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Computed tomography and enamel thickness of maxillary molars of Plio-Pleistocene hominids from Sterkfontein Swartkrans and Kromdraai (South Africa) An exploratory study.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 89133-143 (1992)
Computed Tomography and Enamel Thickness of Maxillary
Molars of Plio-Pleistocene Hominids From Sterkfontein,
Swartkrans, and Kromdraai (South Africa): An Exploratory Study
GABRIELE A. MACHO AND J. FRANCIS THACKEXAY
Departnient of Palaeontology and Palaeoenuzronmental Studies,
Transuaal Museum, Pretoria 0001, South Afrrca
KEY WORDS
CT-scanning, Enamel thickness, Enamel distribution, k'-'lro-k'"iistocene maxiiiary molars, South M i c a , Huiiiiriid
ABSTRACT
This paper is one in a series which explores the possibility of
using the non-destructive CT technique to identify patterns in tooth enamel
distribution and structure of hominid molars from Plio-Pleistocene sites in
South Africa, notably Swartkrans, Sterkfontein, and Kromdraai. Whereas
previous investigators have emphasised gross differences in absolute and
relative or average enamel thickness between hominid taxa, the present
study highlights differences in enamel thickness over functionally significant
regions of the crown. Differences in the distribution of enamel in A. robustus,
A. africanus, and Homo sp. are identified through the use of bivariate and
multivariate analyses, and are interpreted in terms of dietary regimes.
0 1992 Wiley-Liss, Inc.
Attention has recently been drawn to differences in tooth enamel thickness among
Plio-Pleistocene hominids (Beynon and
Wood, 1986; Conroy, 1991; Conroy and Vannier, 1991; Gantt, 1977, 1983; Grine and
Martin, 1988; Kay, 1981;Martin, 1985; Wallace, 1972,1978). Several of these researchers have concluded that Ausfralopithecus robustus and A. boisei have "hyper-thick
enamel, while A. africanus and Homo are
characterized by relatively thinner enamel.
Furthermore, these studies confirmed that
enamel is not equally distributed over the
entire tooth crown, but thicker in areas of
greater functional demands (Molnar and
Gantt, 1977; Molnar and Ward, 1977). Most
of the studies on enamel thickness relied on
measurements taken on naturally broken or
worn teeth. Martin (1985:260) stressed that
such measurements have "limited value." So
far a total of only six Plio-Pleistocene hominid teeth from East and South African sites
have been sectioned (Grine and Martin,
1988). Despite the greater accuracy of results offered from this destructive approach,
it is at present unlikely that many more pre0 1992 WILEY-LISS, INC
cious fossil teeth will be sacrificed for sectioning. It is therefore desirable to take
maximum advantage of non-destructive
techniques in order to determine variability
in enamel thickness and distribution within
and between taxa.
Zonneveld and Wind (1985) pointed out
the potential of computed tomography ICT'!
for palaeoanthropology with special reference to measuring enamel thickness. From
sagittal scans they recorded a maximum
thickness of 2.6 mm in M3 of SK 48 (A.robustus from Swartkrans), and 3.3 mm in M2 of
TM 1517 (A.robustus from Kromdraai). Recent research has been conducted to investigate further the possibility of using CTscans on isolated teeth. In Grine's (1991a,b)
study of extant primates, measurements obtained from CT-scans were compared with
those taken from SEM images of the same
specimens. By contrast, Conroy (Conroy,
1991; Conroy and Vannier, 1991) took mea-
Received May 28,1991; accepted March 10, 1992.
G.A. MACHO AND J.F. THACKERAY
134
TABLE 1. List of horninid maxillary molars from Swartkrans, Sterkfontein, and Kromdraai included in this study
iL = left. R = right)
Cat.
No.
Side
M’
M”
M“
No.
