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Enamel thickness and the helicoidal occlusal plane.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 94:327-337 (1994)
Enamel Thickness and the Helicoidal Occlusal Plane
GABRIELE A. MACHO AND MARGIT E. BERNER
Hominid Palaeontology Research Group, Department of Human Anatomy
and Cell Biology, The University of Liuerpool, Liuerpool L69 3BX,
England (G.A.M.);Department of Anthropology, Natural History Museum
Vienna, Burgring 7, A-1014 Vienna,Austria (M.E.B.)
KEY WORDS
Maxillary molars, Mastication, F’unctional adaptation
ABSTRACT
In the present study 38 unworn maxillary molars (M1
= 16,
M2 = 12, M3= 10) of modern humans from a Slavic necropolis were sectioned
through the mesial cusps in a plane perpendicular to the cervical margin of
the crown. Five slightly worn M’s and one slightly worn M3 were also used
thus increasing the total sample to 44,but measurements made on the worn
areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns
were recorded in order to analyze whether changes in these dimensions in
anteroposterior direction can be related to the helicoidal occlusal plane. Uniand multivariate analyses revealed that the distribution of enamel thickness
within and between maxillary molars corresponds to a helicoidal occlusal
wear pattern. Enamel thickness along the occlusal basin increases from anterior t o posterior, which may lead to rapid development of a reverse curve of
Monson in first molars when compared to posterior teeth. However, although
these overall differences together with the serial, especially delayed eruption
pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars
indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the
paracone, i.e., the lingual slope of the paracone, than under the Phase I1 facet
of the protocone, i.e., the buccal slope of that cusp. These proportional differences are most pronounced in M3, as evidenced by uni- and multivariate
statistics. It thus appears that the pattern of enamel thickness distribution
from M1 to M3 follows a trend towards providing additional tooth material in
areas that are under greater functional demands, that is, corresponding to a
lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In
addition, the heights of the dentine horns in anteroposterior direction change
in a way that lends support to the hypothesis that the axial inclination of
teeth could be one of the most important factors for the development of the
helicoidal occlusal plane. Finally, the changes in morphology and enamel
thickness distribution from first to third upper molars found in this study
suggest that molars could be “specialized in their function, i.e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly. 0 1994 Wiley-Liss, Inc.
Received December 21,1992; accepted January 19,1994
Address reprint requests to Dr. Gabriele A. Macho, Hominid
Palaeontology Research Group, Department of Human Anatomy
and Cell Biology, The University of Liverpool, P.O. Box 147,
Liverpool L69 3BX, England.
0 1994 WILEY-LISS, INC
328
G.A.MACHO AND M.E.BERNER
Differences in enamel thickness among characterized by a change of slopes of ocprimate taxa have usually been regarded as clusal surfaces in anteroposterior direction
evolutionary responses to functional de- with first maxillary molars typically exhibmands through dietary specializations (An- iting more pronounced wear on lingual
drews and Martin, 1991; Gantt, 1983; Jolly, cusps, whereas third upper molars are more
1970; Kay, 1981; Shellis and Hiiemae, 1986; worn buccally, while mandibular teeth show
Simons, 1976). However, differences also oc- complementary patterns of wear. When
cur within teeth. Molnar and colleagues viewed from the anterior, the unworn left
(Molnar and Gantt, 1977; Molnar and Ward, and right dentition is aligned along an imag1977) first studied the distribution of inary curve whose concavity is facing upenamel thickness within a single tooth wards ad palatum; this curve is commonly
crown and established that the areas that referred to as the curve of Monson. Once
are under greater functional demands ex- wear commences this curve becomes flat
hibit thicker enamel than do regions which and, eventually, reversed. In worn dentiare usually less worn. They suggested that tions one could thus define the helicoidal
the unequal distribution of enamel '' . . . in- occlusal plane as exhibiting a reversed curve
creases grinding efficiency and resistance to of Monson in first molars, a flat plane in
