene hominids. In GN van
Vark and WW Howells (eds.):Multivariate Statistical
Methods in Physical Anthropology. Dordrecht: D. Reidel Publishing Company, pp. 351-375.
Butler PM (1939) Studies of the mammalian dentition:
Differentiation of the postcanine dentition. Proc. Zool.
SOC.London 109B;l-36.
Conroy GC (1991) Enamel thickness in South African
Australopithecines: Noninvasive evaluation by computed tomography. Palaeont. Afr. 28:53-59.
199
Corruccini RS, and Holt BM (1989) The dentinoenamel
junction and the hypocone in primates. Hum. Evol.
4t253-262.
Dahlberg AA (1945) The changing dentition of man. J.
Am. Dent. Assoc. 32r676-690.
Dean MC (1987) Growth layers and incremental markings in hard tissues. A review of the literature and
some preliminary observations about enamel structure
in Purunthropus boisei. J. Hum. Evol. 16t157-172.
Dean MC, and Wood BA (1981) Developing pongid dentition and its use for ageing growth studies. Folia Primatol. 36r111-127.
DuBrul EL (1974) Origin and evolution of the oral apparatus. Front. Oral Physiol. 1:l-30.
Enlow DH (1975a) Rotations of the mandible during
growth. In JA McNamara (ed.):Determinants ofMandibular Form and Growth. Monograph No. 4: Craniofacial Growth Series. University of Michigan: Centre
for Human Growth and Development, pp. 65-76.
Enlow DH (197513) Handbook of Facial Growth. Philadelphia: W.B. Saunders Company.
Gantt DG (1983)The enamel of neogene hominoids: Structural and phyletic implications. In RL Ciochon and RS
Conuccini (eds.): New Interpretations of Ape and Human Ancestry. New York: Plenum Press, pp. 249-298.
Grine FE (1991) Computed tomography and the measurement of enamel thickness in extant hominoids:
Implications for its palaeontological application.
Palaeont. Afr. 28:61-69.
Grine FE, and Martin LB (1988) Enamel thickness and
development in Australopithecus and Puranthropus.
In FE Grine (ed.): Evolutionary History of the
“Robust” Australopithecines. New York: Aldine de
Gruyter, pp. 3-42.
Hannam AG, and Wood WW (1989) Relationships between the size and spatial morphology of human masseter and medial pterygoid muscles, the craniofacial
skeleton, and jaw biomechanics. Am. J . Phys. Anthrop. 80t429-445.
Hylander WL, Johnson KR, and Crompton AW (1987)
Loading patterns and jaw movements during mastication in Mucaca fusciculuris: A bone-strain, electromyographic, and cineradiographic analysis. Am. J.
Phys. Anthrop. 72t287-314.
Kanazawa E, Sekikawa M, and Ozaki T (1983) Threedimensional measurements of the occlusal surface of
upper first molars in a modern japanese population.
Acta Anat. 116:90-96.
Kanazawa E, Morris DH, Sekikawa M, and Ozaki T
(1988) Three-dimensional measurement of the occlusal surface of the upper first molar in South African samples. J. Anthrop. SOC.
Nippon 96:405415.
Kay RF (1981) The nut-crackers: A new theory of the
adaptations of the ramapithecinae. Am. J. Phys. Anthrop. 55t141-151.
Kimbel WH, White TD, and Johanson DC (1984) Cranial morphology ofAustralopithecus afarensis: A comparative study based on a composite reconstruction of
the adult skull. Am. J. Phys. Anthrop. 64t337-388.
Koolstra JH, van Eijden TMGJ, Weijs WA, and Naeije M
(1988) A three-dimensional mathematical model of
the human masticatory system predicting maximum
possible bite forces. J. Biomech. 21 r563-576.
200
G.A. MACHO AND M.E. BERNER
Korenhof CAW (1960) Morphological Aspects of the Human Upper Molar. Uitgeversmaatschappij Neerlandia: Utrecht.
Korenhof CAW (1982) Evolutionary trends of the inner
enamel anatomy of deciduous molars from Sangiran
(Java, Indonesia). In G Kurten (ed.): Teeth: Form,
Function and Evolution. New York: Columbia University Press, pp. 350-365.
Ladenbauer-Ore1 H. (1971). Fundberichte aus Osterreich. Band 7. Ferdinand Berger & Sohne OHG: Horn.
