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Patterns of tooth crown size and shape variation in great apes and humans and species recognition in the hominid fossil record.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 125:303–319 (2004)
Patterns of Tooth Crown Size and Shape Variation in
Great Apes and Humans and Species Recognition in the
Hominid Fossil Record
Jeremiah E. Scott1* and Charles A. Lockwood1,2
1
2
Department of Anthropology, Arizona State University, Tempe, Arizona 85287
Institute of Human Origins, Arizona State University, Tempe, Arizona 85287
KEY WORDS
dental variation; taxonomy; Hominidae; Homininae; Ponginae
ABSTRACT
It has been suggested that patterns of
craniodental variation in living hominids (Gorilla, Homo,
Pan, and Pongo) may be useful for evaluating variation in
fossil hominid assemblages. Using this approach, a fossil
sample exhibiting a pattern of variation that deviates
from one shared among living taxa would be regarded as
taxonomically heterogeneous. Here we examine patterns
of tooth crown size and shape variation in great apes and
humans to determine 1) if these taxa share a pattern of
dental variation, and 2) if such a pattern can reliably
discriminate between samples that contain single species
and those that contain multiple species. We use parametric and nonparametric correlation methods to establish
the degree of pattern similarity among taxa, and randomization tests to assess their statistical significance. The
results of this study show that extant hominids do not
share a pattern of dental size variation, and thus these
taxa cannot be used to generate expectations for patterns
of size variation in fossil hominid species. The hominines
(Gorilla, Homo, and Pan) do share a pattern of shape
variation in the mandibular dentition; however, Pongo is
distinct, and thus it is unclear which, if either, pattern
should be expected in fossil hominids. Moreover, in this
case, most combined-species samples exhibit patterns of
shape variation that are similar to those for single hominine species samples. Thus, although a common pattern of
shape variation is present in the mandibular dentition, it
is not useful for recognizing taxonomically mixed paleontological samples. Am J Phys Anthropol 125:303–319,
2004. © 2004 Wiley-Liss, Inc.
Delineating species is a primary goal of paleoanthropology, as it is a necessary first step in the generation
and testing of phylogenetic and adaptationist hypotheses. Therefore, much research has been devoted to
quantitative methods of identifying taxonomically heterogeneous fossil assemblages, particularly in the
hominid1 fossil record (e.g., Lieberman et al., 1988;
Kimbel and White, 1988; Kelley and Etler, 1989;
Miller, 1991, 2000; Wood, 1991, 1993; Cope and Lacy,
1992; Cope, 1993; Martin and Andrews, 1993; Plavcan,
1993; Teaford et al., 1993; Kramer et al., 1995; Richmond and Jungers, 1995; Lockwood et al., 1996; Grine
et al., 1996; Kelley and Plavcan, 1998; Donnelly and
Kramer, 1999; Plavcan and Cope, 2001; Wood and
Lieberman, 2001). Such investigations typically use
extant species as models for interpreting levels of variation in fossil assemblages. In the case of fossil hominids, levels of variation in samples of Gorilla and
Pongo, the most sexually dimorphic living hominids,
are commonly considered to represent the upper limit
of intraspecific variation, and thus fossil hominid samples that exceed the limits set by these taxa are some-
times argued to contain multiple species (e.g.,
Stringer, 1986; Teaford et al., 1993).
However, in some cases, degrees of variation by
themselves may not correctly identify taxonomically
heterogeneous samples. One problem with relying
on the variational limits of extant species is the fact
that some fossil taxa may have possessed levels of
variation that exceed those observed in samples of
living species. Kelley (1993; see also Kelley and Xu,
1991; Kelley and Plavcan, 1998) argued that there is
no reason to expect that the maximum level of hominid sexual dimorphism is sampled among extant
species (but for a contrary view, see Kay, 1982; Cope
and Lacy, 1992; Martin and Andrews, 1993; Plav-
Charles A. Lockwood is currently at the Department of Anthropology, University College London, Gower Street, London WC1E 6BT,
UK. E-mail: c.lockwood@ucl.ac.uk
*Correspondence to: Jeremiah E. Scott, Department of Anthropology, Box 872402, Arizona State University, Tempe, AZ 85287-2402.
E-mail: jeremiah.scott@asu.edu
Received 10 March 2003; accepted 22 July 2003.
1
The taxonomy employed in this paper follows that of Groves
(1986), who included Gorilla, Homo, Pan, and Pongo in the family
Hominidae, while separating Pongo from the African apes and humans into the subfamilies Ponginae and Homininae, respectively.
©
2004 WILEY-LISS, INC.
DOI 10.1002/ajpa.10406
Published online 12 May 2004 in Wiley InterScience (www.
interscience.wiley.com).
304
J.E. SCOTT AND C.A. LOCKWOOD
can, 1993; Teaford et al., 1993). Since sexual dimorphism is a major component of intraspecific variation (e.g., Plavcan, 1990; Wood et al., 1991; Albrecht
and Miller, 1993), levels of dimorphism that exceed
those observed in modern hominids could produce
levels of variation in fossil species that may be incorrectly regarded as excessive for a single species.
In other words, assumptions of process uniformitarianism may hold (e.g., the biological mechanisms
that produce sexual dimorphism), but still result in
cases that violate assumptions of pattern uniformitarianism (e.g., a level of sexual dimorphism in a
fossil primate that exceeds that of all known living
primates).
Another factor complicating the use of magnitudes
of variation is the fact that some fossil assemblages
may contain multiple taxa without exhibiting excessive variation (Godfrey and Marks, 1991). Tattersall
(1986) warned that there is a tendency among paleontologists, and paleoanthropologists in particular,
to underestimate taxic diversity in the fossil record,
a consequence of the fact that many closely related
species do not exhibit marked skeletal or dental
differentiation, making it difficult to distinguish between them. Cope (1993) and Plavcan (1993) demonstrated this problem empirically, showing that
levels of variation in dental dimensions for some
pooled-species samples of sympatric cercopithecids
do not exceed those of single-species reference samples. Plavcan (2002, p. 580) further noted that, because a fossil hominid sample generally must exceed
Gorilla and Pongo in levels of character variation in
order for the single-species hypothesis to be rejected,
two species would have to “differ in body size by as
much as a factor of 2 before a mixture of the two
would be universally accepted as excessively variable.” Not all closely related sympatric primate species exhibit such a size difference (e.g., species of
Cercopithecus and Hylobates), and thus, low levels of
variation in a fossil sample cannot result in a strong
rejection of a multiple-species hypothesis (Kimbel,
1991; Plavcan and Cope, 2001).
Wood et al. (1991) suggested that an alternative to
using the degree of variation is to use the pattern of
variation. Whereas degree refers to magnitude and
is generally character-specific (i.e., different characters possess different magnitudes of variation), the
pattern of variation is the magnitude of variation of
characters in relation to each other (Fig. 1). Wood et
al. (1991) argued that if extant hominids share a
pattern of variation, then it might be reasonable to
expect fossil hominids to exhibit such a pattern as
well. Thus, the presence of a pattern deviating from
that of extant hominids should be taken as evidence
for the presence of multiple taxa in a fossil sample
(Wood, 1993; Kramer et al., 1995).
The expected advantage of such an approach can
be seen in Figure 1. For example, even though a
fossil species that possesses an extreme level of sexual dimorphism may possess a relatively high level
of variation, its pattern of variation may not be
Fig. 1. Variability profiles illustrating difference between
pattern of variation and degree of variation. a: Two taxa in which
average degree of character variation (as measured by the coefficient of variation, or CV) is different but pattern is similar. b:
Two taxa which are similar in average degree of variation but
which differ in pattern (adapted from Sokal and Braumann,
1980).
unusual (Fig. 1a). Conversely, a fossil sample containing two taxa of similar size may not vary as
much in degree as a single highly dimorphic extant
species, but its pattern of variation may diverge
considerably from the common pattern displayed by
closely related extant taxa (Fig. 1b).
The utility of the pattern approach in discriminating between fossil assemblages that contain a single
species vs. those that contain multiple species depends on two factors. First, the extant comparative
taxa must share a pattern of variation. Second,
mixed-species samples must exhibit a pattern of
variation that differs from that seen in single-species samples. In the case of living hominid species,
several studies investigated the degree to which taxonomic patterns of craniodental variation within
this clade are similar or different (Lieberman et al.,
1985; Oxnard et al., 1985; Oxnard, 1987; O’Higgins
et al., 1990; Plavcan, 1990, 2002; Wood et al., 1991;
O’Higgins and Dryden, 1993; Kramer et al., 1995;
Uchida, 1996, 1998a,b; Lockwood, 1999; Wood and
Lieberman, 2001; Ackermann, 2002), while relatively little attention has been paid to how the patterns of variation for mixed-taxon samples compare
to those for single-taxon samples.