wear which prolongs life" (Molnar and second molars, and a normal curve of MonWard, 1977: 562). Bearing in mind that lin- son in third molars. This definition, howgual cusps of upper molars usually interdig- ever, implies that the helicoidal occlusal
itate between the lingual and buccal cusps of plane is entirely a function of wear of serilower molars it is not unexpected that lin- ally erupting molars: its expression could
gual cusps of hominid maxillary molars only be enhanced by consumption of an
have thicker enamel than do buccal cusps abrasive diet, thinner and less abrasion re(Macho and Thackeray, 1992). In mandibu- sistant enamel, which would reverse the
lar molars, buccal cusps fulfill a complemen- curve more rapidly, and by a particularly
tary function and they have relatively delayed eruption pattern of molars, as is the
thicker enamel too. Also, second and third case in humans. On the other hand, it has
maxillary molars have been found to have been proposed that the helicoidal occlusal
substantially thicker enamel than M's, both plane is a structural orofacial feature, which
absolutely and relatively (Macho and could be of functional importance (Osborn,
Berner, 1993). This is probably indicative of 1982; Richards and Brown, 1986; Smith,
the greater wear resistance of posterior mo- 1983, 1986). For instance, most researchers
lars and may also attest to the increasing regard differences in axial inclination of
bite force magnitudes from anterior to poste- teeth to account for a helicoidal occlusal
rior (Koolstra et al., 1988; Mansour and Rey- plane, even in the absence of wear.
nick, 1975a,b; Molnar and Ward, 1977; 0sThe present study on the patterning of
born and Baragar, 1985; Ward and Molnar, enamel thickness within and between max1980).
illary molars aims to appraise the models €or
In light of the reported evolutionary re- the development of the helicoidal plane.
sponse of enamel thickness to functional de- More specifically, it explores whether the
mands, the present study examines whether appearance of the helicoidal occlusal plane
the distribution of enamel within and may come about through a) increased absoamong molars can be reconciled with a heli- lute wear resistance of later erupting molars
coidal pattern of wear. The helicoidal pat- only, or b) whether the greatest wear resistern of occlusion has been regarded as an tance of molars changes from anterior to
orofacial feature typical of Homo (Tobias, posterior in such a way as to provide evi1980, 19911, but recent studies conclusively dence for the helicoidal occlusal plane being
demonstrated that it also occurs in Plio- a structural feature of the orofacial skeleton.
Pleistocene hominids and nonhuman pri- In a previous study, using the same set of
mates, especially in chimpanzees (Osborn, data, we were mainly concerned with abso1982; Smith, 1983,1986; Ward, 1981; White lute differences in enamel thickness beand Johanson, 1982). The helicoidal plane is tween molars (Macho and Berner, 1993).
ENAMEL THICKNESS AND THE HELICOIDAL PLANE
329
This investigation will examine whether
systematic differences in enamel patterning
can be found between molars.
MATERIALS AND METHODS
Thirty-eight unworn modern human maxillary molars (M‘ = 16, M2 = 12, M3 = 10)
were used in the present study, as well as
five slightly worn M’s and one slightly worn
M3; measurements made on the worn areas
of the latter teeth were coded as missing
values (see also Macho and Berner, 1993).A
total of 39 individuals is represented in the
sample. The molars 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, 19711, and are now housed in
the Department of Anthropology, Natural
History Museum Vienna, Austria.
Following Martin (1983) the molars were
cut through the tips of the mesial cusps in a
plane perpendicular to the cervical margin
of the crown with an ISOMET 11-1180 low
speed annular saw producing a 300 pm wide
cut. In mechanical sections the dentine
horns can be easily visualized when they
have been narrowly missed; this provides
the opportunity to grind down the tooth to
the optimal plane of section until the maximum dentine content is achieved. While employing this latter procedure may not be advisable when sectioning valuable fossils,
because it results in loss of tooth material
and, in some instances, morphology, molars
used in this study could be subjected to such
a procedure. Hence, in cases where the tips
of the dentine horns were not cut precisely
the teeth were ground and polished with
aluminum oxide paste on a rotating diamond disc to the optimum plane of section.
