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Functional and phylogenetic implications of molar flarevariation in Miocene hominoids
Michelle Singleton*
Department of Anatomy, Midwestern University, 555 31st Street, Downers Grove, IL 60515, USA
Received 14 November 2002; accepted 22 May 2003
Abstract
Comparative analyses of molar shape figure prominently in Miocene hominoid evolutionary studies, and incomplete
understanding of functional and phylogenetic influences on molar shape variation can have direct consequences for the
interpretation of fossil taxa. Molar flare is a shape trait whose polarity, phylogenetic distribution, and functional
significance have been sources of contention. To clarify the determinants of molar flare variation in the hominoid
radiation, a combination of statistical methods was employed to investigate the eff ects of diet, phylogeny, and geologic
age upon several measures of molar shape, to identify interactions among these factors, and to estimate their relative
influence. Classic indices of molar crown shape and cusp relief are highly significantly associated with diet and show no
clear phylogenetic or temporal patterning. Correlations with diet are insignificant when phylogenetic eff ects are
controlled, a result which is interpreted as an artifact of the distribution of folivory in the Miocene hominoid radiation.
Possession of pronounced molar flare was found to be the primitive condition for Miocene hominoids, but molar flare
reduction cannot be considered a crown hominoid synapomorphy. Molar flare is strongly correlated with geologic age
but diff ers significantly among dietary categories when the eff ects of time are controlled. Among contemporaneous taxa,
hard-object feeders consistently show the highest levels of flare. Molar flare reduction is hypothesized to arise from
realignment of cusp positions to maximize molar shearing and increase working occlusal surface area, while variation
in flare among contemporaneous taxa may be due, at least in part, to enamel thickness variation. The pronounced
molar flare of Otavipithecus is interpreted as a primitive retention, although alternative dietary and phylogenetic
interpretations cannot be excluded. A dramatic reversal of molar flare reduction in Mio-Pliocene hominins is
interpreted as a synapomorphy of the crown hominin clade, thus supporting the hominin status of the Lukeino
hominine. The last common ancestor of the Pan-Homo clade is predicted to have possessed relatively non-flaring
molars, and implications of this hypothesis for early hominin recognition are discussed. 2003 Elsevier Ltd. All rights reserved.
Keywords: Molar flare; Temporal trends; Diet; Hard-object feeding; Otavipithecus; Lukeino molar; Early hominins
* Corresponding author. Tel.: +1-630-515-6137; fax: +1-630-971-6414
E-mail address: [email protected] (M. Singleton).
Journal of Human Evolution 45 (2003) 57–79
0047-2484/03/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0047-2484(03)00086-1
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Introduction
The predominance of teeth in the primate fossil
record dictates that comparisons of molar mor-
phology figure prominently both in determinationsof phylogenetic affinity and paleodietary recon-
structions (Kay and Hiiemae, 1974; Kay, 1975;
Rosenberger and Kinzey, 1976); thus, a clear
understanding of the determinants of molar shape
variation is crucial to the accurate reconstruction of
fossil primate paleobiology. Comparative analyses
of extant and fossil primates have yielded many
useful generalizations concerning relationships be-
tween molar shape and diet (Kay and Hiiemae,
1974; Kay, 1975, 1978, 1984; Rosenberger and
Kinzey, 1976; Kay and Covert, 1984; Benefit, 1987,1999; Benefit and McCrossin, 1990). Frugivorous
primates are known to possess short, broad molars
with low crowns and minimal cusp relief, while
folivorous taxa exhibit long, narrow and high-
crowned molars with high cusp relief (Kay and
Hiiemae, 1974; Kay, 1975, 1978; Rosenberger and
Kinzey, 1976; Benefit, 1987, 1993, 1999; Benefit
and McCrossin, 1990; Kay and Ungar, 1997). At
the same time, key molar characters such as molar
enamel thickness and molar cusp relief are known
to be subject to significant phylogenetic eff ects
(Kay, 1978; Teaford, 1994; Dumont, 1995). Theintersection of these lines of inquiry is the crux of
phylogenetic character analysis, and the conclusion
that a particular molar shape character reflects pat-
terns of recent common ancestry, functional
demands of diet, or some combination thereof can
profoundly influence interpretations of individual
fossil primate taxa.
Many aspects of molar shape remain poorly
understood. Molar flare is one such character
whose polarity, phylogenetic distribution, and
functional significance have been ongoing sourcesof contention, particularly in relation to two prob-
lematic Miocene hominoid fossils. The Lukeino
molar (KNM-LU 335) is an isolated hominine1 M1
or M2 germ from the late Miocene Lukeino For-
mation (Tugen Hills, Kenya). Initially attributed
to Hominidae (Andrews in Pickford, 1975), early
descriptions of this specimen stressed its pro-nounced basal flare and overall resemblance to
robust australopiths (Andrews in Pickford, 1975;
Pickford, 1978). Subsequent assessments empha-
sized its phenetic similarities to Pan (Corruccini
and McHenry, 1980; McHenry and Corruccini,
1980; Hill and Ward, 1988) and posited Lukeino
as a possible morphotype for the last common
ancestor of the Pan-human clade (Hill and Ward,
1988). Ungar et al. (1994) found statistically sig-
nificant diff erences between Lukeino and Pan
in cusp and fissure arrangement as well as flare
but, lacking data on molar flare polarity, hesitated
to draw firm conclusions concerning the
taxon’s ancestral status. The recent attribution of
KNM-LU 335 to Orrorin tugenensis, a 6 Ma
hominine taxon claimed to post-date the
Australopithecus-Homo divergence (Senut et al.,
2001), has renewed interest in the Lukeino molar
and gives new impetus to establish the polarity of
molar flare within the crown hominoid clade.
The middle Miocene stem hominoid Otavi-
pithecus namibiensis (Andrews, 1992; Conroy et al.,
1992; Singleton, 2000; Ward and Duren, 2002) isanother fossil taxon whose interpretation would be
aided by a clearer understanding of molar flare
variation. Otavipithecus possesses an idiosyncratic
molar morphology characterized by pronounced
molar flare, thin enamel, high dentine horn pen-
etrance, and lack of diff erential wear; its dietary
adaptations are uncertain (Conroy et al., 1992;
Singleton, 2000). The hypothesis that Otavi-
pithecus is related to the early Miocene East
African hominoid Afropithecus (Andrews, 1992) is
only weakly supported by parsimony analysis(Singleton, 2000), and the majority of characters
shared by these taxa are common to all
post-Proconsul Miocene hominoids. Although
Singleton (2000) identified the shared possession of
mandibular molar flare as a potential afropithecin
(Andrews, 1992) synapomorphy, the presence of
pronounced molar flare in Aegyptopithecus and
related Oligocene catarrhines raises the possibility
that molar flare is the basal catarrhine condition
and thus uninformative concerning stem hominoid
1 For the purposes of this paper, Hominidae signifies the
crown great ape clade while Homininae is defined as the sister
group to the Ponginae, the clade comprising Pongo and its
extinct Eurasian sister taxa. Hominini is the sister group to the
recent African ape clade and encompasses members of the
crown clade (hominins) as well as ape-grade hominines post
dating the Pan-Homo divergence (stem hominins).