305
PATTERNS OF DENTAL VARIATION IN HOMINIDS
TABLE 1. Samples used in this study1
P. troglodytes4
2
G. g. gorilla
Lower
I1
I2
C
P3
P4
M1
M2
M3
Upper
I1
I2
C
P3
P4
M1
M2
M3
3
P. paniscus4
P. p. pygmaeus
troglodytes
schweinfurthii
verus
74
76
74
82
82
86
80
68
36
38
40
40
40
40
40
36
62
58
58
70
66
66
72
58
38
36
26
42
46
62
46
30
66
58
64
86
84
88
86
82
82
80
76
88
74
102
102
70
36
38
40
38
38
40
40
38
64
58
56
66
66
70
68
54
40
42
32
52
50
76
54
36
48
46
48
86
82
86
84
70
H. sapiens
Nubians5
Fort Center6
30
36
46
42
44
76
46
36
30
30
30
40
40
44
46
42
22/27/19
55/57/49
64/69/57
72/76/57
82/84/81
113/117/111
92/92/91
49/48/48
34
30
42
48
42
72
46
28
267
34
42
44
44
44
58
40
52/54/43
55/56/48
104/103/91
118/117/115
127/120/116
162/165/155
127/127/122
34/34/33
1
All samples contain equal numbers of males and females except where noted.
Data are from Mahler (1973).
3
Data are from Plavcan (1990).
4
Data provided by D.C. Johanson.
5
Sample measured by first author.
6
Data are from Cucina and Iscan (2000). Sex ratio is unknown. Sample sizes for each tooth are listed as follows: mesiodistal/
buccolingual/crown area.
7
Male to female ratio ⫽ 9:17 (see text for discussion).
2
The present study examines the dentition of living
hominids to determine whether or not there is a
shared pattern of variation among these taxa, and
evaluates the utility of using such a pattern to identify mixed-species samples. While previous studies
of pattern have focused on sexual dimorphism, this
study examines intraspecific variation as a whole.
This difference in focus is important because, although there is a relationship between sexual dimorphism and intraspecific variation, particularly
when levels of dimorphism are high and intrasexual
variation is low (Fleagle et al., 1980; Plavcan, 1994),
a measure of sexual dimorphism is not an exact
proxy for a measure of variation, particularly when
levels of dimorphism are low (Plavcan, 1994; Josephson et al., 1996; Rehg and Leigh, 1999), as they are
in the dentition.
MATERIALS AND METHODS
Samples and measurements
The species and sample sizes used in this study
are listed in Table 1. Dental metrical data include
mesiodistal (MD) and labiolingual (LL)2 or buccolingual (BL) dimensions for each maxillary and
mandibular tooth. In general, the left tooth is
preferred unless it is missing, in which case the
antimere is substituted. Eight single-taxon samples are examined: Gorilla gorilla gorilla, Pongo
pygmaeus pygmaeus, Pan troglodytes troglodytes,
P. t. schweinfurthii, P. t. verus, P. paniscus, and
2
For the sake of simplicity, the breadth measurement of the incisors
and canines will be referred to as its postcanine equivalent, the
buccolingual diameter, for the remainder of this paper.
two geographically restricted populations of Homo
sapiens: a population of Meroitic Nubians from
Sudan, and a precontact Florida Indian population. Dental metrical data for G. g. gorilla and P.
p. pygmaeus are from Mahler (1973) and Plavcan
(1990), respectively. D.C. Johanson provided the
data sets for the species and subspecies of Pan (see
Johanson, 1974). The Nubian sample, measured
by the first author, derives from a collection
housed at Arizona State University, while the
Florida Indian population data set is from Cucina
and Iscan (2000).
Each sample contains an equal number of males
and females, except for the Florida human population, in which the sex ratio is unknown. In the Nubian human population, the sex ratio is balanced for
all teeth except for I1. In this case, the male to
female ratio is approximately 1:2. However, balancing the sex ratio does not significantly alter the level
of variation in MD and BL, and thus all teeth are
included in the analyses.
In measuring the Nubian sample, the measurement definitions follow those laid out in Mahler
(1973) and Plavcan (1990). For the postcanine teeth,
the MD dimension is defined as the “greatest dimension of the tooth in the mesiodistal axis” (Plavcan,
1990, p. 40). The BL dimension for the upper premolars, upper molars, and lower premolars is taken
as the greatest breadth across the paracone (or paraconid) and protocone (or protoconid) (Plavcan, 1990).
This procedure differs slightly from those outlined in
Mahler (1973) and Johanson (1974) in that the BL
dimension does not necessarily lie perpendicular to
the MD dimension.
306
J.E. SCOTT AND C.A. LOCKWOOD
For the lower molars, both Johanson (1974) and
Plavcan (1990) took two BL measurements, one
across the trigonid and one across the talonid. In
apes, maximum breadth (the BL dimension used in
this study) usually corresponds to the latter measurement. However, in humans, the position of maximum breadth is much more variable, and thus the
BL dimension in the Nubian sample is taken as the
maximum breadth perpendicular to the MD dimension. For the incisors and canines, the MD dimension is as defined for the postcanine teeth. The BL (⫽
LL) dimension is the maximum width of the tooth
perpendicular to the MD dimension.
Both Mahler (1973) and Plavcan (1990) commented on the irregular morphology of the canine
and P3 in apes, which makes the MD and BL dimensions somewhat difficult to define for these teeth.
Since this analysis focuses on variation, differences
in measuring technique should not affect this study,
as long as the researchers from whom the data sets
employed in this study derive were consistent in
their methodology. Furthermore, the canines and P3
should be among the most variable teeth in the ape
samples. The difficulties encountered in measuring
ape canines and P3s do not represent a problem for
the Nubian sample.
The MD measurements for the Nubian sample are
not corrected for interstitial wear. In order to minimize the effect of wear on levels of dental variation,
heavily worn teeth are excluded from the analysis.
Mahler (1973), Plavcan (1990), and Cucina and
Iscan (2000) also did not estimate corrected MD
length, but Johanson (1974) did. The effects that
both interstitial wear and correcting for interstitial
wear have on magnitudes and patterns of variation
is unclear, although presumably one of the goals of
correcting for wear is to reduce irrelevant variation.
The effect that interobserver error has on this analysis is addressed below (see Discussion).
Each dimension of each tooth in the Nubian sample was measured three times in order to determine
the degree of intraobserver error. The dimension
used in the final analysis is an average of these three
measurements. Measurement error in the Nubian
sample is comparable to that for other studies, i.e.,
less than 2%.
Patterns of variation in single-taxon samples
Analyses are conducted on patterns of variation in
size and shape: MD and BL dimensions together,
MD dimensions only, BL dimensions only, tooth
crown areas (calculated as the product of MD and
BL), and tooth crown shape indices (calculated as
the natural logarithm of the ratio of MD to BL).
Cucina and Iscan (2000) presented only the summary statistics for MD and BL dimensions and
crown areas, and thus this sample is not included in
the analyses for patterns of shape variation. Patterns are examined for all maxillary and mandibular teeth together and for each jaw separately. Thus,
there are 15 pattern comparisons between each pair
of taxa, except for those including the Fort Center
human population, for which there are only 12 intertaxic comparisons due to the fact that patterns of
shape variation for this sample are not examined.
Size variation is quantified using the coefficient of
variation (CV), a measure of relative variation that
expresses the standard deviation of a sample as a
percentage of the mean (CV ⫽ 100s/X), facilitating
comparisons of variability between taxa or characters of different sizes (Simpson et al., 1960; Yablokov, 1974; Sokal and Braumann, 1980; Sokal and
Rohlf, 1995). The CV is widely used in studies of
variation in both extant and fossil taxa, particularly
in the primate fossil record (Gingerich, 1974; Fleagle
et al., 1980; Kimbel and White, 1988; Cope and Lacy,
1992; Plavcan, 1994; Lockwood et al., 1996, 2000;
Arsuaga et al., 1997; Kelley and Plavcan, 1998). For
crown shape, because it is a dimensionless index,
the standard deviation (s) is used as the measure of
variation.
Pearson’s product-moment correlation coefficient
(r) and Spearman’s coefficient of rank correlation (rs)
are used to express the degree of similarity among
the patterns of variation for each taxon. The former
is used to compare the patterns produced by the raw
CVs (and standard deviations, in the case of shape
variation) for each taxon, while the latter is used to
compare the patterns generated by the rank orders
of those CVs, i.e., the CVs are ranked from 1 to p for
p variables, 1 being the least variable and p being
the most variable (Lockwood, 1999; Plavcan, 2002).
There are two reasons for using both r and rs. First,
using the raw CVs has the advantage that similarities and differences in magnitude can be incorporated into the analysis of pattern. However, the disadvantage to using raw CVs is that high magnitudes
for a particular character or small group of characters can obscure subtler patterns of variation (Fig.
2). In hominid dentitions, high variation in the canines and P3 may represent a possible confounding
factor, i.e., high product-moment correlations may
be due to the common difference in the level of
variation between these teeth and the others. Thus,
the rank correlation test is also performed in order
to negate the effect that the absolute magnitude of
variation has on the product-moment correlations.