Measurements of enamel thickness derived
from such sections are more accurate than
those obtained from “clean” sections (Spoor
et al., 19931,thus substantially reducing the
confounding influence of obliquity of section
on enamel thickness measurements, as
pointed out by Martin (1983).
Each tooth was photographed under a
Wild-Photomakroskop at a magnification of
1:8, and measurements of enamel thickness
were recorded in millimeters from enlarged
1
L
1
B
Fig. 1. Cross-section through maxillary molar showing the seven measurements of enamel thickness studied as well as the heights of the protocone and paraeone
dentine horns.
photographs (1:lO) using a Vernier sliding
caliper (Fig. 1). The reference plane 0-P was
drawn parallel to the crown base through
the lowest point of the dentinoenamel junction (DEJ) between the cusps (Macho and
Berner, 1993; Macho and Thackeray, 1992).
In addition to the enamel thickness measurements (Fig. l, see also Macho and
Berner, 1993 for definition) the heights of
the dentine horns of the protocone and the
paracone were taken; they were measured
as the perpendicular distance from the reference plane 0-P to the dentine horn. Since a
previous study revealed the occurrence of
Carabelli’s cusps to have no influence on the
measurements recorded (Macho and Berner,
1993) this additional cuspule was not analyzed further. Similarly, the effects of sexual
dimorphism on the measurements appear
negligible (Macho and Berner, 1993).
In order to test for intra- and interobserver 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. Uni- and
multivariate statistics were carried out using the SPSS statistical program on the IBM
mainframe computer of the University of Vienna. Since the patterning of enamel between teeth of the same individuals was
found to be comparable to intertooth differences in the remaining independent sample
(Macho and Berner, 1993), molars derived
from the same individuals were not analyzed separately.
G.A. MACHO AND M.E. BERNER
330
TABLE I . Descriptive statistics of measurements (in
between M2 and M3. Enamel thickness at
the intercuspal fissure gave nonsignificant
ANOVA
results in all analyses. These results, and
Measurement
M’
M2
M3
F-value
P
those of other tests documented below, sugETl
gest that the statistical significance of the
1.18
7.90
0.0010
X
1.56
1.41
ANOVA results was mainly due to the
0.31
0.17
SD
0.27
markedly smaller dimensions in first maxilN
21
12
10
Ef2
lary molars.
1.60
2.17
2.09
19.27
0.0000
X
Nonetheless, inspection of mean values in
0.27
0.34
0.13
SD
16
12
10
N
Table 1, as well as further statistical tests
ET3
described below, indicate that the pattern1.30
1.62
1.59
13.02
0.0000
x
ing of enamel thickness distribution
0.19
0.19
0.22
SD
21
12
11
N
changes systematically from anterior to posEx4
terior. With regard to the cusp tip of the
1.18
1.17
1.11
0.14
0.8674
X
protocone and its buccal slope the greatest
0.25
0.47
0.38
SD
21
12
11
N
mean values were yielded in M2s, while the
ET5
comparable measurements of the paracone
1.25
1.73
1.78
30.41
0.0000
X
0.15
0.26
0.27
SD
were greatest in third upper molars (Table
21
12
11
N
1).To illustrate this observation the average
ET6
amount of enamel thickness of these dimen1.04
1.62
1.64
26.82
0.0000
X
0.30
0.24
0.16
SD
sions was calculated for the protocone (i.e.,
17
12
11
N
[ET 2 + ET 31/21 and the paracone (i.e., [ET
ET7
1.34
1.01
1.08
7.92
0.0012
5 + ET 6]/2). (Enamel thickness measureX
0.17
0.30
0.32
SD
ments ET 1 and ET 7 were not included in
11
21
12
N
the
calculations since ET 1 would correH1
X
22.79 18.20 16.65
17.09
0.0000
spond to a Phase I wear facet only after the
3.65
3.05
2.55
SD
cusp has worn down considerably, whereas
21
12
11
N
ET 7 is not associated with a wear facet at
H2_
1.29
0.2859
X
22.40 21.55 20.30
all.) Figure 2 shows that enamel thickness
SD
3.58
3.03
3.87
increases in the anteroposterior direction
N
21
12
11
but,
more importantly, that the disparity beLVariablescoded with “ E T refer to enamel thickness measurements
tween protocone and paracone decreases. In
whereas “ H corresponds to the height dimensions of the dentine
horns of the protocone and the paracone, respectively. Results of
other words, while in first maxillary molars
ANOVA and probability levels are also listed.