M. Singleton / Journal of Human Evolution 45 (2003) 57–7958
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relationships (Benefit, 2000; Singleton, 2000). Fur-
thermore, Benefit (1993, 1999, 2000) has found
pronounced molar flare to be functionally corre-
lated with frugivory and hard-object feeding incercopithecoid primates and has implied that a
similar relationship may obtain among Miocene
hominoids. In either case, the value of molar flare
as a phylogenetic character would be negated and
support for the Afropithecin Hypothesis signifi-
cantly reduced. In addition to these most common
sources of homoplasy—functional convergence
and phylogenetic inertia—a third factor may influ-
ence the distribution of molar flare among
Miocene hominoids, namely time. Molar func-
tional morphology in Miocene catarrhines has
been shown to be subject to temporal trends
(Ungar and Kay, 1995; Kay and Ungar, 1997), and
the poor resolution of Miocene hominoid phylo-
genies makes it difficult to separate the eff ect of
phylogenetic propinquity from that of geologic
age. If present, such temporal trends could con-
found both phylogenetic and paleodietary infer-
ences for Miocene hominoids and early hominins.
To resolve these questions, this study examines
the determinants of molar flare variation in the
hominoid radiation and tests a series of inter-
related hypotheses:
1) Molar flare is a functional correlate of diet. If
this is the case, statistically significant diff er-
ences in molar flare should be observed
among members of major dietary categories
independent of phylogenetic relatedness.
2) Molar flare is a phylogenetic character whose
distribution is solely dependent upon patterns
of common ancestry. If this is the case, molar
flare is expected to show clear phylogenetic
trends and to correlate with measures of phylo-
genetic propinquity. The distribution of molar
flare is expected to be independent of diet;
alternatively, molar flare and diet may show a
pattern of phylogenetic correlation.
3) Molar flare is subject to temporal trends. If this
is the case, molar flare is expected to be
strongly correlated with geologic age and inde-
pendent of phylogenetic propinquity.
These hypotheses are not, of course, mutually
exclusive. Therefore a combination of statistical
methods is employed to investigate potential
interactions among functional, phylogenetic, and
temporal eff ects and estimate their relative influ-
ence on the distribution of molar flare in Miocenehominoids. Implications of these results for
interpretations of Otavipithecus and the Lukeino
hominine are considered, and the role of molar
flare in early hominin recognition is subsequently
explored.
Materials and methods
Sample composition and data collection
The study sample comprised eighteen extant
and fossil catarrhine taxa (Table 1, Appendix A)
including fossil and extant members of the crown
hominoid clade, large-bodied stem hominoids,
and basal catarrhines. The Oligocene catarrhine
Aegyptopithecus zeuxis was included as a phylo-
genetic and temporal outgroup. Because closely
related taxa are expected to have both similar
molar proportions and similar dietary patterns,
each genus was represented by a single species.
Where data were available for two or more con-
generic species, interspecific comparisons wereconducted to insure that species sampling would
not influence genus-level results. Following com-
mon usage, taxa were classified as either folivorous
or frugivorous, with frugivores further subdivided
into soft-fruit frugivores and frugivorous mixed
hard-object feeders (Martin, 1990). These cat-
egories subsume considerable dietary variation;
hard-object feeding, in particular, encompasses a
wide range of food items with hard, brittle, or
abrasive consistencies. This classification neverthe-
less has proven useful in summarizing the func-tional capacities of primate teeth and the
mechanical properties of primate diets (Martin,
1990).
Only taxa for which diet could be established
with reasonable certainty were included in this
analysis. Dietary assessments for fossil taxa (see
Table 1) were made on the basis of dental micro-
wear analysis (Teaford and Walker, 1984; Teaford
et al., 1996; Ungar, 1996; Palmer et al., 1998, 2000;
King, 2001), shearing quotient analysis (Ungar
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and Kay, 1995; Teaford et al., 1996; Kay and
Ungar, 1997; Palmer et al., 2000), and molar
functional morphology. In no case was molar
shape, as measured in this paper, factored into
determinations of diet. Microwear analysis, which
provides direct evidence of diet consistency and
composition (Kay, 1984; Teaford et al., 1996), was
given somewhat greater weight than other factors.
Therefore, Aegyptopithecus, which shows enamel
pit sizes and frequencies most similar to modernhard-object feeders (Teaford et al., 1996), is classi-
fied accordingly; Sivapithecus, whose microwear
patterns are significantly diff erent from modern
hard-object feeders but indistinguishable from Pan
troglodytes (Teaford and Walker, 1984), is
grouped with soft-fruit frugivores. Approximate
geologic ages of fossil taxa were drawn from
published chronostratigraphic analyses (Pickford,
1978, 1998; Retallack, 1991; Kappelman, 1992;
Andrews et al., 1996, 1997; Andrews and Bernor,
1999; Kordos and Begun, 2001), and taxa were
grouped into five temporal sub-epochs corre-
sponding to the late Oligocene; early, middle, and
late Miocene; and present (see Table 1).
Quantification of molar shape was based upon
six linear measurements—maximum crown length
(ML), maximum mesial breadth (MMB), maxi-
mum distal breadth (MDB), mesial intercuspal
breadth (MCB), lingual metaconid height
measured from the cervix (MHT), and lingualnotch height measured from the cervix
(DHT)—taken on minimally worn second man-
dibular molars. All measurements were made on
original specimens with the exception of those for
Lufengpithecus and one Proconsul specimen, which
were taken from research quality casts. Measure-
ments were taken by the author with digital
calipers and recorded to the nearest 0.01 mm.
Molar size was calculated as the geometric mean of
linear measurements (Mosimann, 1970). Following
Table 1
Study sample and summary statistics for indices of molar shape
Molar Flare Crown Shape Cusp Relief
n Age (Ma) Sub-EpochMean S.D. Range Mean S.D. Range Mean S.D. Range
Frugivores
Pan troglodytes troglodytes 32 0 Recent 0.34 0.05 0.23–0.46 0.93 0.05 0.85–1.04 0.73 0.05 0 .61–0.83
Hylobates lar carpenteri 25 0 Recent 0.37 0.06 0.26–0.49 0.92 0.05 0.85–1.02 0.73 0.07 0 .58–0.85
Dendropithecus macinnesi ‡ 7 18 Early 0.45 0.06 0 .34–0.53 0 .87 0.06 0 .76–0.94 0 .66 0.04 0 .60–0.70
Dryopithecus laietanus‡ 6 10 Late 0.38 0.09 0.25–0.47 0.90 0.06 0.83–1.00 0.70 0.04 0.65–0.75
Limnopithecus evansi ‡ 6 19 Early 0.36 0.07 0 .30–0.47 0 .90 0.04 0 .85–0.96 0 .70 0.05 0 .65–0.79
Proconsul nyanzae‡ 9 18 Early 0.48 0.04 0 .41–0.52 0 .88 0.03 0 .84–0.93 0 .67 0.08 0 .56–0.79
Sivapithecus sivalensis‡ 7 9 Late 0.41 0.01 0.39–0.43 0.92 0.08 0.80–1.02 0.69 0.05 0.61–0.75
Folivores
Gorilla gorilla gorilla 24 0 Recent 0.37 0.06 0.27–0.50 0.91 0.04 0.85–0.98 0.60 0.05 0 .50–0.70
Nyanzapithecus pickfordi ‡ 1 15 Middle 0.30 – – 0.74 – – 0.50 – –
Oreopithecus bambolii ‡ 2 8 Late 0.32 0.07 0.28–0.37 0.78 0.00 0.78–0.78 0.64 0.14 0.54–0.75Rangwapithecus gordoni ‡ 7 19 Early 0.43 0.06 0 .33–0.51 0 .83 0.04 0 .76–0.86 0 .61 0.05 0 .54–0.67
Simiolus leakeyorum‡ 2 15 Middle 0.47 0.03 0 .45–0.49 0 .79 0.01 0 .78–0.79 0 .53 0.01 0 .52–0.54
Hard-Object Feeders
Pongo pygmaeus pygmaeus 23 0 Recent 0.41 0.05 0.30–0.49 0.96 0.04 0.90–1.04 0.73 0.04 0 .63–0.82
Aegyptopithecus zeuxis‡ 11 33 Oligocene 0.56 0.06 0.45–0.64 0.97 0.06 0.90–1.09 0.75 0.09 0.53–0.89
Afropithecus turkanensis‡ 2 17 Early 0.56 0.08 0 .51–0.62 0 .87 0.04 0 .84–0.89 0 .61 0.09 0 .55–0.67
Equatorius africanus‡ 2 15 Middle 0.45 0.01 0.45–0.46 0.95 – – 0.65 0.03 0.63–0.67
Lufengpithecus lufengensis‡ 2 8 Late 0.44 0.06 0.40–0.48 0.96 0.06 0.92–1.00 0.66 0.04 0.63–0.69
Ouranopithecus macedoniensis‡ 3 9 Late 0.43 0.03 0.40–0.45 0.94 0.03 0.91–0.97 0.68 0.06 0.61–0.73
‡Fossil taxon.