However, it is still possible that the canines and P3
will affect the rank correlations. Therefore, for heuristic purposes, all comparisons of patterns of size
variation are also carried out with the canines and
P3 excluded. These teeth are not excluded for comparisons of patterns of shape variation, as they are
not consistently the most variably shaped teeth (see
Results).
The null hypothesis for all analyses is no pattern
similarity between taxa, i.e., no correlation (r or rs ⫽
0). Developing criteria for what constitutes “similarity” or “difference” is somewhat problematic.
Clearly, a high correlation for a comparison between
two taxa would suggest the presence of a shared
pattern; however, placing a boundary between cor-
PATTERNS OF DENTAL VARIATION IN HOMINIDS
Fig. 2. Variability profiles illustrating difference between using (a) raw CVs and (b) a rank order of those CVs. In a, taxon A’s
pattern is more highly correlated with taxon B’s pattern than is
taxon C’s pattern using Pearson’s product-moment correlation
coefficient, even though A has a divergent pattern of variation in
characters 7–12 in comparison to B and C. This is due to fact that
A and B possess similar magnitudes of variation in characters 5
and 6. In b, the patterns of B and C are more highly correlated
with each other than either is with pattern for A using Spearman’s coefficient of rank correlation because they share the same
rank order pattern. Note that in b, the profiles of B and C overlap
completely, despite fact that they differ in overall magnitudes of
variation.
relations that indicate similarity vs. those that indicate difference is rather subjective. For example, it
is reasonable to assert that two taxa share a pattern
when a comparison between the two yields a correlation of r ⫽ 0.85, but a moderate correlation of r ⫽
0.60 is more difficult to interpret. In this study, we
employ two criteria with which to judge pattern
similarity. First, we use statistical significance of
the correlation coefficient. However, as discussed
below, some correlations (e.g., r ⫽ 0.42) have the
potential to achieve statistical significance without
being very high due to the large number of data
points in the comparison. Therefore, we arbitrarily
recognize correlations greater than or equal to r or
rs ⫽ 0.67 as indicative of pattern similarity.
Randomization tests are used to determine the
statistical significance of the correlation coefficients,
generally following methods outlined in Leigh
(1992) and Lockwood et al. (2000). The raw CVs or
rank-ordered CVs of one taxon in each pairwise comparison are randomly reordered 1,000 times and the
307
correlation coefficients for each permutation are calculated, creating a distribution of correlation coefficients with which to evaluate the probability of obtaining the r and rs values generated from the actual
pairwise comparisons. Probabilities are expressed
as (P ⫹ 1)/(R ⫹ 1), where P equals the number of
randomized correlation coefficients that exceed the
values generated by comparisons between extant
taxa, and R is the total number of correlation coefficients calculated from the randomized CVs and
ranks. In this study, correlations with a probability
of 0.05 or less are considered statistically significant.
All tests for significance are one-tailed, as there is no
biological reason to expect any two species in this
study to exhibit patterns of variation that are highly
negatively correlated, i.e., the opposite of one another.
It should be noted that the number of data points
included in each comparison determines the magnitude at which correlation coefficients achieve statistical significance. For example, in a comparison that
includes 16 data points, a correlation coefficient of
approximately3 r ⫽ 0.42 represents the lowest statistically significant value using a randomization
test with 1,000 permutations and an alpha of 0.05 or
less. However, reduction of the number of data
points from 16 to 8 raises the minimum statistically
significant correlation to approximately r ⫽ 0.62.
Thus, correlations for comparisons with relatively
few data points might be high (i.e., r ⬎ 0.60), but not
statistically significant. Inspection of the magnitudes
of correlation coefficients is conducted to reveal cases
where this might affect interpretation of results.
Patterns of variation in combinedspecies samples
In cases where some or all taxa are found to share
a pattern, combined-species samples are created in
order to evaluate whether taxonomically mixed samples exhibit patterns that allow them to be recognized as such. Combined-species samples are composed of two species that share a pattern; they
contain equal numbers of each species and equal
numbers of each sex of each species. Clearly, most
taxonomically mixed fossil assemblages are neither
species- nor sex-balanced. However, this situation is
the one most likely to result in a divergent pattern
in a mixed-species sample. If these samples do not
result in unusual patterns, there would be little
reason to simulate alternative sex and species sample compositions.
Patterns of variation for combined-species samples are compared to those of single taxa, as outlined
above. For patterns of variation to be considered
useful for detecting mixed-species fossil assemblages, correlation coefficients for comparisons be-
3
The word “approximately” is used because each randomization test
generates a unique set of permutations. Consequently, the minimum
value needed to achieve significance may be slightly more or less than
the actual values given in the example above.
308
J.E. SCOTT AND C.A. LOCKWOOD
TABLE 2. Pearson product-moment correlations between CV tooth crown area patterns of variation for each
nonhuman taxon (maxillary and mandibular dentitions)1
G. g. gor.
G. g. gor
P. p. pyg.
P. pan.
P. t. trg.
P. t. sch.
P. t. ver.
All teeth
No canines
or P3
0.93***
0.93***
0.91***
0.89***
0.89***
0.63*
0.57*
0.29
0.10
0.34
P. p. pyg.
All teeth
No canines
or P3
0.91***
0.93***
0.91***
0.93***
0.48*
0.09
0.08
0.42
P. pan.
All teeth
No canines
or P3
0.96***
0.95***
0.92***
0.37
0.20
0.38
P. t. trg.
P. t. sch.
All teeth
No canines
or P3
All teeth
No canines
or P3
0.98***
0.96***
0.74**
0.66**
0.93***
0.54*
1
G. g. gor., G. g. gorilla; P. pan., P. paniscus; P. p. pyg, P. p. pygmaeus; P. t. sch., P. t. schweinfurthii; P. t. trg., P. t. troglodytes; P. t.
ver., P. t. verus.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
tween combined-species samples and single-species
samples must be consistently lower than those for
comparisons between single-species samples.
RESULTS
The effect of the canines and P3
The most striking result for the intertaxic pattern
comparisons using the raw CVs is how strongly canine size variation affects the product-moment correlations. Table 2 displays the product-moment correlations for crown area patterns of variation
(maxillary and mandibular teeth together) in the
nonhuman taxa. There is a large discrepancy between the correlations that include the canines and
P3 and those that exclude them. For example, when
the canines and P3 are included in the comparison
between P. t. troglodytes and Pongo, the correlation
is r ⫽ 0.93, indicating that the two taxa share a
pattern. However, when the canines and P3 are excluded, r ⫽ 0.09, indicating no similarity in pattern.
This result occurs because the disparity between the
levels of variation in the canines and the rest of the
teeth produces a correlation that is essentially based
on two data points (Fig. 3).
The combination of high variation in the canines
and P3 and low variation in the remaining teeth does
represent a pattern of variation that is shared
among the nonhuman taxa (Wood et al., 1991). However, the statistical significance is unreliable. Also,
knowing that the canines are always more variable
than the other teeth in sexually dimorphic species is
not particularly useful for identifying taxonomically
mixed fossil hominid assemblages, as the canines in
such samples are likely to be the most variable teeth
if the constituent taxa are sexually dimorphic in
canine size. The primary goal of this study is to
determine the degree of overall pattern similarity
throughout the dentition among living hominids. In
this respect, the product-moment correlations that
include the canines and P3s are uninformative, as
they obscure the degree of pattern similarity in the
noncanine dentitions. For example, the high product-moment correlation for the comparison between
P. t. troglodytes and Pongo conceals the lack of a
Fig. 3. Crown area coefficients of variation (maxillary and
mandibular dentitions together) for P. p. pygmaeus plotted
against those for P. t. troglodytes. Each point represents the CV
for the crown area of a particular tooth in one taxon plotted
against the CV for the same tooth in the other taxon (e.g., the CV
for M1 in P. p. pygmaeus vs. the CV for M1 in P. t. troglodytes).
Note marked separation of canines from all other teeth. Pearson
product-moment correlation for all data points is r ⫽ 0.93. With
canines (and P3s) removed, r ⫽ 0.09.
shared pattern of crown area variation in the noncanine teeth between these taxa, a significant result
of this study. All other comparisons of size variation
using product-moment correlations produce similar
results, i.e., when the canines are included, the correlations are high, but when the canines are excluded, the correlations generally drop, often considerably (e.g., Table 2).
While the canines and P3 also affect the rank
correlations (see Tables 3– 6), the effect is not as
marked, as it is in the product-moment correlations.