the mean values are clearly separated with
hardly any overlap in variation, at M3 the
average paracone enamel thickness is
RESULTS
within one standard deviation of the enamel
Descriptive statistics of all measurements thickness of the protocone (Fig. 2 ) .
In order t o test further for significant difare given in Table 1. Except for two dimensions, enamel thickness a t the intercuspal ferences in enamel thickness distribution
fissure (ET 4)and the height of the dentine within and among molars paired t-tests
horn of the paracone (H 2 ) , all analyses of were computed between the corresponding
variance (ANOVAs)yielded statistically sig- measurements of the lingual and the buccal
nificant differences between maxillary mo- side for each tooth category separately.
lars. Enamel thickness along both tooth Comparisons for differences in dentine horn
walls (ET 1 and ET 7 ) is largest in M’s, heights are also included. Table 2 lists the
whereas M2s and M3s have a considerably t-values and the significance levels,
thicker enamel cover at the occlusal basin whereby negative coefficients indicate that
than do first maxillary molars. Comparisons the measurement on the buccal side is larger
of measurements between teeth by Stu- than that on the lingual moiety. Enamel
dent’s t-test confirmed the overall similari- thickness along the lingual wall (ET 1-ET 7 )
ties of second and third molars; only ET 1 was thicker throughout the series although
was significantly different at the 5% level the tests gave statistically significant remillimeters) taken’
ENAMEL THICKNESS AND THE HELICOIDAL PLANE
T
M'
M2
Ma
Fig. 2. Mean value and standard deviation of average
enamel thickness of the protocone (ET 2 + ET 3112 and
the paracone (ET 5 + ET 6)/2 for each tooth type separately.
sults in first and second maxillary molars
only. Common to all molars, protocones have
statistically thicker enamel over the cusp
tips than paracones (ET 2-ET 6). In contrast, proportions of enamel thickness under
the grinding and the shearing surfaces of
the occlusal basin (ET 3-ET 5) change considerably from anterior to posterior. First
maxillary molars are endowed with statistically thicker enamel under the Phase I facet
(variable ET 3) than under the Phase I1
facet (variable ET 5). In posterior molars the
overall enamel coverage over the occlusal
basin not only increases, but it exhibits a
shift in proportions; the lingual slope of the
paracone (variable ET 5) becomes much
thicker than the comparable measurement
on the protocone (variable ET 3; Table 2). In
other words, the enamel on the buccal side of
the occlusal basin exceeds that of the lingual
side in posterior molars. Changes in dentine
horn heights (Hl-H2) from anterior to posterior also show a reversal in proportions.
While first upper molars exhibit higher protocone dentine horns, second and third molars
have more prominent paracone dentine horns.
Finally, discriminant analyses were car-
331
ried out to examine the relationships between enamel thickness measurements
within and between teeth, and to aim for
maximum separation between teeth. These
analyses may elucidate biological trends although, owing to the small sample sizes,
they cannot provide conclusive answers.