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Benefit (1993), molar flare (MFR) was calculated
as the ratio of mesial intercuspal breadth (MCB) to
maximum mesial breadth (MMB):
MFRϭMCB
MMB
This ratio has the unfortunate property that its
value decreases as molar flare increases; therefore,
a simple linear transformation was applied to yield
the index of molar flare (MF):
MF ϭ1ϪMFR
whose value ranges between the theoretical ex-
trema of 0 (no flare) and 1 (maximum flare).
Technically, this index summarizes the relative
proximity of the mesial cusp apices to each other
and to the crown margin, but approximation of
cusp apices also results in sloping of the crown wall
from the apex towards the root and bulging of the
wall, typically most pronounced at or near the
cervix (Benefit, 1993). Whether characterized as
“flaring”, “bulging”, or “sloping”, it is this aspect
of molar shape whose distribution and variation
has concerned most authors (Benefit, 1993; Coffing
et al., 1994; Ungar et al., 1994). The index of molarflare (MF) and similar ratio-based indices (Benefit,
1993; Ungar et al., 1994) are simple metrics, re-
flecting only the relative magnitude of flare. They
are insensitive to variations in slope and curvature
or the presence of beveling—tapering of the crown
towards the cervix (Ungar et al., 1994). These
limitations are off set by ease of computation and
ready availability of data, and previous analyses
have shown such indices to discriminate among
taxa (Benefit, 1987, 1993; Ungar et al., 1994). The
index of molar flare thus provides a useful tool inthe investigation of molar flare variation.
Ratios of crown shape (MMB/ML) and cusp
relief (DHT/MHT) also were computed. These
indices summarize aspects of molar shape known
to be strongly correlated with diet (Kay, 1975,
1978; Benefit, 1987; Benefit and McCrossin, 1990)
and thereby serve as benchmarks against which to
judge the potential usefulness of molar flare as a
dietary indicator. Indices of crown shape and cusp
relief were found to be significantly correlated
(r = 0.75, p<0.001). Molar flare was not signifi-
cantly correlated with either crown shape or cusp
relief and none of the indices were significantly
correlated with molar size. Table 1 gives summarystatistics for the three indices of molar shape.
Because ratio-based variables frequently violate
statistical assumption of normality (Atchley et al.,
1976), individual values were subsequently con-
verted to natural logarithms and mean values were
calculated by taxon.
Analysis
Statistical analysis was performed using the
SPSS 8.0.0 statistical software package. Relation-
ships among molar shape variables, diet, geologic
age, and phylogenetic propinquity were explored
using correlation analysis, analysis of covariance,
and multiple analysis of covariance. The influence
of phylogeny was examined only for the subset of
taxa for which a well-resolved phylogeny was
available, namely those included in the parsimony
analysis of Begun et al. (1997). This subsample
comprised extant and fossil large-bodied hominoid
taxa, with Aegyptopithecus included as an out-
group. A cladogram reflecting the probable
pongine status of Lufengpithecus (Fig. 2c in Begunet al., 1997) was accepted as a working hypothesis
of large-bodied Miocene hominoid relationships,
and nodes along the spine of the cladogram were
ranked to yield an ordinal measure of phylogenetic
propinquity (Fig. 1). Character state distributions
were explored using MacClade 3.07 (Maddison
and Maddison, 1992). A variety of methods are
available for the conversion of continuous quanti-
tative data to discrete character states suitable for
cladistic character analysis (see Singleton, 1998).
An initial analysis (Singleton, 2000) employed sim-ple gap coding (Mickevich and Johnson, 1976;
Thorpe, 1984), a conservative technique that some-
times fails to recognize statistically significant dif-
ferences among taxa and yields small numbers of
character states, each encompassing a broad range
of morphologies (Thorpe, 1984). In the present
study, indices of molar shape were converted to
discrete character states using homogeneous subset
(HS) coding, which recognizes all statistically sig-
nificant diff erences among taxa (Simon, 1983;
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Goldman, 1988). The HS character distributions
are qualitatively similar to those of the prior study,
but permit a finer-grained analysis of phylogenetic
trends in molar shape variation.
Because phylogenetic eff ects not infrequently
produce spurious associations among ecological
and morphological variables (Martins and
Hansen, 1996), standardized phylogenetic indepen-
dent contrasts (Felsenstein, 1985) were computed
for all non-terminal nodes using PDTREE
(Garland et al., 1999; Garland and Ives, 2000) withbranch lengths set equal to unity (speciational
model). Corresponding hypothetical ancestral
dietary patterns (Fig. 1) were reconstructed
manually by Farris optimization (Farris, 1970;
Brooks and McLennan, 1991), and analysis of
variance was performed to test for diff erences
among dietary groups controlling for the eff ects of
phylogeny. The Farris procedure reconstructs the
hypothetical last common ancestor of Miocene
hominoids as a hard-object feeder (Node 1, State
a) contra the accepted view that soft-fruit frugivory
is the primitive hominoid condition (Andrews
et al., 1997; Benefit, 2000). Therefore, alternate
codings of Node 1 were tested to rule out
methodological artifacts.
Results
Phylogeny and diet
Analysis of variance showed significant diff er-
ences among diet categories for all three indices of
molar shape (Table 2). Indices of crown shape and
cusp relief were highly significantly diff erent
among categories (p%0.001). Post hoc pairwise
comparisons showed folivores to diff er signifi-
cantly from both frugivores and hard-object feed-
ers in crown shape and cusp relief (Bonferroni
adjusted p<0.01), while the latter two groups were
Fig. 1. Working hypothesis of large-bodied hominoid phylogenetic relationships based upon Begun et al. (1997, Fig. 2c). Numeralsindicate clade rank of trunk nodes. Hypothetical ancestral dietary patterns (a–c) are reconstructed by Farris optimization ( Farris,1970; Brooks and McLennan, 1991). Reconstruction of the hypothetical last common ancestor of Miocene hominoids as a hard-objectfeeder (Node 1, State a) is contra the accepted view that soft-fruit frugivory is the primitive hominoid condition (Andrews et al., 1997;Benefit, 2000).