Returning to the example above, the rank correlation between the crown area patterns of variation for
309
PATTERNS OF DENTAL VARIATION IN HOMINIDS
TABLE 3. Summary of Spearman rank correlations among groups of taxa for patterns of size variation
in mesiodistal and buccolingual diameters1
Excluding canines and P3
All teeth
Maxillary
Mandibular
All teeth
Maxillary
Mandibular
0.49**
0.23–0.76
0.47*
0.18–0.76
0.53*
0.23–0.82
0.49**
0.09–0.65
0.42
⫺0.04–0.64
0.48
⫺0.03–0.78
0.50**
0.32–0.76
0.49*
0.18–0.76
0.56*
0.23–0.78
0.54**
0.28–0.65
0.50*
0.16–0.64
0.58*
0.12–0.78
0.49**
0.32–0.76
0.44*
0.28–0.69
0.52*
0.23–0.76
0.52**
0.30–0.65
0.50*
0.27–0.60
0.58*
0.12–0.78
0.72***
0.58–0.76
0.66**
0.62–0.69
0.75**
0.50–0.76
0.58**
0.34–0.65
0.50*
0.44–0.54
0.64*
0.12–0.64
0.82***
0.72–0.84
0.79***
0.72–0.92
0.80***
0.68–0.90
0.68***
0.60–0.77
0.70**
0.59–0.88
0.59*
0.57–0.91
2
All taxa (24)
Median
Range
Hominines (17)
Median
Range
Pan ⫹ Homo (11)
Median
Range
Pan (3)
Median
Range
P. troglodytes3 (3)
Median
Range
Homo sapiens3 (1)
Value4
Nonhuman (12)
Median
Range
0.48**
0.66***
0.42–0.76
0.43*
0.62**
0.30–0.76
0.53*
0.58*
0.34–0.82
0.49**
0.35
0.49**
0.09–0.65
0.42
⫺0.04–0.64
0.57*
0.35
⫺0.03–0.74
1
Comparisons are interspecific only, unless otherwise noted.
Indicates number of within-group comparisons.
Comparisons are subspecific.
4
Note that comparison is between only two populations of H. sapiens, and thus there is no median or range.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
2
3
TABLE 4. Summary of Spearman rank correlations among groups of taxa for patterns of size variation in mesiodistal diameters1
Excluding canines and P3
All teeth
Maxillary
Mandibular
All teeth
Maxillary
Mandibular
0.43*
⫺0.12–0.68
0.33
⫺0.10–0.86
0.35
⫺0.24–0.95
0.38
⫺0.005–0.81
0.36
⫺0.07–0.79
0.49
⫺0.03–0.94
0.41
0.003–0.68
0.38
⫺0.10–0.86
0.31
⫺0.24–0.76
0.40
0.005–0.81
0.43
0.07–0.79
0.49
⫺0.03–0.89
0.33
0.006–0.63
0.33
0.19–0.69
0.26
⫺0.10–0.62
0.40
0.005–0.81
0.46
0.07–0.71
0.49
⫺0.03–0.89
0.59**
0.44–0.63
0.57
0.38–0.69
0.60
0.26–0.62
0.40
0.32–0.55
0.36
0.07–0.54
0.37
0.03–0.66
0.80***
0.68–0.81
0.90***
0.83–0.98
0.74*
0.55–0.76
0.64*
0.63–0.84
0.86*
0.75–0.96
0.54
0.43–0.94
0.49*
0.24
0.59**
0.42–0.68
0.48
0.29–0.86
2
All taxa (24)
Median
Range
Hominines (17)
Median
Range
Pan ⫹ Homo (11)
Median
Range
Pan (3)
Median
Range
P. troglodytes3 (3)
Median
Range
Homo sapiens3 (1)
Value4
Nonhuman (12)
Median
Range
0.86**
0.68*
0.26–0.95
0.35
0.37
0.12–0.57
0
0.23
⫺0.07–0.79
0.83*
0.48
⫺0.03–0.94
1
Comparisons are interspecific only, unless otherwise noted.
Indicates number of within-group comparisons.
3
Comparisons are subspecific.
4
Note that comparison is between only two populations of H. sapiens, and thus there is no median or range.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
2
P. t. troglodytes and Pongo is rs ⫽ 0.47 with the
canines and P3, and rs ⫽ 0.01 without them. Thus,
the difference between the product-moment correlation and the rank correlation in the comparison that
includes the canines and P3 is large (r ⫽ 0.93 vs. rs ⫽
0.47), while the difference between the two types of
correlations without the canines and P3 is very small
(r ⫽ 0.09 vs. rs ⫽ 0.01). In both cases, the canines
(and P3 to a lesser extent) are driving the correlations up. However, in the case of the rank correla-
310
J.E. SCOTT AND C.A. LOCKWOOD
TABLE 5. Summary of Spearman rank correlations among groups of taxa for patterns of size variation in buccolingual diameters1
Excluding canines and P3
All teeth
Maxillary
Mandibular
All teeth
Maxillary
Mandibular
2
All taxa (24)
Median
Range
Hominines (17)
Median
Range
Pan ⫹ Homo (11)
Median
Range
Pan (3)
Median
Range
P. troglodytes3 (3)
Median
Range
Homo sapiens3 (1)
Value4
Nonhuman (12)
Median
Range
0.62**
0.33–0.82
0.55
0.29–0.86
0.65*
0.14–093
0.45
⫺0.08–0.70
0.35
⫺0.07–0.79
0.40
⫺0.37–1.00
0.64**
0.44–0.76
0.52
0.33–0.69
0.69*
0.40–0.93
0.46
0.23–0.61
0.32
0–0.57
0.49
0.09–1.00
0.65**
0.44–0.76
0.52
0.33–0.69
0.71*
0.40–0.93
0.46
0.35–0.61
0.32
0–0.57
0.49
0.20–1.00
0.75***
0.66–0.76
0.52
0.33–0.55
0.86**
0.74–0.93
0.53*
0.37–0.56
0.29
0–0.32
0.71
0.37–1.00
0.80***
0.65–0.84
0.62
0.60–0.88
0.86**
0.67–0.93
0.63*
0.38–0.71
0.43
0.39–0.82
0.71
0.37–0.89
0.66**
0.81**
0.63**
0.33–0.82
0.57
0.29–0.86
0.62
0.56
0.14–0.93
0.62*
0.36
⫺0.08–0.70
0.75*
0.35
⫺0.07–0.79
0.37
0.23
⫺0.37–1.00
1
Comparisons are interspecific only, unless otherwise noted.
Indicates number of within-group comparisons.
3
Comparisons are subspecific.
4
Note that comparison is between only two populations of H. sapiens, and thus there is no median or range.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
2
TABLE 6. Summary of Spearman rank correlations among groups of taxa for patterns of size variation in crown areas1
Excluding canines and P3
All teeth
Maxillary
Mandibular
0.51*
0.12–0.76
0.38
⫺0.17–0.79
0.57
0.12–0.86
0.51*
0.12–0.71
0.38
⫺0.17–0.71
0.51*
0.12–0.71
All teeth
Maxillary
Mandibular
0.37
⫺0.07–0.75
0.29
⫺0.18–0.75
0.40
⫺0.26–0.94
0.55
0.12–0.86
0.40
0.09–0.75
0.29
⫺0.18–0.75
0.54
⫺0.26–0.94
0.38
⫺0.17–0.71
0.55
0.12–0.86
0.47
0.09–0.75
0.50
⫺0.18–0.75
0.54
⫺0.26–0.94
0.59*
0.51–0.64
0.38
0.21–0.52
0.74*
0.52–0.76
0.35
0.10–0.40
0.07
⫺0.18–0.29
0.37
⫺0.14–0.60
0.71**
0.62–0.86
0.76*
0.69–0.95
0.69*
0.57–0.79
0.59*
0.47–0.73
0.64
0.54–0.93
0.49
0.31–0.54
0.54*
0.38
0.62
0.43
0.60
0.57*
0.43–0.76
0.40
0.21–0.79
0.67*
0.40–0.83
2
All taxa (24)
Median
Range
Hominines (17)
Median
Range
Pan ⫹ Homo (11)
Median
Range
Pan (3)
Median
Range
P. troglodytes3 (3)
Median
Range
Homo sapiens3 (1)
Value4
Nonhuman (12)
Median
Range
0.56*
0.36
⫺0.07–0.63
0.10
⫺0.18–0.68
0.40
⫺0.14–0.77
1
Comparisons are interspecific only, unless otherwise noted.
Indicates number of within-group comparisons.
3
Comparisons are subspecific.
4
Note that comparison is between only two populations of H. sapiens, and thus there is no median or range.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
2
tion test, the effect is not as great, and the difference
in pattern between the two taxa is more apparent.
While the noncanine/P3 product-moment correlations provide a clearer picture in terms of overall
degree of pattern similarity among taxa than do the
product-moment correlations that include the canine and P3, these results are comparable to those
for the noncanine/P3 rank correlation tests. For this
reason, and those discussed above, the remainder of
this paper will focus on rank correlations.
311
PATTERNS OF DENTAL VARIATION IN HOMINIDS
TABLE 7. Summary of Spearman rank correlations among
groups of taxa for patterns of shape variation1
Patterns of variation in size
Tables 3– 6 summarize the results of the rank
correlation tests for patterns of variation in size.