Firstly, enamel thickness measurements
that gave statistically significant results in
the ANOVA were used (Tables 3,4;Fig. 3).A
plot of the two functions shows that M's are
separated along the first axis, whereas function I1 discriminates between second and
third molars (Fig. 3). Inspection of the coefficients in each function reveals that discrimination of first maxillary molars is due to
proportionally thinner enamel a t the occlusal basin with thicker enamel a t the tooth
walls, when compared with posterior molars
(Table 3; see also Table 1).Discrimination
between second and third molars resulted
from third molars exhibiting proportionally
thicker enamel at the paracone and the buccal tooth wall. On the basis of these calculations 92% of all teeth were correctly assigned to their tooth category, whereby the
correct prediction of M's was 100% (Table
4).It is noteworthy that there is only one
clear outlier, that is, one second molar lay at
the extreme end of M3s (Fig. 3). Exclusion of
this specimen from the analysis resulted in
the second function becoming significant at
the 0.1% probability level and led to complete separation of second molars; only one
third molar was misclassified. Unfortunately, owing to the young age of the outlying specimen it could not be conclusively determined whether the specimen had
congenitally missing third molars. Be it as it
may, both discriminant analyses agreed insofar as the variables important for discrimination between teeth remained the same,
but discrimination became more complete
when the extreme outlier was excluded.
That this finding was not an artefact due to
the distinctiveness of M's could be confirmed on a separate analysis between second and third maxillary molars only. In a
Wilk's stepwise procedure on the same set of
variables the results are virtually identical
(Table 5 ) . Again, measurements on the buccal side contrasted with those on the lingual
side and the resulting coefficients led to a
G.A. MACHO AND M.E. BERNER
332
TABLE 2. Results ofpaired t-tests between corresponding measurements of the lingual and buccal side for each tooth
category, separately
Paired t-tests
ET 2-ET 6
ET 3-ET 5
t-value
P
t-value
P
ET 1-ET 7
t-value
P
M'
M'
M3
0.00
0.00
0.35
4.82
6.57
0.98
0.00
0.00
0.00
6.49
7.07
7.49
TABLE 3. Standardized discriminant function
coefficients, eigenualues, and percentage o f variance based
on discriminant analysis between maxillary molars and
using six measurements of enamel thickness
Measurement
Function
I
-.
Function I1
0.11
-0.35
-0.02
-0.46
-0.61
0.58
4.79
92.61
1.20
0.40
0.07
-0.46
0.09
-1.13
0.38
7.39
ET 1
ET 2
ET 3
ET 5
ET 6
ET 7
Eigenvalue
% variance
TABLE 4. Correct assignment of maxillary molars based
on SLX variables of enamel thickness recorded from sections
of teeth
M'
MZ
M3
N
Predicted group membership
Ml
M*
M3
16
12
9
16
0
0
0
11
2
0
1
7
86% correct classification of molars to their
tooth category.
DISCUSSION
This study is the first to investigate
whether the distribution of enamel thickness found within and among human maxillary molars can be reconciled with the helicoidal plane of occlusion and its associated
wear pattern. Resolving this question is important for an understanding of the development of a helicoidal occlusal plane during an
individual's lifetime, as well as for designing
models to explain its evolutionary history.
More specifically, the following questions
were addressed in this study:
Is the helicoidal occlusal plane entirely a
function of differential wear
between molars?
Although the universal occurrence of the
helicoidal occlusal plane in Homo sapiens
1.90
-2.25
-2.10
0.07
0.05
0.05
H 1-H 2
t-value
0.39
-3.47
-3.65
P
0.70
0.01
0.00
has not been disputed, there are detailed
differences in molar wear patterns among
various modern populations, notably between prehistoric and modern hunter-gatherers and agriculturalists (Smith, 1983,
1984, 1986; also for review of older literature). This observation led to the recognition
that the abrasiveness of diet plays an important role in the development of the helicoidal
plane, which is further influenced by
enamel thickness and delayed eruption of
posterior molars (Butler, 1972; Osborn,
1982; Smith, 1986). For example, thinenamelled molars are more rapidly worn
than thick-enamelled teeth and would thus
have developed a reversed curve of Monson
by the time later erupting teeth come into
occlusion. In modern humans the eruption is
especially delayed, which could account for
the ubiquitous occurrence of the helicodial
occlusal plane. Indeed, the marked differences in overall enamel thickness between
M's and M2slM3s found in this study (see
also Macho and Berner, 1993) would lead to
a rapid reversal of the curve of Monson in
first molars and to a prolonged existence of a
normal curve in posterior teeth. However,
the teeth studied not only differ in their
overall amount of enamel over the tooth
crown, but also show a distinct pattern of
enamel thickness distribution among teeth.
