Kroeber Anthropological Society Papers, Nos. 71-72, 1990
Comparative Dental Metrics and the Radiation of New World Monkeys:
A Preliminary Analysis
Walter Carl Hartwig
Very little is known about the quantitative relationships of tooth morphology within New World mon-
keys as a whole. Previous studies have focused on nonmetric data or have presented basic statistics of
individual tooth measurements without comparisons among genera. This study presents selected com-
parisons of measurements of the upper postcanine dental battery of modern platyrrhines and
demonstrates that metric indices are valuable as taxonomic markers for the New World monkey radia-
tion. Callitrichines, pithecines, and atelines show diagnostic patterns and ranges of variation in a
number of premolar and molar comparisons. These indices are also usefulfor interpreing the wealth of
fossil teeth being recovered in South America, and should help to fill the void of comparative odonto-
metrics in platyrrhine studies.
INTRODUCTION
Living New World monkeys comprise six-
teen genera of large, medium and small-bodied
arboreal primates. The behavioral ecology of
some groups of extant platyrrhines, such as
marmosets and tamarins, has been studied
extensively (Sussman and Kinzey 1984), while
that of other groups, such as uakaris, has not
(Fleagle 1988). Platyrrhines have radiated into a
wide variety of arboreal habitats and foraging
niches which are diagnostic at the subfamily level
and in some cases at the generic level. Unfortu-
nately, modem habitats are difficult to recognize
in the South American fossil record, which is
biased against preservation of the predominant
tropical forest ecosystem. Fossil platyrrhine
tooth morphology is thus one of the only
indicators available for evolutionary analyses.
Nonetheless, comparative quantitative data on
modern dentitions are conspicuously lacking.
This study introduces a few selected metric indi-
ces of modern platyrrhine upper teeth in order to
illustrate useful diagnostic patterns and ranges of
variation. A comprehensive series of dental met-
nc indices will greatly facilitate the systematic
study of fossil New World monkeys, as has been
done for other primate groups.
MATERIALS AND METHODS
The dental measurements used in this study
were taken on the collections at the Field Museum
of Natural History in Chicago, Illinois. The
number of specimens for each genus is listed in
Table 1. At the time data were collected, no spe-
cimens of Brachyteles were available. All graphs
and statistics were generated from standard
mesio-distal (M-D) and bucco-lingual (B-L) tooth
measurements taken in the occlusal plane with
hand-held digital read-out calipers.
Strict absolute measures of tooth magnitude
cannot possibly stand alone as analytical tools,
for they cannot represent the qualitative shape dif-
ferences in cusps within and among genera. This
study is an independent analysis of the size pat-
tening in platyrrhine upper postcanine teeth, and
in no way encompasses the many other analyses
of intra-platyrrhine variation and relationship. Its
value lies primarily as a check of hypotheses gen-
erated from nonmetric studies and as a means of
sconng various dental parameters and indices as
taxonomic markers. As noted above, compara-
tive odontometry is practically nonexistent in
platyrrhine studies. Ranges of variation are
known for only a few genera (Rosenberger et al.
in press).
RESULTS
Figures 1-7 illustrate a variety of dental
metric parameters and indices. In each case the
data points are coded according to genus name
(see Table 1 for codes). The most obvious
message in these graphs is that tooth size regres-
sion in platyrrhines is highly consistent, a result
common in primate families as a whole but not
superfamilies or suborders. The correlation coef-
ficients and regression values clearly indicate that
no platyrrhine genus has departed markedly from
any other or away from the expected dependent
varable regression. Thus, the adaptive radiation
of New World monkeys, while diverse ecologi-
cally, has been conservative along the postcanine
dental battery.
58
Table 1. List of genera and the number of each used in this study. The abbreviations refer to the data
points in Figures 1-7.
Genus
Number of
specimens
Alouatta
Aotus
Ateles
Cacajao
Callicebus
Callimico
Callithrix
Cebuella
Cebus
Chiropotes
Lagothrix
Leontopithecus
Pithecia
Saguinus
Saimiri
34
13
24
7
12
7
74
6
11
6
7
12
10
74
2
Figure 1 plots the summed area of P3 and P4
against the summed area of M1 and M2. M3 is
excluded in order to compare callitrichines, which
do not have third molars, with other platyrrhines.