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not significantly diff
erent. By contrast, the signifi-cance value for molar flare was borderline
(p = 0.044). Hard-object feeders did show greater
mean molar flare than frugivores and folivores, but
diff erences among the three diet classes were not
statistically significant. Of the three indices exam-
ined, molar flare distinguishes least clearly among
diet categories.
Homogeneous subset coding of crown shape
yielded a 2-state parsimony uninformative
character; cusp relief was coded as a 5-state
character exhibiting moderate homoplasy (consist-
ency index = 0.67) and no clear phylogenetic
pattern (retention index = 0). Neither index was
significantly correlated with clade rank. The index
of molar flare shows a highly significant negative
correlation with clade rank (Spearman rank
correlation rs =0.689, p<0.01), indicating a
phylogenetic trend toward molar flare reduction.
Homogeneous subset coding of molar flare (Fig. 2)
yielded a 7-state character exhibiting minimal
homoplasy (ci = 0.86) and clear phylogenetic pat-terning (ri = 0.67). Pronounced molar flare (States
0–2) is observed in Aegyptopithecus and the stem
hominoids Proconsul , Afropithecus, and Equato-
rius, suggesting that this condition is primitive for
large-bodied Miocene hominoids. Members of the
extant hominoid clade show a progressive reduc-
tion in flare (States 3–6), which is most marked
in Pan and Oreopithecus. Analysis of variance
on standardized phylogenetic independent
contrasts—nodal values adjusted for the eff ects
Fig. 2. Phylogenetic distribution of molar flare. Molar flare shows minimal homoplasy (ci = 0.86) and clear phylogenetic patterning(ri = 0.67). Pronounced molar flare (States 0–2) characterizes the outgroup and stem hominoids. The crown hominoid clade ischaracterized by reduced molar flare (States 2–6)s which is most evident in Oreopithecus and Pan.
Table 2
ANOVA of indices of molar shape by diet category
F p Pairwise Comparisons‡
Molar Flare 3.87 0.044 Not Significant
Molar Shape 13.54 0.000 Folivore p<0.01
Cusp Relief 11.20 0.001 Folivore p<0.01
‡Bonferroni adjusted post hoc comparisons among folivores,
frugivores, and hard object feeders.
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of phylogeny—found no significant diff erences
among dietary groups for any of the indices of
molar shape and therefore fails to support an
adaptive functional association between these
measures of molar shape and diet. Recoding
the hypothetical last common ancestor (Fig. 1,
Node 1) as a soft-fruit frugivore (State b) had no
eff ect upon results.
Geologic age and diet
Indices of crown shape and cusp relief are
uncorrelated with geologic age. Molar flare is
significantly negatively correlated (rs =0.59,
p = 0.01), and a plot of mean molar flare against
geologic age (Fig. 3a) shows a clear decrease in
molar flare values through time. A simple sign test
Fig. 3. Molar flare against time. (a) Log molar flare against geologic age (Ma) for 18 catarrhine taxa. Molar flare decreases throughtime, but taxa show substantial dispersion about the line of best fit. (b) Average molar flare (log mean molar flare) across taxa for fivegeologic sub-epochs: Oligocene; early, middle, and late Miocene; and recent time. Decreasing average values demonstrate a cleartemporal trend toward molar flare reduction.
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(Sokal and Rohlf, 1981) of this trend is not signifi-
cant, probably due to the dispersion of data points
about the line of best fit. Computing average
molar flare values by geologic sub-epoch (Fig. 3b)
demonstrates the strength of the trend in central
tendency (rs =1.00, p<0.000), but leaves toofew data points to allow meaningful significance
testing. The source of dispersion in taxon means is
suggested by Fig. 4, which plots taxon flare values
against geologic sub-epoch, labeling taxa by diet
category. Within sub-epochs, taxa segregate by
diet with hard-object feeders tending to show the
highest molar flare values and folivores the lowest.
Within diet categories, mean flare values decrease
through time, resulting in parallel trends toward
flare reduction for the three dietary groups. This
pattern is supported by an analysis of covariance(ANCOVA) of molar flare across dietary cat-
egories controlling for the eff ect of geologic age
(Table 3). The ANCOVA model is statistically
significant (p<0.01), as are the two major eff ects:
geologic age and diet; interaction terms are insig-
nificant. Geologic age and diet both account for
substantial proportions of variance in molar flare,
as indicated by Eta2 values in excess of 0.40.
Pairwise comparisons of estimated marginal
means—mean values adjusted for the eff ects of
geologic age—find significant diff erences among
dietary categories (Table 3). Hard-object feeders
have significantly greater molar flare than both
folivores and frugivores (unadjusted p<0.05). With
Bonferonni adjustment of probability values,
frugivores are no longer significantly diff erent
from hard-object feeders (p = 0.07), but the latter
are still significantly diff erent from folivores
(p = 0.02).
Fig. 4. Molar flare against time (sub-epoch) with taxa labeled by dietary category. Hard-object feeders consistently show more flaringmolars than contemporaneous soft-fruit frugivores and folivores. The three dietary groups exhibit parallel trends toward decreasedmolar flare through time.
Table 3
ANCOVA of molar flare by diet controlling for time
Eff ect F p Eta2*
Model§ 7.67 0.003 0.62
Geologic Age (Sub-Epoch) 10.43 0.006 0.43
Diet 5.25 0.020 0.43
Folivore Frugivore HO
Feeder
Folivore –
Frugivore NS –
Hard Object Feeder 0.009‡ 0.024‡ –
NS = not significant.*Proportion of total variance explained by eff ect.§Interaction terms are not significant.‡Probability not adjusted for multiple comparisons.
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Phylogeny and geologic age
Geologic age and phylogenetic propinquity (as
measured by clade rank) are strongly correlated
(rs =0.68, p = 0.01). To separate the eff ects of
time and phylogeny, clade rank was incorporated
into a multiple analysis of covariance model(MANCOVA), permitting the simultaneous test-
ing of dietary, temporal, and phylogenetic eff ects.
The MANCOVA model (Table 4) is statistically
significant (p<0.001), as are the eff ects of time and
diet (p<0.05). The phylogenetic eff ect (clade rank)
is insignificant, as are all interaction terms. This
result is mirrored by Eta2 values which show clade
rank to account for only a miniscule proportion
(0.01) of total variance in molar flare, while time
and diet again account for substantial proportions
of variance (Table 4). The observed increase inEta2 values compared to the ANCOVA analysis is
attributable to the reduced phylogenetic sample
which excludes several basal catarrhine taxa (most
notably Simiolus) whose molar flare values do not
conform well to the pattern of parallel trends in
decreasing flare. Pairwise comparisons of esti-
mated marginal means—mean values adjusted for
the eff ects of geologic age and clade rank—yield
results identical to those for the ANCOVA
analysis.
Discussion
Crown shape and cusp relief
Indices of molar crown shape and cusp relief are
strongly correlated both with diet and with each
other and neither shows strong temporal or phylo-
genetic patterning, yet analyses controlling for the
eff ects of phylogeny fail to support a functional
association between these features and diet. The
latter result is likely to be an artifact both of
the under-representation of folivorous taxa in the
sample available for phylogenetic analysis and
the phylogenetic distribution of folivory among
Miocene catarrhines. Because optimization of the
dietary character fails to reconstruct any ancestralnode as possessing the folivorous state, potential
functional associations between folivory, repre-
senting one extreme of the hominoid dietary spec-
trum, and measures of molar shape are rendered
eff ectively invisible to the methods employed here.