This seems to indicate that in the course of
hominid evolution enamel thickness has responded to the greater functional demands
by providing greater wear resistance to lingual cusps of M's and to buccal cusps of posterior molars. While diet, overall enamel
thickness between teeth, and time of eruption may contribute to establishing a pronounced helicoidal pattern of wear, it cannot
be assumed that these factors could have
provided sufficient selective pressure to account for the patterning of enamel thickness
ENAMEL THICKNESS AND THE HELICOIDAL PLANE
333
+3
2
1
+I
.5
2
F
n
*2
2
C
3
i
2
2
2
U
2
2
1
1
’ 1
1
1
2
I1
2
3
#
3
0
n
1
1
1
II
3
3
3
1
1 1
-1.5
3
3
2
1
-3
-4
-2
0
+2
+4
Function I
Fig. 3. Plot of discriminant functions I and I1 based on analysis of six measurements of enamel
thickness. Tooth types are listed by their numbers and centroids are marked by asterisks.
between maxillary molars observed in this
study.
Is the helicoidal occlusal plane a
structural feature of the
orofacial skeleton?
It is commonly accepted that the expression of the helicoidal plane of occlusion need
not necessarily involve a clear reversal of
slope from anterior to posterior; changes in
wear plane angles from first to third molars
alone qualify for the dentition to be classified as helicoidal (e.g., Osborn, 1982; Smith,
1983, 1984, 1986). The present study revealed that the posterior molars analyzed
are endowed to resist proportionally more
wear on their buccal cusps than do anterior
molars.
Although enamel distribution at the cusp
tips within maxillary molars follows a pattern found in other primates, i.e., lingual
cusps displaying thicker enamel t h a n buccal
G.A. MACHO AND M.E. BERNER
334
TABLE 5. Results of a Wilk's stepwise discriminant analysis between second and third muxcllary molars and using sir
measurements of enamel thickness
Variable
entered
ET 1
ET 2
ET 3
ET 5
ET 6
ET 7
Eigen value
Significance
Variable
removed
Wilk's
lambda
F-value
1
3
0.83
0.49
4.02
4.17
4
0.59
3.95
-0.68
2
0.65
4.90
-1.20
1.04
0.02
cusps (Macho and Thackeray, 1992; Molnar
and Gantt, 1977; Molnar and Ward, 19771,
enamel at the lingual slope of the paracone
increases from anterior to posterior,
whereas the comparable measurement on
the protocone decreases. Thus, there is a
clear change in wear resistance at the occlusal basin from lingual anteriorly to buccal posteriorly (Table 2). That this pattern of
change is not paralleled by the enamel
thickness at the cusp tips is not surprising
given that lingual upper cusps intercuspate
between the lingual and the buccal lower
cusps regardless of differences in relative
mandibular arch width between first and
third molars (e.g., Richards and Brown,
1986; Smith, 1986). Therefore, while the lingual slope of the paracone assumes greater
functional importance than the comparable
area on the protocone in anteroposterior direction, the cusp tip of the paracone is nonetheless not directly involved in the chewing
cycle. In light of these considerations the
distribution of enamel thickness found in
this study closely matches a pattern of
greatest wear along the occlusal surface defined as helicoid. It is interesting that chimpanzees, which have also been reported to
develop a helicoidal occlusal plane, albeit to
a lesser extent (Smith, 1986), might show a
comparable pattern of enamel thickness distribution within and among teeth. Out of
three first maxillary molars of Pan sectioned by Martin (19831, the buccal slope of
the protocone (his measurement 3'" had
thicker enamel than the lingual slope of the
paracone (his measurement "h) in two
cases, and they were equal in one. On the
other hand, all second upper molars had
thicker enamel under the Phase I wear facet
of the paracone than under the Phase I1
Standardized discriminant
coefficients I
1.39
0.70
facet of the protocone, although the only
third molar sectioned by Martin did not
show any disparity between the two sides.