Site
Saecies
Sterkfontein
Sterkfontein
Sterkfontein
Sterkfontein
Sterkfontein
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Swartkrans
Kromdraai
Kromdraai
Kromdraai
Kromdraai
1
2
3
4
5
6
7
8
9
10
11
12
13
i4
15
16
17
18
19
20
21
22
23
~
Sts
Sts
Sts
Sts
TM
SK
SK
SK
SK
SK
SK
SKX
SK
SK
SK
SK
SK
SK
SKW
Kb
Kb
TM
TM
8
24
28
37
1511
36
41
89
98
102
105
268
829
832
839
839
3977
3975
14129
5063
5383
1601
1603
L
R
R
L
R
R
X
X
X
x
x
x
x
X
X
X
R
L
L
L
R
R
L
R
R
R
L
L
x
X
X
x
X
X
X
x
x
X
X
X
X
A. africanus
A. ufricarzus
A. africaizus
A. africanus
A. africanus
A. robustus
A. robustus
A. robustics
A. robustus
A. robustus
A. robustus
Homo sp.
A. robustus
A. robustus
A. robustus
A. robustus
A. robustus
A. robustus
A. robustus
A. robustus
A. robustus
A, robustus
A. robustus
~~
Catalogue specimen numbers correspond to specimen numbers 1-23 included in Figures 2 and 3
tocene hominids from South African sites
were selected for analysis from collections
housed at the Department of Palaeontology
and Palaeo-environmental Studies, Transvaal Museum, Pretoria (Table 1).The hominid sample from Sterkfontein comprise
specimens collected by Robert Broom and
John Robinson between 1936 and 1958.
Those from Swartkrans were obtained by
Broom and Robinson, and more recently by
C.K. Brain. The material from Kromdraai
was collected by Broom, supplemented by
isolated hominid teeth from Elisabeth
Vrba’s excavations. The specimens from
Sterkfontein derive from Member 4 and
have been attributed to A. africanus, while
all except one tooth from Swartkrans and all
those from Kromdraai have been assigned to
A. robustus. SKX 268 from Swartkrans has
been described as Homo (Grine 1989).
A Siemens SOMATOM DR3 scanner a t
the Department of Radiology, Hillbrow Hospital, Johannesburg, was used. One millimeter slices (i.e., the thinnest possible slice
width) were taken with a high resolution
filter at 125 kV and a scanning time of 7
seconds. The images were reconstructed
MATERIALS AND METHODS
from a pixel matrix of 512’ and 720 projecIsolated unworn or slightly worn perma- tions. The window settings were kept connent maxillary molars M1-M3 of Plio-Pleis- stant for all images.
surements directly from CT-scans of fossil
hominid teeth. Whereas Conroy maintained
that his study demonstrated the viability of
the CT-scanning technique, Grine contended that it could not be used to provide
reliable and accurate measures of enamel
thickness.
Measurement errors on CT scans stem
from different sources and the percentage
error may vary considerably (Koehler et al.,
1979; Ruff and Leo, 1986; Zonneveld, 1983).
While recognizing difficulties associated
with the recording of measurements from
hard copy outputs of CT-scans, we place emphasis on patterning in enamel thickness
within teeth and between individuals,
rather than on absolute values. In this investigation we make use of the non-destructive CT scanning technique (Computed Tomography) to explore variability in tooth
enamel distribution of hominids represented at Swartkrans, Sterkfontein and
Kromdraai in the Transvaal Province,
South Africa; specimens from these sites
have been commonly attributed to A. africanus, A. robustus, or Homo.
PATTERNING IN ENAMEL THICKNESS
PROTOCONE
135
PARACONE
Fig. 1, Cross-section through maxillary molar showing seven linear dimensions of enamel thickness
(variables 1-7) used in this exploratory study. Each measurement was scaled to the reference plane A-B
being 10 mm.
The basal area of each tooth was orientated horizontally and mounted on a microscope slide on a soft plasticine base. For purposes of orientation two very small, low
density markers were placed on tips of the
paracone (mesio-buccal cusp) and protocone
(mesio-lingual cusp). The microscope slide
with the mounted tooth was placed on another slide whose lateral edge was placed
beneath the paracone-protocone plane. This
arrangement facilitated accurate alignment
of both cusp tips on the beam of the CTscanner. A l m m slice was then taken in the
bucco-lingual plane, perpendicular to the
basal area. In order to test for accuracy of
orientation, additional scans were taken at
lmm increments, mesially and distally from
the plane initially selected. Bucco-lingual
diameters. which had previously been recorded on original specimens, were checked
against corresponding diameters of the images on the screen with the built-in device.