Each analysis for a given set of measurements is
presented separately, and each table includes the
correlations for each pattern of dental variation both
with and without the canines and P3. The median
rank correlations and rank correlation ranges are
presented for: 1) all hominids, 2) all hominines (⫽
African apes and humans), 3) Pan and Homo, 4)
Pan, 5) the subspecies of P. troglodytes, 6) the two
human populations, and 7) the nonhuman taxa.
Groups 1– 6 are clades, and group 7 is a grade.
Comparisons summarized for groups 1– 4 and 7 are
interspecific only, while those for groups 5 and 6 are
intraspecific only. For example, in the rank correlation summary for all hominines, comparisons between subspecies of P. troglodytes are not included,
but comparisons between P. paniscus and each subspecies of P. troglodytes are. This is done because
correlations between subspecies of P. troglodytes are
generally high, and thus including them with the
interspecific comparisons would bias the median
rank correlations upward.
The most important conclusion to be drawn from
Tables 3– 6 is that, in general, the median rank
correlations for each group of taxa are low and the
ranges are wide, indicating that extant hominids as
a group do not share a stable pattern of size variation in the dentition. This conclusion is not affected
by the presence or absence of the canines and P3 in
the analysis. Moreover, the nonhuman comparisons
that include the canines and P3 demonstrate that
low rank correlations are not restricted to comparisons that include Homo, as might be predicted based
on the fact that humans and apes differ in canine
dimorphism (Wood et al., 1991). The highest group
medians come from comparisons within the genus
Pan; however, the rank correlations for these comparisons are not as high as one might expect, ranging from rs ⫽ 0.21 to rs ⫽ 0.98.
Significantly, removal of the canines and P3 from
the analysis results in surprisingly low rank correlations between subspecies of P. troglodytes. Likewise, the two human populations are not very similar to each other in most comparisons. In fact, the
Nubian population shares more patterns of variation with some nonhuman samples than it does with
the Fort Center population.
Although relatively high correlations are found
between some taxa (usually nonhuman) in each
group, other correlations within that group can be
quite low. For example, in a comparison between
patterns of variation in BL dimensions (maxillary
and mandibular teeth together) for Gorilla and
Pongo on one hand and Gorilla and P. t. schweinfurthii on the other, the rank correlations are rs ⫽
0.82 and rs ⫽ 0.76, respectively. However, a comparison between Pongo and P. t. schweinfurthii yields a
rank correlation of rs ⫽ 0.49, indicating that there is
All teeth
Maxillary
Mandibular
0.59**
0.22–0.88
0.46
0.07–0.90
0.71*
0.33–0.95
0.70**
0.53–0.88
0.56
0.24–0.90
0.76*
0.67–0.95
0.76***
0.53–0.84
0.57
0.33–0.88
0.76*
0.67–0.90
0.76***
0.54–0.84
0.57
0.38–0.79
0.86**
0.67–0.90
0.70**
0.61–0.78
0.64*
0.57–0.71
0.69*
0.62–0.79
0.55*
0.22–0.84
0.46
0.07–0.83
0.70*
0.33–0.90
2
All taxa (18)
Median
Range
Hominines (12)
Median
Range
Pan ⫹ Homo (7)
Median
Range
Pan (3)
Median
Range
Pan troglodytes3 (3)
Median
Range
Nonhuman taxa
(12)
Median
Range
1
Comparisons are interspecific only, unless otherwise noted. Fort
Center human population is not included in analysis of patterns
of shape variation. See Materials and Methods.
2
Indicates number of within-group comparisons.
3
Comparisons are subspecific only.
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
a substantial difference between these two taxa, and
thus, each is similar to Gorilla in a different way.
While the canines, P3, and first and second molars of
the nonhuman taxa are generally stable in their
ranks relative to each other, the other teeth exhibit
wide variation in their distributions in different
taxa. In fact, removal of the canines and P3 eliminates any suggestion of a shared pattern in many of
the nonhuman grade comparisons. Thus, patterns of
size variation in living hominids do not appear to be
consistent.
Patterns of variation in shape
Table 7 summarizes the results for comparisons
between patterns of variation in shape. The results
are presented as in Tables 3– 6 with the following
differences: 1) the canines and P3 are not excluded
from the analysis, and 2) the Fort Center human
population is not included in the comparisons (see
Materials and Methods). Table 7 reveals that comparisons among the hominine patterns of shape
variation are characterized by relatively high median rank correlations with narrow ranges when all
teeth are examined together and when the mandibular dentition is considered separately, indicating
the presence of a shared pattern among these taxa.
All rank correlations among hominine taxa are statistically significant at P ⬍ 0.05. When Pongo is
included in the comparisons (Table 7: all taxa, all
nonhuman taxa), the median rank correlations are
lower and the ranges are relatively wider, indicating
a difference between the orangutan and the hominines. Thus, there are two patterns of shape variation in the dentition of extant hominids: a hominine
312
J.E. SCOTT AND C.A. LOCKWOOD
TABLE 8. Sexual dimorphism in mandibular canine shape1
ISD
t-test
P. p.
pyg.
G. g.
gor.
P.
pan.
P. t.
trg.
P. t.
sch.
P. t.
ver.
H. s.
(N)
0.896
*
0.984
0.98
0.982
0.984
1.007
0.989
1
Canine shape is calculated as MD/BL. G. g. gor., G. g. gorilla; H.
s. (N), H. sapiens (Nubian population); P. pan., P. paniscus; P. p.
pyg., P. p. pygmaeus; P. t. sch., P. t. schweinfurthii; P. t. trg., P. t.
troglodytes; P. t. ver., P. t. verus.
ISD, index of sexual dimorphism ⫽ (male mean/female mean).
*Significance level of a t-test of difference between males and
females: P ⬍ 0.001.
parison with Pongo produces a rank correlation that
achieves statistical significance is Gorilla (rs ⫽ 0.69;
P ⬍ 0.05). The rank correlations between the orangutan and all other taxa are not statistically significant, and range from a high of rs ⫽ 0.60 to a low of
rs ⫽ 0.33, with a median of 0.45 (n ⫽ 5 comparisons).
One factor contributing to the difference between
the two subfamilies is shown in Table 8: in comparison to the hominines, the mandibular canine of the
orangutan exhibits a relatively high level of sexual
dimorphism in shape, with females having relatively narrow canines in comparison to males.
Fig. 4. Patterns of crown shape variation in mandibular dentition of (a) all extant hominines (⫽ African apes and humans)
and (b) all extant hominids. Rank 1 is least variable; rank 8 is
most variable. Note difference between Pongo and hominines in
rank of mandibular canine (b).
pattern and a pongine pattern. Furthermore, the
low median correlations exhibited by the hominine
maxillary dentition pattern comparisons imply that
the pattern exhibited when both jaws are examined
together is largely a product of the pattern in the
mandibular dentition. Therefore, only the mandibular pattern is considered hereafter.
Figure 4 illustrates the patterns of mandibular
tooth crown shape variation for each subfamily. The
hominine pattern (Fig. 4a) is characterized by high
variation in incisor and premolar shapes, and low
variation in canine and molar shapes. This pattern
can be further subdivided based on the rank of P3: P.
paniscus, P. t. troglodytes, and P. t. schweinfurthii
form one group (rs ⱖ 0.79 among all taxa) in which
the P3 is the most variable tooth, while Gorilla,
Homo, and P. t. verus form a second group (rs ⱖ 0.90
among all taxa) which is characterized by a P3 that
is intermediate in its level of shape variation. Intertaxic comparisons between these two subgroups
have rank correlations that range from rs ⫽ 0.62 to
rs ⫽ 0.76.
The orangutan pattern is similar to that of the
second group of hominines, with the major distinction being in the rank of the canine (Fig. 4b). In
Pongo, the canine is the most variable tooth,
whereas in hominines it is among the least variable
teeth. The only hominine taxon with which a com-
Similarities and differences among
individual taxa
Tables 9 and 10 list the patterns of variation that
are the most similar and most different, respectively, among individual taxa. Only the results for
analyses that include the canines and P3 are reported, although those for the analyses in which the
canine and P3 are excluded are generally comparable. The most similar taxonomic patterns of variation represent the top 25% of all 342 interspecific
comparisons (in this case, rs ⱖ 0.67). The least similar taxonomic patterns represent the bottom 25% of
all interspecific comparisons (rs ⱕ 0.34). Intraspecific comparisons with rank correlations that fall
within these ranges are also listed.
The most obvious conclusion to be drawn from
Tables 9 and 10 is that the two human samples are
very distinct from the nonhuman taxa in the majority of their patterns, particularly in comparison to
Gorilla, Pongo, and P. paniscus. While this is partly
due to the difference in sexual dimorphism in the
canines and P3 between humans and apes, the human patterns of variation are not more similar to
those for the ape samples when these teeth are
removed from the analyses.
In addition, as mentioned above, the human populations share very few patterns with each other.