The measurements provided by Martin
(1983) for Pongo and Gorilla do not show
such a pattern of enamel thickness distribution among molars. Hence, the data on
enamel thicknesses available give credence
to Smith's proposal that Homo and Pan
show similarities in their masticatory apparatus and that differences in wear plane angles from first to third molars between humans and chimpanzees are of degree rather
than kind.
What is the evolutionary and functional
importance of the helicoidal
occlusal plane?
Neumann (1922) was the first to draw at-
tention to the inclination of tooth crowns for
the development of the occlusal plane. Subsequently, this interpretation not only
gained support from most anthropologists
(Osborn, 1982; Richards and Brown, 1986;
Smith 1983,1984,1986; Ward, 1981),but its
general acceptance provided a sensible explanation for the more frequent occurrence
of the helicoidal occlusal plane in Homo as
compared to other hominids (White and Johanson, 1982). Since the Plio-Pleistocene,
the foreshortening of the dental arcade in
hominids resulted in molars coming to lie
mostly posterior to the root of the zygomatic
arch and medially to the masseterlpterygoid
complex; and both factors appear to be important for the development of a helicoidal
plane (Smith, 1986). Moreover, the reduction of the dental arches and their retraction
under the skull necessitated axial inclination of the molar roots. Although axial tilt of
molars has also been reported for early hom-
ENAMEL THICKNESS AND THE HELICOIDAL PLANE
inids (Ward et al., 1982) it is less pronounced than it is in humans (Dempster
et al., 1963). The present findings indirectly
provide evidence for the evolutionary history of axial inclination of molars in the coronal plane in Homo. The height of the protocone dentine horn decreases substantially in
an anteroposterior direction whereas the
height of the paracone dentine horn remains
virtually the same from the first t o the third
upper molar (Table 1). Bearing in mind that
a) the alveolae into which the teeth are implanted are relatively straight, and that b)
the occlusal planes form a functional masticatory unit, such disparities in cusp heights
can only be counteracted by tilting the axes
of posterior teeth in order for both cusps to
reach the occlusal plane; even so, the occlusal plane of posterior molars is still somewhat oblique (Smith, 1986). Therefore, it
seems probable that a trend towards pronounced axial inclination of the teeth in the
course of evolution has been paralleled by
differential changes in cusp heights in order
for the masticatory complex to remain functional.
However, posterior molars of hominids
are not only tilted in a coronal plane, as discussed above, but also in a sagittal one. Human lower third molars, in particular, have
undergone a forward tilt during evolution as
a result of the displacement of the temporomandibular joint in relation to the occlusal
plane (Osborn, 1987). These changes not
only facilitated the generally more pronounced development of the curve of Spee in
humans when compared to other hominoids,
but it also rendered third molars functional
despite their disadvantageous position (Osborn, 1987). The curve of Spee refers to the
curved occlusion of upper and lower teeth in
lateral view, with the concavity pointing upwards. Robinson (1956) appears to be the
first to make mention of this curve when
discussing taxonomic differences between
Plio-Pleistocene hominids. Nonetheless, the
importance of the curve of Spee for the development of the helicoidal plane is equivocal and has been explained in two different
ways. Pleasure and Friedman (1938:1614)
suggested that the ". . . reversal of pitch in
the last molar is employed because it endows the dentures with balancing contact on
335
the non-working side." Subsequent researchers occasionally took up this idea to
explain the helicoidal occlusal plane in the
dentition (e.g., Hall, 1976), but recently Richards and Brown (1986) refuted this hypothesis and maintained that nonfood side
contacts decrease as wear progresses; this
results in wear to occur predominantly on
the chewing side. Alternatively, Osborn
(1982) proffered a mechanism that placed
functional importance on the curve of Spee,
suggesting that molars on the working side
function in a smooth grinding movement because of this curve. Moreover, an important
consequence of this complex interrelationship between the helicoidal occlusal plane
and the curve of Spee is that molars function
in series rather than simultaneously (Osborn, 19821, and third molars retain their
functional importance (Osborn, 19871, while
the crusWshear ratio of the force increases
on posterior molars (Osborn, 1993).