The recent recognition of the Nyanzapithecinae
(Harrison, 2000)—a clade comprising the foli-
vorous basal catarrhines Rangwapithecus, Nyanza-
pithecus and Turkanapithecus, but excluding the
late Miocene folivore Oreopithecus (Harrison and
Rook, 1997; Alba et al., 2000; Begun, 2001;Harrison, 2002) —implies that folivory in associ-
ation with high molar cusp relief has evolved
independently a minimum of three times: in the
common nyanzapithecine ancestor, in Oreo-
pithecus, and in Gorilla. As leaves are most ef-
ficiently broken down by shear, and high cusp
relief has been shown to maximize shear stress on
food particles in the earliest stages of mastication
(Spears and Crompton, 1996), evolution of high
molar cusp relief in non-cercopithecoid catarrhines
may safely be inferred to be a true adaptation tofolivory.
The case of molar cusp relief cautions against
uncritical acceptance of independent contrast
results, which may sometimes be biased by the
phylogenetic eff ects they are intended to eliminate.
It does not necessarily follow, however, that
other indices of molar shape have similar adaptive
value. The potential significance of crown
shape variation, for example, is far from obvious.
Molar elongation in folivores has been related to
Table 4
MANCOVA of molar flare by diet controlling for time and
phylogeny
Eff
ect F p Eta
2*
Model§ 15.33 0.00 0.89
Phylogeny (Clade Rank) 0.04 0.84 0.01
Geologic Age (Sub-Epoch) 7.20 0.03 0.48
Diet 5.82 0.03 0.59
Folivore Frugivore HO
Feeder
Folivore –
Frugivore NS –
Hard Object Feeder 0.014‡ 0.035‡ –
NS = not significant.*Proportion of total variance explained by eff ect.§
Interaction terms are not significant.‡Probability not adjusted for multiple comparisons.
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maximization of shearing crest length; conversely,
the broader molars of frugivores and hard-object
feeders have been hypothesized to reflect a greater
emphasis on crushing or grinding capacity (Kay,1975, 1977). But Benefit (1987), using an alternate
index of molar shape, found no relationship
between crown shape and either shearing crest
development or degree of folivory. This discrep-
ancy is not surprising given the arbitrary atomiza-
tion of form inherent in the use of ratio-based
shape variables. More comprehensive and nuanced
approaches to shape analysis may ultimately be
required to adequately characterize crown shape
variation and clarify its functional significance.
But so long as the primary goal is dietary infer-
ence, the precise adaptive value of features associ-
ated with diet is less important than the strength of
the correlation (Anthony and Kay, 1993), and the
present findings support the long-standing practice
of using simple indices to summarize functional
aspects of molar shape. The index of crown shape
should be particularly valuable where molar speci-
mens are too worn to permit more sophisticated
measures of functional capacity.
Molar flare
In contrast with crown shape and cusp relief,
which are influenced primarily by functional de-
mands, variation in molar flare appears to be the
product of a complex interaction of functional,
phylogenetic, and temporal eff ects. Polarity deter-
minations based on cladistic character analysis are
in concurrence with hypotheses that possession of
pronounced molar flare is the basal catarrhine
condition and thus primitive for the Miocene
hominoids (Benefit, 1993, 2000; Singleton, 2000).
The crown hominoid clade is characterized bydecreased molar flare; however, this is attributable
to temporal eff ects rather than cladogenetic
character evolution. Molar flare is correlated with
both phylogenetic propinquity and geologic age,
but MANCOVA results establish that time is the
significant eff ect and the influence of phylogeny on
molar flare variation is negligible. The relationship
between molar flare and geologic age manifests as
a trend toward decreased mean molar flare
through time, with functional separation in molar
flare values among contemporaneous taxa creating
parallel trends toward decreasing flare for foli-
vores, frugivores, and hard-object feeders,
respectively. When adjusted for the eff
ects of time,hard-object feeders show significantly more flaring
molars than either folivores or soft-fruit frugi-
vores. Thus, molar flare does contain a dietary
signal, but one which can be interpreted only in an
appropriate temporal context.
The processes responsible for the observed dis-
tribution of molar flare in Miocene hominoids are
unclear. Any plausible evolutionary scenario must
simultaneously explain: 1) the marked decrease in
average molar flare over time; and, 2) the persist-
ence of systematic variation in molar flare across
dietary categories. An explanation for the first
phenomenon may be found in the presence of a
comparable temporal trend in hominoid molar
shearing capacity (Ungar and Kay, 1995; Kay and
Ungar, 1997). Kay and Ungar (1997) showed that
while functional diff erences in shearing capacity
between folivores and frugivores remain more or
less constant through time, average molar shearing
quotient values have increased steadily over the
course of hominoid evolution. They attributed this
pattern to a “Red Queen” eff ect (Van Valen, 1973),
hypothesizing that continuous selective pressurefor increased molar shearing capacity—perhaps
due to interspecific competition or coevolution of
plant defenses—led to an “upshift” in average
shearing quotients across dietary categories over
time. The complementarity of these trends—
decreasing molar flare and increasing molar
shearing—suggests that molar flare reduction may
have been a correlated eff ect of selection for
shearing capacity.
Assuming constraints on molar crown size, in-
creased shearing capacity is most easily achievedby shifting the centrally located cusps of the primi-
tive catarrhine molar towards the crown periphery,
simultaneously reducing molar flare and increasing
both shearing crest length and total working oc-
clusal surface area. This scenario has considerable
intuitive appeal, but is difficult to test with cur-
rently available data. The convention of reporting
shearing quotients as analysis-specific residuals
(Kay, 1975) precludes the compilation of pub-
lished values into a larger data set. A ratio-based
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index of relative shearing capacity (Total M2
Shearing Crest Length/M2L) calculated from one
published data set (Kay and Ungar, 1997) is only
moderately correlated with the index of molar flare
(n = 11, r =0.66, p = 0.03); exclusion of Oreo-
pithecus, a conspicuous outlier (Fig. 5), renders the
relationship statistically insignificant (r =0.53,p = 0.12). Still, it is premature to reject a func-
tional relationship between molar shearing capac-
ity and flare without rigorous testing using larger
taxonomic samples and more refined measures of
molar shape and functional capacity.
Explaining the persistence of molar flare vari-
ation across dietary categories through time is
more problematic, in part because the functional
significance of molar flare variation among
contemporaneous taxa is not firmly established.
Benefit (1993; 2000) noted the presence of pro-nounced molar flare in fossil and extant Old World
monkeys whose diets incorporate fruits and seeds
and hypothesized that buccolingual approxima-
tion of molar cusps yielded an “enhanced func-
tional capacity” for frugivory and hard-object
consumption (Benefit, 1993: 123). This supposition
finds support in research relating the biomechanics
of molar shape to material properties of food.
Both soft fruits and hard, brittle foods such as
nuts and tubers are most efficiently broken down
between low-relief cusps and restricted occlusal
basins arrayed to create reciprocal “mortar and
pestle” configurations (Lucas and Luke, 1984).
Because broad-based cusps both maximize stress
concentration at the cusp tip and dissipate stresses
generated within the tooth, flaring molars are
ideally suited for safe and efficient breakdown of hard or stiff food items (Lucas and Luke, 1984;
Strait, 1997). While the possession of pronounced
molar flare is clearly conducive to hard-object
consumption, the maintenance of hard-object feed-
ing capabilities even as absolute molar flare
decreases does not admit the same sort of straight-
forward, adaptationist explanation possible for
molar cusp relief (Kay and Cartmill, 1977; Kay
and Covert, 1984; Anthony and Kay, 1993). If the
modern hominoid dentition reflects a functional
tradeoff
between increased relative occlusal surfacearea and stress dissipation, we must look to iden-
tify compensatory morphological and behavioral
adaptations to explain the persistence of hard-
object feeding adaptations (Singleton, In Press).