Figure 2 is an expanded reproduction of this
comparison which excludes Alouatta in order to
compare the smaller genera more clearly. This
comparison cleanly separates the platyrrhine gen-
era into subfamily groupings. Callitrichines, the
smallest in size, naturally are grouped at the
bottom of the regression. Two genera, Leonto-
pithecus and Callimico, grade into the cebine and
pithecine range, while the large-bodied atelines
complete the top of the regression.
Figures 3 and 4 illustrate the relative sizes of
P3 and P4 against a linear scale of the summed
M-D lengths of P2-M2. This scale is used in or-
der to more clearly separate longer from shorter
toothrows. The high correlation coefficients are
logical; i.e., one would expect premolar area to
be highly correlated with the M-D length of the
postcanine toothrow, even if M3 is excluded.
The more interesting application of these graphs
is what they reveal about the relationship between
p3 and P4. In all platyrrhines, P3 and P4 have
maintained a constant size relationship with one
another such that even in genera which show
slightly higher (e.g., Chiropotes and Cacajao
among the pithecines, Leontopithecus among the
callitrichines) or lower (e.g., Aotus and Calli-
cebus among the pithecines) premolar areas
compared to the molar area regression (Figures 1-
2), it is the p3-P4 complex as a whole which has
Abbreviation
A
0
T
J
M
x
U
B
H
L
R
p
G
S
changed (or conversely, remained the same as the
M1-M2 complex changed). The platyrrhine with
the largest teeth, Alouatta, has enlarged upper
molars but premolariform premolars with areas
predicted by this regression. Similarly, the
smallest platyrrhine genus, Cebuella, tracks con-
sistendy along the regression. Its greatly reduced
body size, therefore, has not affected the relative
size of the postcanine dental battery in any un-
usual way.
Figures 5 and 6 document one of the few
diagnostic indices for the dental radiation of
platyrrhines as a whole. The vertical axis is an
index calculated by dividing the M-D length of
P2-M2 by the area of Ml plus M2 (Figure 5) or
by the area of P3 plus P4 (Figure 6). The hori-
zontal axis is used as a scale for increasing
toothrow length. The indices reflect the range of
variation in molar (Figure 5) or premolar (Figure
6) battery size across taxa, and document two
trends. First, callitrichines are much more vari-
able than other platynrhines im this relationship, as
indicated by their more vertical distribution.
While the range of variation in their postcanine
toothrow length is absolutely low, relative widths
of P3-M2 are not. Second, larger platyrrhines are
progressively less variable in "these indices than
smaller platyrrhines, which suggests a tighter al-
lometric relationship betweezi increase in tooth
length and are  As  brw legth increases,
the area indices and rangepof variation decrease,
especially for molrs (Figur 5). Platyrrhine up-
per postcanines widen at least as much as they
59
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61
Figure 3. The x-axis is the total of the M-D lengths of P2-M2. The high correlation (r = .989) is expec-
ted because the variables are partially dependent upon one another. Deviations m  this regiessin are
therefore informative, such as the relatively greater premolar area of Cebus compared to other platy-
rrhines of similar toothrow lengths.
E
E
I.
C
nl
IL
a.
0
t.
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30
AO
P2-M2 LENGTH SCALE (mm)
Figure 4. Same as Figure 3 but using P4 area instead of P3. The graph does not appear to be infor-
mative since the area of a tooth should be closely related to the length of the toothrow (r = .986).
Comparison with Figure 2 indicates, however,, that this simple regression reveals differences in molar
widths between Aocus/Callicebus and PithecialChiropotes.
E
E
w
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P2-M2 LENGTH SCALE (mm)
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62
Figure 5. The y-axis represents the M-D lengths of P2-M2 divided by the occlusal area of M1-M2. The
logarithmic profile indicates decreased variation in this index from callitrichines through atelines, espe-
ciaily Alouatta.       ,
1.3 a
4     o? o "
LT L~ L
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P2-M3 LENGTH (mm)
Figure 6. The y-axis represents the M-D lengths of P2-M2 divided by the occlusal area of P3-P4. The
logarithmic profile is silar but not as uniform as in Figure 5.
of
P2-M3 LENGTH (mm)
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63
Figure 7. This regression combines the indices of Figures 5 and 6 and demonstrates their independence
from one another by maintaining the same logarithmic profile.
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40
3.
P2-M3 LENGTH (mm)
lengthen, but a threshold appears to have been
reached in large-bodied atelines (Alouatta, Ate-
les).