The thick enamel of posterior molars together with results yielded in studies in related fields lend further support t o such a
complex model of mastication. Microwear as
well as enamel thickness studies in Pan suggest that the amount of compression and
shearing loads applied to food items differs
between molars (Gordon, 1982, 1984; see
also Macho and Berner, 1993). For example,
the increase in pit frequencies on crushing1
grinding surfaces from 30% in first molars
to over 50% in third molars implies that
compression is highest in third molars,
whereas shearing is lowest. Gordon (1982)
also noticed that shearing facets are inclined more relative t o the occlusal plane
and concluded that the flatter occlusal morphology in third molars in Pan relates to the
qualitatively different wear processes occurring in posterior molars. It is relevant that
changes of patterning of enamel thickness
from M1 to M3 in this protohominid and
those reported for Homo in the present
study are comparable (see also Macho and
Berner, 1993; Martin, 1983). The thicker
enamel cover over the occlusal basin reported here together with the decrease in
dentine horn heights evince that cusps of
posterior molars are less pronounced than
those of anterior teeth. Moreover, it should
be borne in mind that enamel deposition re-
G.A. MACHO AND M.E. BERNER
336
sults in progressive outward expansion of
the cusps (Butler, 1956; Corruccini, 1987a,b;
Kraus, 1952; Macho and Thackeray, 19931,
which results in a proportionally greater occlusal basin for crushing and grinding purposes. In other words, owing to their thicker
enamel, cusp tips of posterior molars are
more displaced in relation to the underlying
dentine horns than are cusp tips of M’s.
Taken together, these differences in morphology as well as the changes in amount
and patterning of enamel thickness from anterior to posterior suggest proportionally
more crushing and grinding posteriorly,
while first maxillary molars would be better
suited to resist shear forces. It is therefore
probable that human molars are “specialized in their function as are those of the
chimpanzee (Gordon, 1982, 1984; Osborn,
1982). Since the hominid palate has shortened and retracted during evolution, this
could only have been achieved by a complex
restructuring of the orofacial skeleton involving axial tilt of the molars in two planes;
this would lead t o a more pronounced curve
of Spee and helicoidal occlusal plane. Ultimate clarification of this hypothesis will
only come from studies that compare surface
patterns of microwear with structural features of enamel.
CONCLUSION
This study is the first to demonstrate that
the distribution of enamel thickness within
and between teeth as well as the changes in
dentine horn heights of the protocone and
the paracone from M1to M3 accord with the
helicoidal pattern of occlusion. While the
findings of this investigation provide evidence for a long evolutionary history of this
trait, they also lend support to the suggestion that some factors, such as axial inclination of teeth, could be particularly important
for its development. Moreover, the helicoidal
occlusal plane has usually been viewed as a
by-product of evolutionary changes that
have occurred in the orofacial skeleton. In
light of the conclusions reached in this
study, however, the possibility that the helicoidal plane could also represent a functional adaptation in itself must be considered. In order to shed light on this proposal
future studies ought to be multidisciplinary
and should combine investigations in comparative anatomy, biomechanical engineering, and microwear processes.
ACKNOWLEDGMENTS
We thank J. Szilvassy for permission to
section the material and for providing computer facilities. 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. We are grateful to B. A. Wood for valuable comments on the manuscript. We
thank the referees for their helpful suggestions.
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