Neither the histological basis of molar flare
variation nor the developmental mechanisms by
which flare reduction was achieved are known, and
the interrelationships of molar flare and other
aspects of molar morphology, such as cingulum
development and enamel thickness, are largely
Fig. 5. Molar flare against relative shearing capacity. Index of relative shear (Total M2 Shearing Crest Length/M2 Length) computedfrom data of Kay and Ungar (1997). Molar flare and molar shear are not strongly linearly related (n = 11, r = 0.66, p = 0.03), andexclusion of Oreopithecus, an obvious outlier, renders the relationship statistically insignificant.
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uninvestigated. Like molar flare, molar cingulum
expression has tended to decrease through time,
and it has been conjectured that pronounced molar
flare arises by absorption of cingulum into theocclusal surface (Andrews in Pickford, 1975;
Strasser and Delson, 1987). That cingulum
development influences measures of molar flare is
indisputable, and the distinction between a
strongly flaring molar and one with well-developed
cingula can be difficult to draw. But cingulum
expression can be highly variable within species
(McCrossin and Benefit, 1997) and its association
with molar flare is inconstant. The mandibular
molars of Dryopithecus fontani , which exhibits
partial buccal cingula (Begun, 2002), are slightly
less flaring (MF = 0.36; unpublished data) than
those of D. laietanus (MF = 0.38), which has less
well-developed cingula (Begun et al., 1990). The
middle Miocene hominoid Otavipithecus namibien-
sis exhibits pronounced molar flare in association
with only moderately developed cingular elements
(Conroy et al., 1992). Thus, while diff erences in
cingulum development may contribute to flare
variation, molar flare reduction cannot be attrib-
uted to simple cingulum reduction.
A more likely source of functional molar flare
variation is enamel thickness. Increased enamelthickness, a functional adaptation to hard-
consistency diets (Teaford, 2000), is known to alter
external crown geometry, decreasing both crown
relief and shearing capacity while maximizing force
dissipation (Kay, 1984; Shellis et al., 1998; Ungar,
1998; Macho and Spears, 1999). This functional
role is evidenced by greater enamel thickness on
working cusps relative to guiding cusps (Molnar
and Ward, 1977; Reid et al., 1998), and may
contribute to the buccal flare which characterizes
the mandibular molars of hard-object feeders.However, the existence of thin-enameled forms
with extreme molar flare such as Otavipithecus
(Conroy et al., 1995; Singleton, 2000) and thick-
enameled forms with reduced flare such as Ourano-
pithecus (Bonis and Koufos, 1993) implies that
relative enamel thickness cannot be the sole deter-
minant of molar flare variation. In much the same
way flare shows functional variation about tem-
porally constrained average values, enamel thick-
ness is known to exhibit functional variation
relative to phylogenetically constrained baselines
(Dumont, 1995). Thus, it seems likely that molar
flare variation among contemporaneous taxa is
due in part to diet-related diff
erences in relativeenamel thickness. However, the reduction in mean
flare values through time almost certainly reflects
a true realignment of cusp positions. Whether
this was accomplished by reorganization of the
topology of the dentinoenamel junction, changes
in patterns of enamel deposition, or some combi-
nation thereof requires further investigation.
Functional and phylogenetic implications
This study documents temporal and functionalpatterns of flare distribution in Miocene homi-
noids and establishes a baseline for the interpret-
ation of molar shape variation in the hominoid
fossil record. Increased understanding of these
patterns should strengthen and refine paleobiologi-
cal inferences for specific hominoid taxa, while
marked divergences from expected values may be
informative concerning hominoid phylogeny and
adaptation.
Otavipithecus namibiensis
Otavipithecus possesses exceptionally flaring
molars (Conroy et al., 1996; Singleton, 2000), both
in absolute terms and relative to other middle
Miocene hominoids (Fig. 6). This finding admits
several possible and non-mutually exclusive inter-
pretations. The first and most obvious conclusion,
that Otavipithecus was a hard-object feeder, is
contradicted by other features of its dentognathic
anatomy. While the mandibular corpus of Otavi-
pithecus is reported to have biomechanical proper-
ties similar to those of Pongo (Schwartz andConroy, 1996), the molars of Otavipithecus are
relatively thin enameled and exhibit little diff eren-
tial wear along the tooth row. These features diff er
from typical primate hard-object feeders such as
Pongo, Cebus or Cercocebus and are inconsistent
with habitual consumption of hard or brittle foods
(Kinzey, 1992; Shellis et al., 1998; Ungar, 1998;
Macho and Spears, 1999). But Otavipithecus also
lacks specializations of the anterior dentition such
as those found in extant pitheciin seed predators
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(Kinzey, 1992; Anapol and Lee, 1994) and the
Miocene stem hominoids Afropithecus and Equa-
torius, both hypothesized to have been hard-fruit
specialists (Leakey and Walker, 1997; Ward et al.,
1999b). Lacking obvious specializations of either
the anterior or posterior dentition, Otavipithecusconforms to neither primate model for hard-object
specialization. If it was a hard-object feeder, it is
one for which there is no known fossil or extant
primate analog, a circumstance under which the
comparative method falters and robust functional
inference is difficult (Kay and Cartmill, 1977).
A second possible conclusion is that molar flare
is a shared derived trait of an Otavipithecus-
Afropithecus clade. Like Otavipithecus, Afro-
pithecus possesses pronounced molar flare both
relative to contemporaneous taxa and relative toexpectations based on observed functional and
temporal trends (Fig. 6). Conceivably, accentua-
tion of molar flare relative to the established
hominoid baseline—rather than pronounced
molar flare per se —is a synapomorphy of the
afropithecin clade (Andrews, 1992; Singleton,
2000). In this scenario, a shift toward increased
molar flare in the common afropithecin ancestor
culminated in the extraordinary molar flare seen in
Otavipithecus. If this shift occurred as a functional
adaptation to sclerocarp exploitation, as seen in
Afropithecus, we must either accept a hard-object
feeding adaptation for Otavipithecus or attribute
its molar morphology to phylogenetic inertia.
A final possibility is that the presence in Otavi-
pithecus of molar flare at levels comparable toearly Miocene non-cercopithecoid catarrhines rep-
resents retention of a primitive condition and is
uninformative concerning its phylogenetic affini-
ties. This interpretation is consistent with the uni-
formly primitive nature of its known postcranial
and cranial elements (Conroy et al., 1993, 1996;
Pickford et al., 1997; Senut and Gommery, 1997)
and would tend to support previous characteriza-
tions of Otavipithecus as a generalized “hominoid
of archaic aspect” (Andrews, 1992; Pilbeam, 1996,
1997; Singleton, 2000). If the extreme molar flareobserved in Otavipithecus signifies persistence into
the middle Miocene of an early Miocene dental
morphotype, it would also give further weight
to the previous suggestion that Otavipithecus
represents a geographically remote relic taxon
(Singleton, 2000) isolated from the environmental
selective pressures which led to increased locomo-
tor and ecological diversity as well as increased
molar efficiency in contemporaneous East Africa
hominoids.
Fig. 6. Molar flare in Otavipithecus. Axes and symbols as in Fig. 4. Otavipithecus exhibits extreme molar flare, both in absolute termsand relative to contemporaneous taxa. Afropithecus also exhibits a degree of molar flare somewhat greater than expected based onfunctional and temporal patterning.