CT images were recorded on film (hard copy)
and the data stored on floppy discs.
Measurements of external dimensions of
each tooth were also taken from hard copies
and checked against the same diameters recorded directly from originals. A further
check of CT-imagery was made by examining the dentine horns below cusps of the protocone and paracone in a series of scans.
Scans that had not been taken through both
cusp tips simultaneously could be identified
when the apical angle of the dentino-enamel
junction (DEJ) was obtuse under one cusp
contrasting with a more acute angle below
the other; a scan further mesially or distally
usually resulted in a slight improvement
and an apparently more accurate delineation of one dentine horn but not of the other.
In such cases the specimen could be re-orientated to correct for the observed disparity.
Of the specimens included in this analysis
(Table 11, comparisons were made between
external distances recorded from the teeth
and those from hard copies of CT images. A
correlation coefficient (r2)of 0.99 was obtained. However, preliminary analyses suggest that small distances may be consistently overestimated; this would be in
accordance with Grine's study of modern
specimens (1991a,b). It is recognised that
such overestimates for particular variables
are not necessarily consistent between
teeth. On the other hand, since errors from
CT scans are likely to by systematic within
any particular tooth, proportional distribution of measurements of enamel within the
tooth should not be affected.
All images were traced and enlarged before measurements were taken with a Kontron Image Analyser. Where necessary the
outlines were adjusted for slight wear (specimens SK 41, SK 832, SK 3975, SK 3977,
SKW 14129 and TM 1511). Seven variables
of enamel thickness, including six previously defined by Beynon and Wood (1986)
and Martin (19831, are shown in Figure 1.
The data were analysed by means of commercially available statistical packages (Lotus 2.1; Statgraphics 4.0) and include bivariate plots, linear regression analyses,
and principal component analyses. All teeth
were scaled t o the same size, the distance
A-B (Fig. 1)being standardised arbitrarily
to 10 mm.
136
G.A. MACHO AND J.F. THACKEMY
All measurements were taken by one of us
(G.A.M.).A selected set of variables was recorded twice to check for consistency. Comparison between these sets of data yielded
correlation coefficients (r2)ranging between
0.97 and 0.99.
Although SK 839 is represented by antimeres, these teeth will be treated as separate entities. This was done not to artificially inflate sample size but rather as an
attempt to judge the degree of variability
occurring within one individual.
RESULTS
In Figure 2, lingual enamel diameters
(variable numbers 1, 3, and 4) are plotted
against corresponding enamel thickness on
the buccal side (numbers 2,6, and 5, respectively). The results demonstrate a considerable degree of overlap between sites. Although this may be due to some extent to
systematic measurement error, it is nonetheless evident that the highest values of
enamel thickness are obtained for specimens of “robust” australopithecines. Deviations from the isometric line are apparent.
Close inspection reveals that this trend is
most strongly expressed for measurement
No. 3 vs. 6 (Fig. 2). Least-squares linear regression analyses were performed between
pairs of variables and using the pooled sample of hominid teeth. The enamel thicknesses of the lingual cusp of the tooth were
chosen as the independent variables. Regression analyses between variables 3 and 6
yield a slope of 0.57. At the occlusal basin
(variables 4 vs. 5) there is also a disparity
between the enamel thicknesses of the lingual and buccal cusps; the slope is 0.72. By
contrast, when comparing lingual and buccal enamel thicknesses at cusp tips (variables 1 and 21, the regression line more
closely follows the isometric line; in this instance, the slope is 0.86.
The proportional distribution of enamel
thickness in the Homo specimen SKX 268
deserves special mention. This molar exhibits thick enamel along the occlusal surface
(variables 1,2,4,and 5) comparable to those
of most other Swartkrans specimens and is
positioned closely to the isometric line (Fig.