Table 9 shows that only two of the comparisons
between the Nubians and the Fort Center population have rank correlations that are greater than or
equal to rs ⫽ 0.67. Conversely, Table 10 shows that
only a single comparison between the human populations falls at or below rs ⫽ 0.34. Thus, the human
populations are moderately correlated. Interestingly, the Nubian population shares more patterns
313
PATTERNS OF DENTAL VARIATION IN HOMINIDS
TABLE 9. Taxonomic patterns of variation that are most similar1
G. g. gor.
P. p. pyg.
P. pan.
P. t. trg.
P. t. sch.
P. p. pyg.
MBA, MBL, ML,
BA, BU, BL,
CAA, CAU,
CAL, CSL
MBA, MBU, MU,
BU, CAA, CAU,
CAL, CSA, CSU,
CSL
MBA, MBL, MA,
ML, BL, CAL,
CSL
MBU, BU, CSA,
CSL
P. pan.
P. t. trg.
P. t. sch.
P. t. ver.
H. s. (N)
CAA, CAL
ML
ML
MBA, MBL, BA,
BL, CAL,
CSA, CSL
MU, BL, CSA,
CSU, CSL
P. t. ver.
MBL, ML, CAL,
CSL
ML
MBA, MBL,
MBU, BA, BL,
CAL, CSL
H. s. (N)
CSA, CSU, CSL
BU
BU, BL, CSA,
CSU, CSL
H. s. (F)
BU
MBA, MBU, MBL,
MA, MU, ML,
BA, BU, BL,
CAA, CAU,
CAL, CSA, CSU,
CSL
MBA, MBU, MBL,
MA, MU, ML,
BA, BL, CAA,
CAU, CAL
MBL, BA, BL,
CAU, CSL
MBA, MBU, MBL,
MA, MU, BL,
CAU, CSA, CSL
BL, CAA, CAL,
CSA, CSL
BA, BU, BL
BL, CSL
CAL
ML, BU
1
Only rank-order comparisons are shown; comparisons using Pearson’s product-moment correlation coefficient are not listed (see text
for discussion). Only comparisons having Spearman rank correlations of rs ⫽ 0.67 or greater are reported. Comparisons of patterns of
size variation include canines and P3. G. g. gor., G. g. gorilla; H. s. (F), H. sapiens (Fort Center, Florida); H. s. (N), H. sapiens (Nubian
population); P. pan., P. paniscus; P. p. pyg., P. p. pygmaeus; P. t. sch., P. t. schweinfurthii; P. t. trg., P. t. troglodytes; P. t. ver., P. t. verus.
BA, buccolingual, both jaws; BL, buccolingual, lower jaw; BU, buccolingual, upper jaw; CAA, crown area, both jaws; CAL, crown area,
lower jaw; CAU, crown area, upper jaw; CSA, crown shape, both jaws; CSL, crown shape, lower jaw; CSU, crown shape, upper jaw;
MA, mesiodistal both jaws; MBA, mesiodistal ⫹ buccolingual, both jaws; MBL, mesiodistal ⫹ buccolingual, lower jaw; MBU,
mesiodistal ⫹ buccolingual, upper jaw; ML, mesiodistal, lower jaw; MU, mesiodistal, upper jaw.
TABLE 10. Taxonomic patterns of variation that are the most distinct1
G. g. gor.
P. p. pyg.
P. pan.
P. t. trg.
P. t. sch.
P. t. ver.
H. s. (N)
P. p. pyg. MU, CSU
P. pan.
P. t. trg.
P. t. sch.
P. t. ver.
H. s. (N)
H. s. (F)
MBL, MU, BL, CSA
MBU, MU, BA, BU,
CAU, CSA, CSU
MU, BL, CAU, CSA,
CSL
CSU
BL, CSA, CSU
MBA, MBU, MA, MBA, MBU, MBL, MA,
MU, ML, CAA,
MU, ML, CAA, CAU,
CAU, CAL
CAL, CSU
MA, ML, CAU
MBA, MBU, MA, MU,
ML, CAA, CAU, CAL
ML
BU, CAU
MBU, MA, MU, MA, MU, ML
ML
MBU, MU
ML, CAA,
CAL
MBL, MA, MU, MA, MU, ML MBA, MBL, MA, MA, ML, CAU
ML, CAA,
MU, ML, CAU
CAU, CAL
MA
1
Only rank-order comparisons are shown; comparisons using Pearson’s product-moment correlation coefficient are not listed (see text
for discussion). Spearman rank correlations range from a high of rs ⫽ 0.34 to a low of rs ⫽ ⫺0.24. Comparisons of patterns of size
variation include canines and P3. G. g. gor., G. g. gorilla; H. s. (F), H. sapiens (Fort Center, Florida); H. s. (N), H. sapiens (Nubian
population); P. pan., P. paniscus; P. p. pyg., P. p. pygmaeus; P. t. sch., P. t. schweinfurthii; P. t. trg., P. t. troglodytes; P. t. ver., P. t. verus.
BA, buccolingual, both jaws; BL, buccolingual, lower jaw; BU, buccolingual, upper jaw; CAA, crown area, both jaws; CAL, crown area,
lower jaw; CAU, crown area, upper jaw; CSA, crown shape, both jaws; CSL, crown shape, lower jaw; CSU, crown shape, upper jaw;
MA, mesiodistal both jaws; MBA, mesiodistal ⫹ buccolingual, both jaws; MBL, mesiodistal ⫹ buccolingual, lower jaw; MBU,
mesiodistal ⫹ buccolingual, upper jaw; ML, mesiodistal, lower jaw; MU, mesiodistal, upper jaw.
of variation with Gorilla (3), P. paniscus (5), P. t.
troglodytes (5), and P. t. schweinfurthii (5) than it
does with the Fort Center population.
The nonhuman species that share the most patterns are Gorilla and Pongo (10) and Gorilla and P.
paniscus (10). The number of patterns that each
subspecies of P. troglodytes shares with Gorilla and
P. paniscus varies from 4 –7. Thus, overall, P. paniscus is more similar to Gorilla than it is to P. troglodytes. Note also that P. t. troglodytes shares all of its
patterns with P. t. schweinfurthii and 11 with P. t.
verus, while only 9 comparisons between the latter
two taxa have rank correlations greater than or
equal to rs ⫽ 0.67.
314
J.E. SCOTT AND C.A. LOCKWOOD
TABLE 11. Combined species samples used in this study1
Lower
I1
I2
C
P3
P4
M1
M2
M3
Upper
I1
I2
C
P3
P4
M1
M2
M3
P. pan. ⫹
P. t. trg.
P. pan. ⫹
P. t. sch.
P. pan. ⫹
P. t. ver.
G. g.
gor. ⫹
P. t.
ver.
60
72
92
84
88
132
92
72
60
60
48
68
76
112
76
56
60
72
92
84
88
152
92
72
132
116
112
164
164
172
160
136
60
60
60
80
80
88
92
72
68
60
84
96
80
140
92
56
64
56
56
80
84
136
92
56
68
60
84
96
84
144
92
56
96
92
88
172
148
172
168
140
52
60
84
88
84
88
96
56
P. pan.
⫹ H. s.
(N)
1
All samples are species- and sex-balanced. Refer to Table 1 for
data sources. G. g. gor., G. g. gorilla; H. s. (N), H. sapiens (Nubian
population); P. pan., P. paniscus; P. t. sch., P. t. schweinfurthii; P.
t. trg., P. t. troglodytes; P. t. ver., P. t. verus.
The nonhuman taxa that have the least number of
patterns in common are P. paniscus and Pongo (2)
and P. troglodytes and Pongo (1 for each subspecies
of P. troglodytes). In fact, comparisons between
these taxa yield more low rank correlations (rs ⬍
0.34) than high correlations (rs ⬎ 0.67), indicating
that the Pan clade is very different from Pongo.
Patterns of shape variation in combinedspecies samples
The hominine pattern of mandibular tooth crown
shape variation represents the only pattern that is
stable across species and genera in this study, and
thus it is the only one that is examined in terms of
its ability to discriminate between single- and multiple-species samples. The combined-species samples used in this study (Table 11) simulate three
types of taxonomically mixed fossil assemblages: 1)
two sister species that overlap in dental size and are
similar in tooth shape (P. paniscus ⫹ one of each
subspecies of P. troglodytes), 2) two species that do
not overlap in size but are similar in shape (P. t.
verus ⫹ G. g. gorilla), and two species that overlap in
size but differ in shape (P. paniscus ⫹ H. sapiens). In
each case, the combined-species sample contains
equal numbers from each species and sex. The latter
two assemblages are extreme cases, and it is likely
that they would be recognized as taxonomically heterogeneous on grounds independent of pattern of
variation (e.g., the high magnitude of variation produced by the P. t. verus ⫹ G. g. gorilla sample would
immediately mark it as taxonomically mixed, while
the obvious differences in canine and P3 morphology
between H. sapiens and P. paniscus would similarly
indicate the presence of two taxa in the assemblage).