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The Lukeino molar
The results of this study clarify flare polarity
within the great ape clade and furnish a temporal
and phylogenetic framework within which to
interpret the pronounced flare of the Lukeino
molar. Table 5 gives molar flare values for Lukeino
(KNM-LU 335) and several early hominin speci-mens. Molar flare in extant hominoids varies
by roughly five percent between molar positions
(Appendix B), thus these specimens should provide
reasonable estimates of flare for their respective
taxa. In the case of the Lukeino molar, a probable
M1 germ whose crown is not yet complete, two
potential sources of bias must be considered.
Because M1 is typically the most flaring molar in
extant frugivores (Appendix B), KNM-LU 335
may overestimate flare relative to the comparative
M2 baseline. However, hominoid molars aretypically broadest at or near the cervix, meaning
the flare index for this specimen is a minimal (i.e.,
conservative) estimate of flare for the completed
tooth crown. As these biases are expected to
off set, the flare index for the Lukeino molar is
deemed sufficiently accurate to allow qualitative
comparisons.
When compared with Miocene and extant
hominoids (Fig. 7), early hominins are shown to
dramatically reverse the Miocene trend toward
molar flare reduction. All hominin specimens
fall well above the extant hominoid range, and
the earliest taxa— Australopithecus anamensis and
A. afarensis —barely overlap the early Miocenerange. Even the relatively non-flaring Homo
ergaster molar exceeds the majority of Miocene
and extant hominoids. The Lukeino molar clearly
groups with early hominins, showing less flare than
Paranthropus but more than H. habilis. By con-
trast, the outgroups Pan and Gorilla exhibit rela-
tively non-flaring molars consistent with temporal
and functional expectations. This strongly suggests
that reduced molar flare is the primitive hominine
condition and that secondary increase in molar
flare is a hominin synapomorphy. Thus, the
Lukeino molar cannot represent the ancestral mor-
photype of the Pan-Homo clade. Instead, it is
expected that the last common ancestor possessed
relatively non-flaring molars and the Lukeino
molar, whether belonging to Orrorin tugenensis or
another as yet unrecognized taxon, is most parsi-
moniously interpreted as representing an early
hominin lineage.
The limited hominin sample employed here
does not permit rigorous comparisons among taxa,
but results support previous conclusions concern-
ing early hominin dietary evolution. Molar flareincrease in early hominins coincides with—and
may be partially attributable to (see Discussion)—
an increase in relative enamel thickness associated
with an ecological shift toward hard-object feeding
at the base of the hominin clade (Ward et al.,
1999a; Teaford et al., 2002). Diff erences in flare
between P. robustus and A. africanus are consistent
with microwear data indicating a greater emphasis
on hard or abrasive foods by the former (Grine,
1986; Ungar and Grine, 1991). Similarly, the de-
crease in flare from Homo habilis, which groupswith the australopiths, to Homo ergaster accords
well with the findings of Teaford et al. (2002), who
document a gradual transition toward softer,
tougher foods (i.e., meat) through the evolution of
genus Homo. Thus, relative molar flare appears to
be a useful indicator of early hominin diets.
The dentognathic morphology and dietary
adaptations of Australopithecus anamensis have
been characterized as intermediate between great
apes and younger hominins (Ward et al., 1999a;
Table 5
Index of molar flare for selected hominin taxa
Specimen‡ Molar Flare
Lukeino KNM-LU 335 0.59A. anamensis KNM-ER 20422 0.57
A afarensis LH 3t 0.64
A. africanus STS 2 0.53
STS 24 0.48
STS 52b 0.53
MLD 2 0.58
Taung 1 0.54
P. robustus SKX 4446 0.61
H. habilis OH 16 0.58
H. ergaster KNM-WT 15000 0.48
‡Measurements of KNM-ER 20422 based on published
photographs and verified against published measurements
(Coffing et al., 1994); all other measurements taken onresearch quality casts.
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Teaford et al., 2002) and the strong flare observed
in this taxon was not anticipated. Its presence in
both A. anamensis and A. afarensis suggests that
enhanced hard-object feeding capacity was already
well established in australopiths by 4 Ma and gives
added weight to the argument that increased
reliance upon hard and abrasive foods such asseeds and underground storage organs was a key
early hominin adaptation (Conklin-Brittain et al.,
2002; Teaford et al., 2002). Thus, based on avail-
able evidence, the Lukeino hominin is a strong
candidate for the earliest “dietary hominin”. This
conceivably pushes the onset of hominin dietary
specialization into the latest Miocene, and other
Mio-Pliocene hominines exhibiting pronounced
molar flare are also expected to belong to the
hominin lineage.
The converse proposition—that Mio-Pliocenehominines possessing non-flaring molars are ex-
cluded from hominin status—is more problematic,
and its validity is partially dependent upon
whether one adopts a stem-based or crown-based
definition of the hominin clade (White, 2002). To
date, no ape-grade African hominine post-dating
the African ape-human divergence has been for-
mally recognized, despite the likelihood that
many such forms existed (McHenry, 2002). As ever
more ancient and primitive taxa— Ardipithecus at
4.4–5.8 Ma (White et al., 1994; 1995); Orrorin at
6 Ma (Pickford and Senut, 2001); and Sahelanthro-
pus at 6–7 Ma (Brunet et al., 2002) —are allocated
to the hominin clade, the temporal range in
which the earliest chimpanzee ancestors and stem
hominins may be expected to occur is increasingly
restricted. Whether this reflects rapid diversifica-tion at the base of the Pan-human clade or is an
artifact of current systematic practices remains to
be seen. Certainly, the recent controversy sur-
rounding the hominin status of Sahelanthropus
(Brunet, 2002; Wolpoff et al., 2002) highlights the
difficulty of drawing clear phylogenetic and taxo-
nomic distinctions near the base of the hominine
radiation, especially when determinations turn
upon only one or two key taxonomic characters
(Brunet, 2002; Wolpoff et al., 2002).
Interestingly, published illustrations of Ardi- pithecus and Sahelanthropus (White et al., 1994;
Brunet et al., 2002) indicate absence of significant
molar flare. While the former shows relative molar
proportions similar to early hominins (Teaford
et al., 2002) and enamel thickness in the latter is
slightly greater than in Pan (Brunet et al., 2002),
dietary adaptations of these taxa are likely to more
closely resemble recent African apes than early
hominins (Teaford et al., 2002). For both Ardi-
pithecus and Sahelanthropus, detailed dietary
Fig. 7. Molar flare in Lukeino and selected early hominins. Axes and symbols as in Fig. 4, interpolating a sub-epoch to accommodateMio-Pliocene hominin taxa. Pan exhibits molar flare consistent with expectations based on functional and temporal patterning. TheLukeino hominine and early hominin taxa reverse the Miocene hominoid trend toward molar flare reduction.
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analyses and direct assessments of molar flare and
enamel thickness are needed to substantiate this
supposition. If proven accurate, the ultimate classi-
fication of these taxa as crown hominins, stemhominins or perhaps even early African apes will
determine the importance of diet to hominin
origins and the utility of molar flare for dis-
tinguishing the earliest members of the hominin
clade.