2). Along the lingual and buccal wall of the
crown (variables 3 and 61, SKX 268 shows
I
I5x93
S ’ 21
3-
.
a
1 -
1.o
2.0
3.0
4.0
VARIABLE 1 (LINGUAL)
3-
2-
1.o
2.0
3.0
4.0
VARIABLE 4 (LINGUAL)
3-
2-
1.o
2.0
3.0
4.0
VARIABLE 3 (LINGUAL)
Fig. 2. Bi-variate plots of pairs of corresponding
variables on lingual and buccal sides of maxillary molars from Swartkrans, Sterkfontein, and Kromdraai. Dimensions of all variables are expressed in arbitrary
numbers. Note the deviation from the isometric line
(dotted). Specimens 1-5: Sterkfontein; specimens 6-19:
Swartkrans; specimens 20-23: Kromdraai.
thinner enamel than most of the other teeth
from Swartkrans. Furthermore, it is the
only specimen which clearly lies above the
isometric line for variables 3 and 6. This indicates that the enamel is relatively thicker
along the buccal wall in proportion t o the
137
PATTERNING IN ENAMEL THICKNESS
TABLE 2 Principal component analysis, agenualues and percentage contributtons of components I-IV to the
totol
Measurement
No.
_ _ --.
--_
1
2
3
4
5
6
7
8 variance
.
Description
.
.
.. ._
.___Protocone tip
Paracone tip
Protocone lingual
Protocone buccal
Paracone lingual
Paracone buccal
Intercuspal fissure
iinrinnr~
PCI
- - ____ __ __ ______ - _0 45
0 45
0 47
0 31
0 29
0 32
0 30
78 41
lingual one, whereas the opposite holds for
other teeth studied.
Principal Components Analysis (PCA) was
employed in order to explore structural differences between specimens (Table 2, Fig.
3). The first principal component (PC) is associated with overall size differences, and
accounts for 78% of the variance. The contributions of PC I1 to PCIV are relatively small,
but nonetheless informative since they discriminate between shapes and patterns of
enamel distribution in individual teeth. PC
I1 contrasts enamel thickness on the cusp tip
(variable 2) and occlusal surface (variable 5)
of the paracone, with enamel thickness on
the lingual slope of the protocone (variable
3). This PC clearly separates Homo SKX 268
from other specimens (Fig. 3). The third PC
reveals general differences between sites:
all specimens from Sterkfontein have negative coefficients: Kromdraai specimens Kb
5383, TM 1601, and TM 1603 also fall in that
category, as do SK 3977, SK 36, and the
antimeres of SK 839, but all other Swartkrans teeth (n = 10) and Kb 5063 have positive coefficients (Fig. 3). The contrasting features of PC 111 are the dimensions of the
buccal side of the paracone (variables 2 and
6) and that of the lingual wall of the protocone (variable 3), on one hand, and the remaining variables of enamel thickness on
the occlusal surface and the protocone, on
the other (Table 2 ) . It would thus seem that
there is a difference in relative enamel
thickness between the lingual and buccal
side of hominid teeth from different sites,
which is not detected by bi-variate analyses.
The fourth PC contrasts enamel thickness
of the fissure between the protocone and
paracone (variable 7) with thickness of vari-
Principal components
PCII
- PCIII
__ PCN
0.07
-0.62
0.71
-0.09
-0.31
0.12
0.01
9.07
-0 41
-0 0 3
-0 01
-0 31
0 06
0 11
0 84
4 58
0.23
-0.52
-0.29
0.64
0.15
-0.23
0.34
5.20
-2
-l
PC II
-2
-l
PC II
-11
-2
~
, ,
I
~
-1
,
1
O
-1
0
1
I
1
PC I1
Fig. 3. Plot of principal components 1 and I1,II and
111, and I1 and IV. Specimens 1-5: Sterkfontein; specimens 6-19: Swartkrans; specimens 20-23: Kromdraai.
138
G.A. MACHO AND J.P THACKEIUY
ables 1 and 4 on the buccal slope of the protocone. Inspection of the plot reveals that
this component separates three of the Kromdraai teeth from other Specimens. TM 1603
is most clearly separated, whereas Kb 5063
and TM 1601 lie adjacent to the main
cluster.