However, they are included here in order to repre-
sent a range of possible interspecific combinations,
and to determine how the patterns for mixed-species
samples behave in extreme circumstances.
The results for the comparisons between singleand combined-species patterns of mandibular tooth
crown shape variation are presented in Table 12,
and the patterns themselves are illustrated in Figure 5. Most comparisons between single- and combined-species samples have rank correlations that
are within the range of those derived from singlespecies comparisons. Perhaps surprisingly, the G. g.
gorilla ⫹ P. t. verus sample exhibits a pattern that
does not differentiate it from single-species samples.
This result is due to the fact that, although there is
a large disparity in dental size between the two taxa,
differences in canine and molar shapes are only
slight.
The one combined-species sample that does exhibit an unusual pattern is the P. paniscus ⫹ H.
sapiens sample. In this case, 4 of the 6 comparisons
fall below the low end of the range for single-taxa
comparisons and fail to achieve statistical significance, thus rejecting the single-species hypothesis
(Table 12). Figure 5 shows that the pattern for this
sample is similar to the orangutan pattern (rs ⫽ 0.81
between these two samples), with the canine being
the most variable tooth; however, with a standard
deviation of s ⫽ 0.203, canine shape variation in the
P. paniscus ⫹ H. sapiens sample is much more than
canine shape variation in the orangutan sample (s ⫽
0.094). As noted above, this result should be expected due to the marked difference in canine morphology between humans and other hominines, i.e.,
humans have canines that are longer labiolingually
than they are mesiodistally, whereas in the other
taxa, the opposite is true. Nevertheless, the rank of
the canine is the only real difference between this
sample and the single-taxon samples, and thus, correlations for two of the comparisons fall within the
range of single-taxon comparisons.
DISCUSSION
Patterns of variation in extant hominids
The results of this study are consistent with previous studies that emphasized the distinctiveness of
taxonomic patterns of dental sexual dimorphism
within the hominid clade (Lieberman et al., 1985;
Oxnard et al., 1985; Oxnard, 1987; Uchida, 1996,
1998a,b). In general, the differences among taxa in
patterns of dental variation outweigh the similarities. Of the 15 patterns examined, only one, the
hominine pattern of mandibular tooth crown shape
variation, is shared across a clade containing multiple genera. Significantly, patterns of variation in
size are not stable among hominids as a group,
among the apes, or even among all subspecies of P.
troglodytes. The latter finding is similar to the observation by Uchida (1998a) that subspecies of G.
gorilla exhibit pattern differences in dental sexual
dimorphism, indicating that some patterns of vari-
315
PATTERNS OF DENTAL VARIATION IN HOMINIDS
TABLE 12. Spearman rank correlations between rank-order mandibular tooth crown shape patterns of variation
for single taxon vs. combined-taxa samples1
P. pan. ⫹ P. t. trg.
P. pan. ⫹ P. t. sch.
P. pan. ⫹ P. t. ver.
G. g. gor. ⫹ P. t. ver.
P. pan. ⫹ H. s. (N)
G. g. gor.
P. pan.
P. t. trg.
P. t. sch.
P. t. ver.
H. s. (N)
Single-species
hypothesis
rejected2
0.76*
0.81**
0.76*
0.86**
0.50
0.95**
0.90**
0.95**
0.76*
0.64*
0.95**
0.90**
0.93**
0.79*
0.67*
0.81**
0.93**
0.90**
0.86**
0.43
0.64*
0.69*
0.74*
0.81**
0.31
0.76*
0.74*
0.76*
0.86**
0.40
0
0
0
0
4
1
G. g. gor., G. g. gorilla; H. s. (N), H. sapiens (Nubian population); P. pan., P. paniscus; P. t. sch., P. t. schweinfurthii; P. t. trg., P. t.
troglodytes; P. t. ver., P. t. verus.
2
Indicates number of combined-species/single-species comparisons in which rank correlation falls below low end of range for
single-taxon comparisons (see Table 6).
*P ⬍ 0.05.
**P ⬍ 0.01.
***P ⬍ 0.001.
Fig. 5. Patterns of crown shape variation in mandibular dentition of combined-hominine species samples. Note that most
samples exhibit patterns of variation that are similar to those of
single hominine taxa (compare to Fig. 4). The exception is P.
paniscus ⫹ H. sapiens sample, which has a pattern of variation
similar to that for P. p. pygmaeus.
ation are specific to populations rather than species.
While the human samples predictably have patterns
that are distinct from those exhibited by the ape
samples, there also appear to be substantial differences between Pan and Pongo, demonstrating that
Homo is not exceptional in possessing a “unique”
pattern (see also Oxnard, 1987; Lieberman et al.,
1985; Oxnard et al., 1985). Thus, the results of this
study indicate that patterns of dental size variation
are not consistent among hominids.
Wood et al. (1991, p. 199) found “two basic patterns” among hominids, one characterized by the
absence of sexual canine dimorphism (Homo), and
the other by its presence (Gorilla, Pan, and Pongo).
With regard to the patterns of dental dimorphism in
the other teeth, Wood et al. (1991, p. 199) described
the taxonomic differences as “relatively minor compared to the presence or absence of canine dimorphisms.” While such a conclusion is true, i.e., high
variation in the canines is the most prominent feature of the ape patterns of dental variation (as it is
among most anthropoids; Plavcan, 1990), the results
of this study and others (e.g., Oxnard, 1987) show
that patterns of size variation in the other teeth
have the potential to vary dramatically among species and even populations, and thus they are not
likely to be useful for assessing variation in fossil
samples of unknown taxonomic composition.4 Moreover, the pattern of high canine variation/low postcanine tooth variation by itself is probably not useful
for evaluating the taxonomic composition of a fossil
sample. This is because the canines are also likely to
be the most variable teeth in a sample that contains
multiple highly dimorphic species. Conversely, if a
fossil sample exhibits a pattern in which canine size
variation is exceeded by one or more of the postcanine teeth (i.e., a pattern that is the opposite of the
one observed in great apes), the pattern is still consistent with one species, if that species follows the
pattern seen in Homo.
The absence of a stable pattern of size variation in
the dentition of extant hominids may be surprising
in light of the fact that some studies suggest that
there is a shared pattern of cranial variation among
these taxa (Wood et al., 1991; Kramer et al., 1995;
Wood and Lieberman, 2001). For example, Wood
and Lieberman (2001) and Plavcan (2002) showed
that different regions of the skull (e.g., the basicranium, face, mandible) exhibit different levels of average variation, i.e., basicranial traits tend to be less
variable then palatal and mandibular traits, suggesting that extant taxa can be used to model cranial dimorphism in fossil species. However, it should
be noted that intertaxic differences have been documented in hominid cranial variation as well (e.g.,
O’Higgins et al., 1990; O’Higgins and Dryden, 1993;
Lockwood, 1999; Ackermann, 2002). For example,
Lockwood (1999) demonstrated that, while P. troglodytes and G. g. gorilla are very similar in their
patterns of cranial dimorphism, these taxa differ
substantially from G. g. beringei. Moreover,
O’Higgins et al. (1990) noted that significant differences can be found among modern human populations. In any event, while both similarities and dif-
4
These results should not be confused with patterns of magnitude,
i.e., patterns in tooth size.
316
J.E. SCOTT AND C.A. LOCKWOOD
ferences in patterns of variation can be identified
among taxa, and each may have implications for
components of variation such as sexual dimorphism,
the differences imply that patterns of variation may
not be useful for detecting mixed-species samples.
It should be noted that, because the differences
between levels of variation in the noncanine teeth
are more subtle than the difference between the
canines and noncanine teeth, the samples used in
this study may not be large enough to detect a
shared pattern. In other words, intertaxic differences in pattern may result from sampling error,
i.e., patterns of variation may fluctuate depending
on sample size. While it is possible that this may be
the case, it is nonetheless clear that, for practical
purposes (e.g., species recognition in the fossil
record), extant hominids lack a shared pattern of
noncanine dental size variation.
One might ask whether interobserver error is a
possible explanation for some of the differences documented in this study, as the data sets for each
genus derive from different sources. However, the
fact that only 19 of the 45 (42.2%) interspecific comparisons (canines and P3 included) within the genus
Pan are greater than or equal to rs ⫽ 0.67 (Table 9),
despite the fact that these samples were measured
by a single researcher (see Materials and Methods;
Table 1), and further, that the P. paniscus sample
shares more patterns with the Gorilla sample than
it does with any of the P. troglodytes samples, suggests that interobserver error is not responsible for
the lack of a shared hominid pattern.
On the other hand, the hominines do share a
pattern of shape variation in the mandibular dentition characterized by low variation in canine and
molar shapes and high variation in incisor and premolar shapes. In Pongo, the pattern is slightly different: the canine is the most variably shaped tooth,
a consequence of the fact that orangutans are sexually dimorphic in canine shape, but hominines are
not. While the hominine pattern could prove useful
in evaluating variation in fossil assemblages, the
fact that Pongo exhibits a different pattern implies
that fossil hominids outside of the hominine clade
might also have possessed different patterns, effectively limiting the application of this pattern to the
hominin fossil record.