Macho GA, and Berner ME (in press) Enamel thickness
and the helicoidal occlusal plane. Am. J . Phys. Anthrop.
Macho GA, and Moggi Cecchi J (1992)Reduction of maxillary molars in Homo sapiens sapiens: A different
perspective. Am. J. Phys. Anthrop. 87t151-159.
Macho GA, and Thackeray JF (1992) Computed tomography and enamel thickness of maxillary molars of
Plio-Pleistocene hominids from Sterkfontein, Swartkrans and Kromdraai (South Africa): An exploratory
study. Am. J. Phys. Anthrop. 89t133-143.
Mansour RM, and Reynick R J (1975a) In vivo occlusal
forces and moments. I. Forces measured in terminal
hinge position and associated moments. J. Dent. Res.
54t114-120.
Mansour RM, and Reynick RJ (197513) In vivo occlusal
forces and moments. 11. Mathematical analysis and
recommendations for instrumentation specifications.
J. Dent. Res. 54t121-124.
Martin LB (1983)The Relationships of the Late Miocene
Hominoidea. Ph.D. thesis, University of London.
Martin LB (1985) Significance of enamel thickness in
hominoid evolution. Nature 314t260-263.
Mayhall JT, and Kanazawa E (1989)Three-dimensional
analysis of the maxillary first molar crowns of Canadian Inuit. Ani. J . Phys. Anthrop. 78:73-78.
Molnar S, and Gantt DG (1977) Functional implications
of primate enamel thickness. Am. J. Phys. Anthrop.
46t447454.
Molnar S,and Ward SC (1977) On the hominid masticatory complex: Biomechanical and evolutionary perspectives. J . Hum. Evol. 6t557-568.
Osborn JW (1987)Relationship between the mandibular
condyle and the occlusal plane during hominid evolution: Some of its effects on jaw mechanics. Am. J.
Phys. Anthrop. 73r193-207.
Rak Y (1983) The Australopithecine Face. New York:
Academic Press.
Ricketts RM (1975) Mechanisms of mandibular growth:
A series of inquiries on the growth of the mandible. In
JA McNamara (ed.): Determinants of Mandibular
Form and Growth. Monograph No. 4: Craniofacial
Growth Series. University of Michigan: Centre for
Human Growth and Development, pp. 77-100.
Sakai T, and Hanamura H (1971)A morphological study
of enamel-dentin border on the Japanese dentition. V.
Maxillary molar. J. Anthrop. SOC.Nippon 79t297-322.
Sakai T, and Hanamura H (1973)A morphological study
of enamel-dentin border on the Japanese dentition.
VI. Mandibular molar. J. Anthrop. SOC.Nippon 81:
2545.
Schwartz J H (1984) The evolutionary relationships of
man and orang-utans. Nature 308:501-504.
Schwartz J H (1990) Lufengpithecus and its potential
relationship to an orang-utan clade. J. Hum. Evol.
19.59 1-605.
Scott GR (1980)Population variation of Carabelli’s trait.
Hum. Biol. 52t63-78.
Shellis RP (1984) Variations in growth of the enamel
crown in human teeth and a possible relationship between growth and enamel structure. Archs. Oral Biol.
29t697-705.
Shellis RP, and Hiiemae KM (1986) Distribution of
enamel on the incisors of Old World monkeys. Am. J .
Phys. Anthrop. 71:103-113.
Simons EL (1976) The nature of the transition in the
dental mechanism from pongids to hominids. J. Hum.
Evol. 5:511-528.
Smith R J (1978) Mandibular biomechanics and temporomandibular joint function in primates. Am. J.
Phys. Anthrop. 49t341-350.
Spoor CF, Zonneveld FW, and Macho GA (in press) Linear measurements of cortical bone and dental enamel
by computed tomography: Applications and problems.
Am. J. Phys. Anthrop.
Statistisches Handbuch fur the Republik Osterreich,
1971. Osterreichisches Statistisches Zentralamt: Wien.
Stloukal M, and Vyhnanek (1976) Slovane z velkomoravskych Mikulcic. Academia: Praha.
Ward SC, and Molnar S (1980) Experimental stress
analysis of topographic diversity in early hominid
gnathic morphology. Am. J. Phys. Anthrop. 53t38S395.
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