The remaining principal components (PC
V, VI, and VII) account for less than 2% of
the total variation, and do not reveal clearcut patterns. Only PC VI appears to differentiate between MI, M2, and M3.
DISCUSSION
It is clear that valuable information can
be obtained from CT-scans provided certain
guidelines are followed (see also Ruff and
Leo, 1986; Spoor, pers. comm.). In this study
we have reported a correlation coefficient
(r2)
of 0.99 for the relationship between external measurements taken from original
specimens and those obtained from hard
copy outputs of CT images. Difficulties of
course arise when examining internal parameters. Although window settings were
kept constant in the present study, differences in CT images may occur on account of
preservational and other factors (depending
on whether roots, bone, matrix or plaster are
attached to the crowns). Here we have refrained from relying on the absolute values
of CT based measurements of enamel thickness, and instead investigated proportions
and patterns of enamel distribution wit,hin
individual teeth.
Thin, finely tapered cervical margins of
the enamel cap form blunt borders on CTscan images and are situated towards the
pulp cavity, especially in cases of isolated
crowns. Measurements of areas and volumes would be especially affected by this
fact (Spoor and Zonneveld, pers. comm.).
Hence, only linear dimensions were recorded in the present study. These diameters are close to the occlusal part where
enamel thickness is expected to be relatively
uniform (Grine and Martin, 1988). However,
distortion of the DEJ a t the cervix may have
also affected the position of the reference
plane A-B, which is parallel to the cervical
border, but the edges on both sides are affected by beam hardening and edge arte-
facts t o a similar extent. Thus, inclination of
the reference plane from the true plane may
be regarded as slight if not negligible.
Damage of the DEJ appears to be a potential cause of error which is difficult to assess
on the scans. Irregularities of the DEJ in
CT-scans are the only indication of possible
breakage a t the DEJ. Although the specimens used in the present study all appear to
have an intact DEJ, the possibility of damage cannot be ruled out completely, Whereas
it is technically possible to obtain accurate
rstimates of enamel thickness fi.~niCTscans (Spoor, pers. comm.), it is questionable whether the technique will advance to
such an extent that morphological details of
the border between the two tissues can be
accurately determined. This must be regarded as one of the major drawbacks of this
new, non-destructive technique, which offers such promising prospects in other respects (e.g., Ruff, 1989; Vannier and Conroy,
1989).
Differing enamel thicknesses within and
between teeth are usually regarded as evolutionary responses to functional demands
through dietary specializations (e.g., Kay,
1981; Molnar and Gantt, 1977; Ward and
Pilbeam, 1983). Tooth morphology suggests
that the “robust” australopithecines were
adapted to a coarser diet than that of “gracile” specimens (Grine, 1981, 1986; Jolly,
1970; Kay, 1975; Robinson, 1954, 1963).
Furthermore, A. robustus and A. boisei have
been described as “hyper-thick enamelled
hominids (Beynon and Wood, 1986; Grine
and Martin, 1988), while H. sapiens and A.
africanus have proportionally thinner
enamel. This condition persists even when
the enamel is scaled to tooth size. Although
Robinson (1956) acknowledged some differences in thickness of the enamel cap between australopithecine species, he claimed
that the differences are not substantial.
Overall, the present study shows a considerable degree of overlap in enamel thickness
of “gracile”and “robust” australopithecines.
A. robustus, as represented by specimens
from Swartkrans, tends to be associated
with high values (Figs. 2, 3). Although our
measurements of specimens in this study
are comparable t o those of published enamel
PATTERNING IN EN AMEI, THICKNESS
thicknesses of Plio-Pleistocenehominids, we
shall refrain from placing too much emphasis on absolute diameters until we have experimentally evaluated the effects of beam
hardening and beam attenuation in enamel
and appraised the absolute measurement
error. More importantly, distinct differences
in the distribution of enamel have been observed within teeth, and between teeth belonging to different taxa.
Inspection of the bi-variate plots (Fig. 2)
reveals that patterns of enamel distribution
are apparer,t?jr c o r n ~ ~ oton A. clfricanxs
(Specimens from Sterkfontein Member 4)
and A. robustus (represented by samples
from Swartkrans and Kromdraai). Furthermore, on the basis of samples included in
this study of enamel thickness, there is no
strong evidence to suggest that there is necessarily more than one species among the
Sterkfontein Member 4 specimens, as suggested by Clarke (1988a,b).