Summary and conclusions
Molar flare variation in Miocene hominoids is
the product of a complex interaction of functional,
phylogenetic, and temporal eff ects. Pronounced
molar flare is the basal catarrhine condition and
primitive for Miocene hominoids. While members
of the crown hominoid clade are characterized by
reduced flare, flare reduction is not a synapomor-
phy of this group. Rather, flare reduction is due to
a temporal trend characterized by decreasing mean
flare through time accompanied by significant
diff erences in molar flare values among dietary
categories. Hard-object feeders consistently show
greater molar flare than contemporaneous soft-
fruit frugivores and folivores, thus molar flare can
be a useful dietary indicator if interpreted in the
appropriate temporal and phylogenetic context.Diet-related variation among contemporaneous
taxa may be linked to variation in relative enamel
thickness. However, decrease in mean flare values
through time is hypothesized to arise from re-
organization of the crown geometry, perhaps in
response to continuous selection for increased
shearing capacity. While molar flare is clearly
conducive to hard-object feeding, the persistence
of hard-object feeding capabilities even as absolute
flare decreases raises questions concerning the
precise adaptive significance of molar flare. Fur-ther research into the developmental bases of flare
reduction, as well as morphological and behavioral
correlates of molar flare, should clarify the evolu-
tionary forces underlying flare variation.
The middle Miocene hominoid Otavipithecus
namibiensis exhibits extreme molar flare, both in
absolute terms and relative to contemporaneous
taxa. This morphology may be indicative of a
hard-object feeding adaptation; alternatively,
increased molar flare relative to the established
Miocene baseline may be an afropithecin synapo-
morphy. However, the pronounced molar flare of
Otavipithecus is most conservatively interpreted as
a primitive retention representing the persistenceinto the middle Miocene of an early Miocene
molar morphotype. A dramatic reversal of the
trend towards molar flare reduction is observed in
early hominins and is interpreted here as a crown-
hominin synapomorphy. The last common ances-
tor of the Pan-human clade is hypothesized to
have possessed relatively non-flaring molars and
the pronounced flare of the Lukeino molar is
considered to support its hominin status. Charac-
terization of molar flare and resolution of the
taxonomic status of basal hominines including
Ardipithecus and Sahelanthropus will determine the
utility of molar flare for distinguishing the earliest
members of the hominin radiation.
Acknowledgements
For access to specimens and curatorial assist-
ance, I thank the following individuals and insti-
tutions along with their curators and staff : Glenn
C. Conroy; Stephen C. Ward; Barbara Brown;
American Museum of Natural History, Division
of Paleontology; National Museum of NaturalHistory, Division of Mammals; Cleveland
Museum of Natural History; Harvard Museum of
Comparative Zoology, Mammal Department;
Yale Peabody Museum; British Museum (Natural
History), Division of Paleontology; Museo di
Geologia e Paleontologia, Firenze; Institut Paleon-
tologic M. Crusafont; Royal Central African
Museum; Geological Laboratory, Aristotle Uni-
versity of Thessaloniki; Kenya National Museums
and the Office of the President of Kenya. For
access to hominin dental casts and collectionsassistance, I thank Ian Tattersall and Ken
Mowbray, Division of Anthropology, AMNH
as well as Terry Harrison and Chris Robinson,
Department of Anthropology, New York
University.
I am grateful to Terry Harrison, David Begun,
Brenda Benefit, Sandra Inouye, and an anony-
mous reviewer, all of whose comments significantly
improved this work. For stimulating discussions of
this and related topics, I thank Mike Plavcan,
M. Singleton / Journal of Human Evolution 45 (2003) 57–79 73
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Peter Ungar, John Hunter, Eric Delson, Steve
Frost, and Kieran McNulty. This work was
supported by the New York Consortium in Evo-
lutionary Primatology; The Boise Fund; Wenner-
Gren Foundation Grant #5988; NSF Dissertation
Improvement Grant #SBR-9523229; and NSF
Research & Training Grant #BIR-9602234
(NYCEP).Appendix A. Fossil specimens
Specimens Locality Specimens Locality
Aegyptopithecus zeuxis Fayum Lufengpithecus lufengensis Lufeng
DPC 1027 RPA 580*
DPC 1028 RPA 584*
DPC 1112
DPC 3837 Oreopithecus bambolii Monte Bamboli
DPC 5391 IGF 4335
DPC 5396 IGF 4350
DPC 6254
DPC 7258 Ouranopithecus macedoniensis Ravin de la PluieDPC 10691 RPL 45
DPC 10700 RPL 55
DPC 11265 RPL 391
Afropithecus turkanensis Kalodirr Proconsul nyanzae Rusinga Island
KNM-WK 17010 KNM-RU 1676
KNM-WK 17024* KNM-RU 1678
KNM-RU 1710
Dendropithecus macinnesi KNM-RU 1947
KNM-RU 1850 Rusinga Island KNM-RU 1982
KNM-RU 1893 KNM-RU 2087
KNM-RU 1901 KNM-RU 1695*
KNM-RU 2015A KNM-RU 1734KNM-RU 2003 KNM-RU 1736
KNM-MW 53 Mfwangano
Rangwapithecus gordoni Songhor
Dryopithecus laietanus KNM-SO 374
IPS 1782 Can Llobateres KNM-SO 420
IPS 1796 KNM-SO 463
IPS 1797 KNM-SO 486
IPS 1802 KNM-SO 908
IPS 9001 KNM-SO 909
IPS 1803/4 La Tarumba KNM-SO 1958
Equatorius africanus Maboko Island Sivapithecus sivalensis
KNM-MB 11660 AMNH 19412 West HasnotKNM-MB 14250 GSI D 118/9 Chinji
GSP 6160 Dinga Kas 226
Limnopithecus evansi Songhor YPM 13806 Hari Talyangar L35
KNM-SO 385 YPM 13811 Hasnot L94
KNM-SO 386 YPM 13814 Hasnot L81
KNM-SO 387 YPM 13825 Hari Talyangar L40
KNM-SO 422
KNM-SO 530
KNM-SO 532
*Research quality cast.
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Appendix B. Flare diff erences among mandibular molar positions by taxon
M1 M2 M3
P. troglodytes troglodytes MFR 0.63 (26) 0.66 (30) 0.66 (26)M1 – 3% 3%
M2 ** – 1%
M3 * NS –
P. paniscus MFR 0.61 (23) 0.68 (22) 0.68 (20)
M1 – 10% 11%
M2 ** – 0%
M3 ** NS –
G. gorilla gorilla MFR 0.65 (12) 0.62 (26) 0.60 (12)
M1 – 3% 8%
M2 NS – 3%
M3 * * –
P. pygmaeus pygmaeus MFR 0.58 (21) 0.61 (26) 0.57 (20)
M1 – 4% 2%
M2 * – 6%
M3 NS ** –
H. lar carpenteri MFR 0.59 (26) 0.63 (34) 0.64 (24)
M1 – 7% 0%
M2 ** – 7%
M3 ** NS -
M1-M2 M1-M3 M2-M3
Average % Diff erence 6% 5% 3%
Results of paired-samples t-tests for diff erences in flare (untransformed flare ratio MFR) among molar positions bytaxon. Sample sizes vary by comparison; mean values are based upon the largest available sample (parentheses) at
each molar position. Lower diagonals show unadjusted significance levels: *p<0.05, **p<0.01; NS not significant.
Upper diagonals show the mean diff erence in flare between positions expressed as a percentage of the smaller mean
value. Percentage mean diff erences range from a maximum of 11% (P. paniscus M1–M3) to a minimum of 0%.
Typical mean diff erences are in the 3–7% range with an average mean diff erence across all taxa and molar
comparisons of approximately 5%.
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