Patterns of variation and species recognition
in the hominid fossil record
There are two ways in which patterns of variation
might be used to suggest taxonomic heterogeneity in
a fossil sample: a comparison of a fossil sample’s
pattern of variation to a shared pattern observable
in extant members of its clade, and comparison of a
fossil sample’s pattern of variation to the patterns
expressed by any extant analogue. In both cases,
results are subject to some statistical concerns, especially sample size. Moreover, interpretation requires that mixed-species samples have unusual
patterns of variation and that extant taxa thoroughly represent possible patterns of variation.
Wood (1993) and Kramer et al. (1995) focused on
patterns of metrical variation in the hominin fossil
record in evaluating variation in Homo habilis. Various studies noted that this taxon, as broadly defined, exhibits a high degree of craniodental variation (Stringer, 1986; Lieberman et al., 1988; Wood,
1992; Grine et al., 1996). Although Miller (1991,
2000) suggested that it is probable that the magnitude of variation observed in H. habilis could be
accommodated within a single, highly dimorphic
species, both Wood (1993) and Kramer et al. (1995)
found that the pattern of metric variation exhibited
by the more complete early Homo crania is unusual
in comparison to modern hominid patterns, leading
them to reject the single-species hypothesis.
Miller (2000) criticized the conclusions of Wood
(1993) and Kramer et al. (1995) on the grounds that
the sample size of early Homo crania is too small to
represent the actual pattern of variation for a population. Indeed, the pattern-based approach to evaluating variation is probably inapplicable to most
fossil samples, which generally consist of sample
sizes less than n ⫽ 10. Nevertheless, there do exist
fossil assemblages of sufficient size to allow for a
relatively accurate approximation of their pattern of
variation (e.g., the Lufeng and Pasalar Miocene
hominoid samples, and the Hadar Australopithecus
afarensis dental and mandibular series). In these
samples, at least, examination of patterns of variation could prove useful in taxonomic debates. However, beyond the problem of insufficient sample size,
the reliability of such a method for identifying taxonomically heterogeneous fossil samples has never
been adequately tested. Even if all hominid species
possess a similar pattern of variation, it is unclear
whether we should expect an assemblage that contains multiple hominid species to exhibit an unusual
pattern.
The results for the pattern comparisons between
single- and combined-species samples in this study
bear directly on this issue. First, the fact that the P.
paniscus ⫹ H. sapiens sample is identified as taxonomically mixed based on its pattern of variation in
4 out of 6 comparisons to single-species samples
confirms that mixed samples can exhibit unusual
patterns. In this case, the two taxa in the sample
differ markedly in a character that typically exhibits
low variation in single-species samples, i.e., canine
shape. However, as noted above, there is probably
little need to examine the pattern of variation in a
sample containing two taxa as dissimilar as P. paniscus and H. sapiens, as differences in dental proportions and morphology would clearly indicate the
presence of two species.
On the other hand, the other combined-species
samples examined in this study exhibit patterns
that cannot be distinguished from those for singlespecies samples, suggesting that this approach has
little power in terms of its ability to recognize fossil
PATTERNS OF DENTAL VARIATION IN HOMINIDS
assemblages that contain more than one species. As
demonstrated by the patterns of variation for the
combined-Pan samples, when two morphologically
similar taxa that share a pattern are mixed together, the resulting sample is likely to exhibit a
pattern that is similar to one for a single-species
sample. Thus, just as “the presence of a single species in a fossil sample . . . cannot be proved through
studies of relative variation” (Cope, 1993, p. 232),
the presence of a single species in a fossil sample
cannot be proved using patterns of dental variation.
Conversely, the presence of an unusual pattern in
a fossil sample does not provide unequivocal evidence of taxonomic heterogeneity. Although all
hominines share a pattern of mandibular tooth
crown shape variation, the possibility that a fossil
member of this clade could have possessed a unique
pattern cannot be dismissed. This conclusion is supported by the fact that the Pongo pattern of mandibular tooth crown shape variation is unique within
the hominid clade. If the Pongo sample used in this
study represented an extinct taxon and living hominids were used to evaluate its pattern of shape variation, one might be led to conclude that two species
were present in the sample on the basis of “unusual”
variation in canine shape. On the contrary, hominid
paleontologists should be prepared to encounter
novel patterns of mandibular tooth crown shape
variation in fossil hominines. Certainly the patterns
of shape variation in the maxillary dentition are
highly variable within the Homininae, as are patterns of size variation among living hominid species.
Kelley (1993) cautioned that imposing the variational limits observed in extant taxa on paleontological species may be a misapplication of the principle
of uniformitarianism, i.e., it confuses pattern with
process, and thus it restricts our ability to identify
novel conditions in fossil taxa. This argument is
based on the observation that the degree of postcanine tooth size dimorphism in the Lufeng fossil hominoid sample appears to exceed that of any extant
primate, assuming that the sample represents one
species, as argued by Kelley and Xu (1991), Kelley
(1993), and Kelley and Plavcan (1998). The implication is “that the greatest sexual dimorphism in tooth
size, and probably body size, found among extant
anthropoid primates does not represent some upper
limit during primate evolutionary history” (Kelley,
1993, p. 446). The extent to which paleontologists
should use variational limits of extant taxa as
guides to interpreting the fossil record is a matter of
debate (e.g., Kay, 1982; Cope and Lacy, 1992; Kelley,
1993; Martin and Andrews, 1993; Teaford et al.,
1993; Kelley and Plavcan, 1998). Regarding patterns of variation, the results of this study indicate
that living taxa do not sample all possible patterns
of dental variation, and thus, novel patterns should
be expected in paleontological taxa (for a similar
conclusion regarding patterns of cranial dimorphism, see Lockwood, 1999; Ackermann, 2002; Plavcan, 2002). Consequently, patterns of varia-
317
tion in fossil samples should be interpreted with
caution.
Phylogenetic information content of
patterns of variation
One of the underlying assumptions of this study is
that patterns of variation are phylogenetically patterned. This issue was more explicitly addressed by
others (e.g., Oxnard, 1987; Wood et al., 1991; Ackermann, 2002; Plavcan, 2002), with most studies concluding that the degree of pattern similarity among
taxa is inversely proportional to degree of phylogenetic separation, i.e., as phylogenetic distance increases, pattern similarity decreases. The results
presented here do not contradict these conclusions.
While most of the patterns examined in this study
are not shared across hominids as a group, the number of patterns shared among taxa appears to be
related to phylogenetic distance to some degree (see
Tables 9 and 10). For example, the number of patterns shared among taxa within the genus Pan is
high, the number of patterns shared between species
and subspecies of Pan and Pongo is low, and the
number of patterns shared between Pan and Gorilla
is intermediate.
The difference in pattern between Pongo and the
hominines in mandibular tooth crown shape variation represents another example of phylogenetic
patterning of a pattern of variation. While this pattern could prove useful in distinguishing between
fossil pongines and hominines, its polarity remains
to be determined. Including Hylobates as an outgroup could clarify this issue; however, the fact that
gibbons are sexually monomorphic suggests that a
broader comparative sample is required (e.g., all
catarrhines or all anthropoids).
CONCLUSIONS
This analysis of dental variation reveals that extant hominids lack a stable pattern of size variation.
While the human patterns are the most distinctive,
significant differences also exist among the nonhuman taxa (e.g., Pan and Pongo). Some of these differences may result from interobserver error, as the
samples for each genus were measured by different
researchers. However, the fact that the rank correlations between P. paniscus and subspecies of P.
troglodytes (measured by the same researcher) are
not particularly high indicates that differences in
measurement technique do not affect the conclusions of this study. Thus, patterns of dental metric
variation cannot be used to identify taxonomically
heterogeneous fossil assemblages, as fossil taxa are
likely to exhibit patterns that are as unique as those
for extant taxa.
The hominines do share a pattern of shape variation in the mandibular dentition. However, the orangutan exhibits a pattern that sets it apart from
the hominines, and thus the extent to which either
pattern can be expected in the fossil record is un-
318
J.E. SCOTT AND C.A. LOCKWOOD
clear. It is also likely that other patterns of shape
variation in the mandibular dentition may characterize fossil taxa. Consequently, the presence of an
unusual pattern in a fossil sample may not be indicative of the presence of multiple taxa. Furthermore,
combined-hominine species samples do not consistently exhibit unusual patterns of mandibular tooth
crown shape variation in comparison to single-species samples, underscoring the fact that the pattern
approach has little power to identify mixed-species
samples using dental variation.
ACKNOWLEDGMENTS
We thank Bill Kimbel, Diane Hawkey, and two
anonymous reviewers for their invaluable comments
and suggestions on numerous aspects of this study.
We also thank Diane Hawkey and the Arizona State
University Department of Anthropology for providing access to the Nubian skeletal collection.
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