In all of the teeth examined in this study,
with the exception of the Homo molar SKX
268, the lingual wall of the protocone exhibits thicker enamel than the buccal slope of
the paracone, whereas the buccal slope of
the protocone and the lingual slope of the
paracone are more similar in enamel thickness. Enamel thickness at cusp tips exhibits
the widest scatter in all of the bivariate plots
shown in Figure 2. This finding could be attributed, at least in part, to the fact that
adjustment had been made for slight wear in
some specimens. Nonetheless, a difference
in enamel thickness has been found between
cusp tips at the protocone and the paracone.
Generally (with the exception of TM 16031,
lingual cusps exhibit thicker enamel. Similar differences have also been found by Beynon and Wood (1986) in east African “robust” specimens, and by Grine and Martin
(1988) in molars of modern humans, A. africanus and A. robustus (P. crassidens). Such
differences reflect greater functional demands on the lingual than on the buccal
cusps in maxillary molars, whereas the opposite holds for mandibular molars (Molnar
and Ward, 1977; Molnar and Gantt, 1977).
It is thus surprising that Conroy (1991)lists
only 9 out of 21 lower hominid molars from
South Africa to possess thicker enamel on
139
the buccal than on the lingual side, while the
present study, also on South African hominids, accords with the expected pattern.
This discrepancy could be due to differences
in technique which need to be investigated
and clarified.
Bearing in mind the reported functional
demand on the buccal and lingual cusps
(Molnar and Gantt, 1977; Molnar and Ward,
1977), it is surprising that the greatest deviation from the isometric line was found for
tooth walls rather than cusp tips. Although
this could xiggect that the entire lingual
cusp was under relatively stronger compressive forces, a note of caution seems to be
called for. Australopithecines are characterized by a high percentage of protoconal cinguli or Carabelli cusps, especially A. africanus (Robinson, 1956; Sperber, 1974; Wood
and Engleman, 1988). Besides, in a sample
of Homo s. sapiens from Sangiran, Korenhof
(1960,1982) showed a relatively high occurrence of complete or incomplete cinguli a t
the DEJ despite the lack thereof at the tooth
crown. A similar morphological situation at
the DEJ may have exaggerated differences
between lingual and buccal walls of teeth
used in the present study.
With regard to enamel thickness at the
occlusal basin, SKX 268 (Homo)falls within
the upper range of values, whereas the buccal and lingual walls of the tooth exhibit relatively thinner enamel when compared to
values from most other specimens. Hence, in
total enamel aredvolume of enamel SKX
268 could be expected to lie at the lower end
of the range for Swartkrans hominid teeth.
While bi-variate analyses appear to show
no major structural differences in enamel
thickness distribution among australopithecine specimens from Sterkfontein, Swartkrans and Kromdraai, multivariate analyses reveal complex patterns of enamel
distribution over the tooth crown between
teeth. Most of the variation between maxillary molars can be attributed to absolute
size differences, although this could have
been artificially inflated because of measurement errors.
Complex proportional differencesin enamel
thickness of sectioned Plio-Pleistocene hominid molars were noted by Grine and Martin
140
G A MACHO AND J.F. TKACKERAY
(1988). The A. africanus specimen STW 284
and A. robustus SKX 21841 exhibit a pattern
similar to that found in humans, whereas
the other A. africanus tooth, Stw 402, is
comparable to A. boisei L10-21 and L398847. Kb 5223 is similar to the latter in its
preserved parts. Although Grine and Martin
seem to acknowledge that humans tend to
be different in their proportions of enamel
thickness from the “robust” australopithecines, it is not clear whether they regard the
results obtained for A . africanus as random
nwing t o the qmall sample si7e, n r whether
their findings have taxonomic implications
(sensu Clarke).
The Homo specimen SKX 268 was most
clearly separated from the other teeth,
mainly due to a proportional difference of
enamel thickness between the lingual half of
the paracone and that of the lingual slope of
the protocone (Table 2, Figs. 2,3);the former
was proportionally thicker than the latter.