DISCUSSION
The evolutionary study of fossil and living
New World monkeys is experiencing strong
growth and development (see Fleagle and Rosen-
berger 1990). A remarkable diversity of fossil
platyrrhines has been recovered in the last five
years (Setoguchi and Rosenberger 1985, 1987;
Luchterhand et al. 1986; Fleagle et al. 1987; Kay
et al. 1987; Kay 1989; Hartwig et al. 1990).
Until recently, documentary work on dentition
(Orlosky and Swindler 1975; Swindler 1976;
Hershkovitz 1977) has been largely descriptive
rather than comparative, and comparative studies
(Rosenberger 1979) have been largely nonmetric.
This is because size based metric analyses can
say nothing about morphology and very little
about function, the two principal variables in
dental studies today. As demonstrated here,
however, metric analyses can illustrate important
trends in the masticatory apparatus and in some
cases serve as taxonomic indicators.
The correlation between premolar (P34) and
molar (M1-2) area is strong for platyrrhines as a
whole (r = .97), as expected. Along this regres-
sion (Figure 1), pithecines (Aotus, Callicebus,
Pithecia, Chiropotes, Cacajao) with broadly sim-
ilar molar areas are diagnostically separated by
different premolar areas. Hypotheses of a close
Aotus - Callicebus phylogenetic relationship
(Rosenberger 1981) are strengthened by the
overlapping ranges and similar regressions of
these two genera (Figures 1 and 2).
Callitrichines lack hypocones on upper mo-
las, so areal calculations based on (M-D) x (B-L)
dimensions are consequently higher than the
actual occlusal surface area. This increases the r-
value for platyrrhine-wide comparisons, but the
slope of the regression for callitrichines would be
the same in either case because all callitrichines
would lose roughly the same amount of relative
occlusal area. One conclusion to draw from the
metric indices as they are presented here is that
dental reduction in callitichines has been primar-
ily nonmetric in distinction (Plavcan and Gomez
1990). In premolar comparisons (Figures 3 and
4), which are free of the problems associated
with hypocone loss, callitrichines, including Ce-
buella, Callimico and Leontopithecus, fall
consistently along the platyrrhine regression. As
Figures 5 and 6 suggest, there may in fact be no
i                                                         I                                  I           I          I                                                         I                      I                      I           I
I
64
metrically diagnostic postcanine features of the
callitrichine radiation.
The radiation of large-bodied atelines, such
as Alouatta and Ateles, is characterized by the
very trend that is lacking in callitrichines. Fig-
ures 5 and 6 indicate that ateline premolar and
molar area are tightly related to the length of the
postcanine toothrow. More importantly, from a
proportional point of view, strict thresholds of
.2-.3 apply to the toothrow length/molar area
ratio (Figure 5), and .4-.5 to the premolar ratio
(Figure 6) as the toothrow length increases.
Some of this decreasing variation can be ex-
plained as a natural logarithmic consequence of
the y-axis index. As tooth area increases in large-
bodied platyrrhines, the denominator of the y-
axis variable increases and so the difference in the
resulting fraction is logarithmically expressed.
Nonetheless, for individuals with similar postca-
nine toothrow lengths, the difference in variation
between callitrichines and atelines reflects the na-
ture of selection on body size decrease in the
former and tooth size increase in the latter.
Figure 7 represents a combination of Figures
5 and 6, in which the vertical axis now indicates
the summed p3-M2 area. The four principal post-
canine teeth as a battery logarithmically increase
in width as length increases, rather than one van-
able (length) being directionally dependent upon a
component of the other (premolars or molars).
CONCLUSION
This analysis is part of a larger treatment of
platyrrhine morphological variation. No attempts
to infer dental function or possible phylogenetic
relationships are made here because the compari-
sons are exclusively size-based. The value of
such comparisons lies in their usefulness for
discerning trends in adaptive radiations and for
tracking patterns within and across taxa. The re-
gressions illustrated in this study show that a
number of taxonomic markers can be demonstra-
ted through the use of metric indices, and they
underscore the need for further data collection
and analyses. A better understanding of the rich
diversity of fossil platyrrhines from the Miocene
depends upon an understanding of the metric re-
lationships among the living forms.
ACKNOWLEDGMENTS
This work was supported financially by a
Thomas J. Dee Fellowship from the Field Mu-
seum of Natural History, a Robert H. Lowie
Fellowship from the University of California at
Berkeley, and a National Science Foundation
Graduate Fellowship. I thank Dr. Bruce Patter-
son for access to specimens under his care, Dr.
A.L. Rosenberger for helpful comments, and
Gary Richards for technical assistance.
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