Two explanations, not mutually exclusive,
are offered for this finding.
The lingual slope of the paracone is a
Phase I facet in primates and thus participates mainly in shearing (Crompton and Hiiemae, 1969; Kay, 1975; Maier and Schneck,
1981, 1982; Mills, 1955, 1963). A more omnivorous diet in Homo would have required
proportionally more puncture-crushing and
shearing than grinding, the latter being essential to break down harder and more fibrous foods. The morphology of Homo teeth
with high, less spaced cusps and more
steeply inclined slopes of wear facets is a
reflection of an altered diet. Hence, a proportionally thickened lingual slope of the
paracone relative to the tooth walls could be
regarded as an evolutionary adaptation to
enhanced shearing and less grinding activity in Homo.
Although the cusps of australopithecine
molars are low and bulbous (Beynon and
Wood, 1986; Grine, 1981, 1986; Robinson,
1956; Wallace, 1972, 19781, those of Homo
are more pronounced. This situation may be
paralleled at the DEJ. No systematic study
has so far been done on this aspect, but
Grine and Martin’s figure 1.10 and 1.11
clearly support this suggestion. Owing to
the more protruding dentine horns in Homo,
1
the reference plane A-B (Fig. 1)is proportionally lower and thus closer to the cervical
border where the enamel thickness tapers.
It is more likely that in australopithecine
teeth, appositional, i.e. full-thickness enamel
over the cusp tips, rather than imbricational
enamel has been measured (Beynon and
Wood, 1987). In Homo the enamel extension
rate decreases markedly towards the cervix,
while the crown formation time is longer
when compared with australopithecines
(Beynon and Dean, 1988; see also Mann et
a1 , 19W for review nf literatvrcl.) This fa.ct
as well as the frequent occurrence of protoconal cinguli in Australopithecus discussed
above may have exaggerated (although not
invalidated) structural differences between
Homo and australopithecine specimens examined. The use of this measurement for
taxonomic purposes is therefore questionable. It would seem that diameters of walls
defined by Beynon and Wood (1986) (i.e.,
their variables LT(L) and LT(B)),which are
taken about lmm below the dentine horns
irrespective of the reference plane A-B,
are more reliable than those of Martin
(1983) (i.e., his variables k and 1; see also
our measurements No. 3 and 6). The latter
pair could be abandoned for taxonomic
purposes.
Grine and Martin (1988) did not report a
distinctive pattern of enamel distribution
for their sample from Sterkfontein. However, the present study demonstrates that
the Sterkfontein material differs structut ally from the majority of Swartkrans molars, but not from those from Kromdraai.
Sterkfontein was separated from the main
cluster of Swartkrans hominid teeth by a
proportional difference of enamel thickness
between the occlusal basin (including the
cusp area of the protocone) on the one hand,
and the buccal half of the paracone and the
lingual wall of the protocone, on the other.
Interestingly, three out of four Kromdraai
specimens, i.e., Kb 5383, TM 1601, and TM
1603, fell on the same half of PC I11 as did
four Swartkrans specimens. Hence, most of
the molars studied from Swartkrans have a
proportionally thicker occlusal basin when
compared with those from Sterkfontein and
Krorndraai.
PATTERNING IN ENAMEL THICKNESS
In his SEM study on “gracile”and “robust”
deciduous australopithecine teeth, Grine
(1981) claimed that Kromdraai molars appeared to have followed a masticatory occlusal course similar to that of A. africanus.
He concluded that on morphological grounds
the Kromdraai hominid molars seem “to be
‘intermediate’ between the ‘gracile’ and
Swartkrans ‘robust’ forms” (Grine, 1981:
223).The pattern of enamel thickness distribution, as found in the present study, substantiates this claim. While there are subtle
differences between the majority of specimens from Sterkfontein and those from
Kromdraai, the hominid teeth from these
sites are more similar to each other than
they are to most of the specimens of A. robustus from Swartkrans with regard to
enamel distribution. However, the Kromdraai specimens investigated are not homogeneous morphologically; i.e., none of the
principal components resulted in a complete
clustering of all hominid teeth from that site.
The proportionally thicker buccal slope of
the protocone can be reconciled with Grine’s
(1981, 1986) findings on tooth wear in A.
robustus. He suggested that the greater
Phase I1 facets in “robust”australopithecine
molars are related to a different diet and a
greater amount of compressive force. The
morphology of the teeth and the microwear
features both led him to conclude that the
dentition of A. robustus was adapted for
grinding and puncture-crushing, The distribution of enamel found in the present study
supports this hypothesis. Although the specimens from Swartkrans have on average
thicker enamel, the major difference seems
to occur along the Phase I1 facets; the areas
underneath these facets are endowed with
proportionally thicker enamel. This could
indicate an evolutionary response to enhanced grinding activity attributable to fibrous diet. Conversely, A. africanus, which
possesses more pointed cusps, also exhibits
proportionally thinner enamel at the grinding facet of the protocone. The distribution
of enamel in A. robustus from Kromdraai is
comparable to that seen in maxillary molars
from Sterkfontein.
In conclusion, structural differences appear to accord, at least in part, with differ-
141
ences between taxa and may be related to
different diets. However, since A. africanus
and A. robustus have in common relatively
thick enamel along the lingual and buccal
wall, a substantial compressive load owing
to puncture-crushing and grinding can be
postulated for both. By contrast, Homo appears to exhibit a greater disparity in
enamel thickness between the occlusal surface, including cusp tips, and walls o f the
molar. In order to remain functional over a
long period of time, a tooth so endowed
should not be subjected to n;assive grinding
forces. Nonetheless, even an omnivorous
diet could demand a considerable amount of
shearing and puncture-crushing and, hence,
thick enamel over cusp tips could be an advantage. However, since only one Homo molar has been used in this investigation, the
results must be regarded as tentative.
From this preliminary study we conclude
that enamel thickness per se, or the volume
of tissue, may not necessarily be a useful
measure to distinguish between hominid
species as postulated by previous anthropologists. Among early hominids, there appears to be considerable overlap in enamel
thickness and overall tooth size and morphology, especially where maxillary molars
are concerned (Corruccini and McHenry,
1980; Wood and Engleman, 1988). On the
other hand, the distribution of enamel thickness over the tooth crown differs between
taxa in a way that may reflect masticatory
differences, indicative of different diets. The
present results support conclusions reached
from previous studies, which postulate that
diets of “robust” australopithecines were
comparatively coarser and more fibrous
than those ofA. africanus and Homo.
The possibility of enamel patterning reflecting evolutionary/functional responses
to different diets needs to be tested further
by utilizing a bigger sample of molars from
extinct taxa and extant species with known
dietary specialization. Furthermore, differences of enamel distribution between molars
within a species need to be investigated. Finally, in order to conclusively elucidate
functional and mechanical properties of
enamel thickness, other aspects of tooth
morphology and histology need to be incor-
142
G.A. MACHO AND J.F. THACKERAY
porated. Such studies would preferably combine investigations of tooth morphology, development, microwear, enamel thickness,
trace elements and stable carbon isotope ratios.
ACKNOWLEDGMENTS
We thank J . Nach for permission to use
the Siemens Scanner at the Hillbrow Hospital, Johannesburg, and R. Harman and R.
van der Riet for their help with the scanning
operation. Furthermore, we are gratef‘d tc
G. Conrog, M.C. Dean. L. Freedman, F.
Grine, D. Panagos, E.E. Sarmiento, F.
Spoor, and F. Zonneveld for valuable advice
andlor comments on the manuscript. We
also thank two anonymous referees for their
thoughtful comments. R. Hill gave permission to use the Kontron Image Analyser at
the Department of Anatomy, University of
the Witwatersrand. This work was financed
in part by the Foundation for Research and
Development. We thank C.K. Brain, Director of the Transvaal Museum, for his support and encouragement.
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swartkrans, kromdraai, tomography, enamel, south, pleistocene, maxillary, hominids, plio, sterkfontein, africa, molar, stud, thickness, exploratory, computer
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