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 havefocused on nonmetric data or have presented basic statistics ofindividual tooth measurements without comparisons among genera. This study presents selected com-parisons of measurements of the upper postcanine dental battery of modern platyrrhines anddemonstrates that metric indices are valuable as taxonomic markersfor the New World monkey radia-tion. Callitrichines, pithecines, and atelines show diagnostic patterns and ranges of variation in anumber ofpremolar and molar comparisons. These indices are also usefulfor interpreing the wealth offossil 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-bodiedarboreal primates. The behavioral ecology ofsome groups of extant platyrrhines, such asmarmosets and tamarins, has been studiedextensively (Sussman and Kinzey 1984), whilethat of other groups, such as uakaris, has not(Fleagle 1988). Platyrrhines have radiated into awide variety of arboreal habitats and foragingniches which are diagnostic at the subfamily leveland in some cases at the generic level. Unfortu-nately, modem habitats are difficult to recognizein the South American fossil record, which isbiased against preservation of the predominanttropical forest ecosystem. Fossil platyrrhinetooth morphology is thus one of the onlyindicators available for evolutionary analyses.Nonetheless, comparative quantitative data onmodern dentitions are conspicuously lacking.This study introduces a few selected metric indi-ces of modern platyrrhine upper teeth in order toillustrate useful diagnostic patterns and ranges ofvariation. A comprehensive series of dental met-nc indices will greatly facilitate the systematicstudy of fossil New World monkeys, as has beendone for other primate groups.

MATERIALS AND METHODS

The dental measurements used in this studywere taken on the collections at the Field Museumof Natural History in Chicago, Illinois. Thenumber of specimens for each genus is listed inTable 1. At the time data were collected, no spe-cimens ofBrachyteles were available. All graphsand statistics were generated from standard

mesio-distal (M-D) and bucco-lingual (B-L) toothmeasurements taken in the occlusal plane withhand-held digital read-out calipers.

Strict absolute measures of tooth magnitudecannot possibly stand alone as analytical tools,for they cannot represent the qualitative shape dif-ferences in cusps within and among genera. Thisstudy is an independent analysis of the size pat-tening in platyrrhine upper postcanine teeth, andin no way encompasses the many other analysesof intra-platyrrhine variation and relationship. Itsvalue lies primarily as a check of hypotheses gen-erated from nonmetric studies and as a means ofsconng various dental parameters and indices astaxonomic markers. As noted above, compara-tive odontometry is practically nonexistent inplatyrrhine studies. Ranges of variation areknown for only a few genera (Rosenberger et al.in press).

RESULTS

Figures 1-7 illustrate a variety of dentalmetric parameters and indices. In each case thedata points are coded according to genus name(see Table 1 for codes). The most obviousmessage in these graphs is that tooth size regres-sion in platyrrhines is highly consistent, a resultcommon in primate families as a whole but notsuperfamilies or suborders. The correlation coef-ficients and regression values clearly indicate thatno platyrrhine genus has departed markedly fromany other or away from the expected dependentvarable regression. Thus, the adaptive radiationof New World monkeys, while diverse ecologi-cally, has been conservative along the postcaninedental battery.

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Table 1. List of genera and the number of each used in this study. The abbreviations refer to the datapoints in Figures 1-7.

Genus Number ofspecimens

AlouattaAotusAtelesCacajaoCallicebusCallimicoCallithrixCebuellaCebusChiropotesLagothrixLeontopithecusPitheciaSaguinusSaimiri

3413247

127

746

1167

1210742

Figure 1 plots the summed area of P3 and P4against the summed area of M1 and M2. M3 isexcluded in order to compare callitrichines, whichdo not have third molars, with other platyrrhines.Figure 2 is an expanded reproduction of thiscomparison which excludes Alouatta in order tocompare the smaller genera more clearly. Thiscomparison cleanly separates the platyrrhine gen-era into subfamily groupings. Callitrichines, thesmallest in size, naturally are grouped at thebottom of the regression. Two genera, Leonto-pithecus and Callimico, grade into the cebine andpithecine range, while the large-bodied atelinescomplete the top of the regression.

Figures 3 and 4 illustrate the relative sizes ofP3 and P4 against a linear scale of the summedM-D lengths of P2-M2. This scale is used in or-der to more clearly separate longer from shortertoothrows. The high correlation coefficients arelogical; i.e., one would expect premolar area tobe highly correlated with the M-D length of thepostcanine toothrow, even if M3 is excluded.The more interesting application of these graphsis what they reveal about the relationship betweenp3 and P4. In all platyrrhines, P3 and P4 havemaintained a constant size relationship with oneanother such that even in genera which showslightly higher (e.g., Chiropotes and Cacajaoamong the pithecines, Leontopithecus among thecallitrichines) or lower (e.g., Aotus and Calli-cebus among the pithecines) premolar areascompared to the molar area regression (Figures 1-2), it is the p3-P4 complex as a whole which has

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changed (or conversely, remained the same as theM1-M2 complex changed). The platyrrhine withthe largest teeth, Alouatta, has enlarged uppermolars but premolariform premolars with areaspredicted by this regression. Similarly, thesmallest platyrrhine genus, Cebuella, tracks con-sistendy along the regression. Its greatly reducedbody size, therefore, has not affected the relativesize of the postcanine dental battery in any un-usual way.

Figures 5 and 6 document one of the fewdiagnostic indices for the dental radiation ofplatyrrhines as a whole. The vertical axis is anindex calculated by dividing the M-D length ofP2-M2 by the area of Ml plus M2 (Figure 5) orby the area of P3 plus P4 (Figure 6). The hori-zontal axis is used as a scale for increasingtoothrow length. The indices reflect the range ofvariation in molar (Figure 5) or premolar (Figure6) battery size across taxa, and document twotrends. First, callitrichines are much more vari-able than other platynrhines im this relationship, asindicated by their more vertical distribution.While the range of variation in their postcaninetoothrow length is absolutely low, relative widthsof P3-M2 are not. Second, larger platyrrhines areprogressively less variable in "these indices thansmaller platyrrhines, which suggests a tighter al-lometric relationship betweezi increase in toothlength 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

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lengthen, but a threshold appears to have beenreached in large-bodied atelines (Alouatta, Ate-les).

DISCUSSION

The evolutionary study of fossil and livingNew World monkeys is experiencing stronggrowth and development (see Fleagle and Rosen-berger 1990). A remarkable diversity of fossilplatyrrhines has been recovered in the last fiveyears (Setoguchi and Rosenberger 1985, 1987;Luchterhand et al. 1986; Fleagle et al. 1987; Kayet 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 descriptiverather than comparative, and comparative studies(Rosenberger 1979) have been largely nonmetric.This is because size based metric analyses cansay nothing about morphology and very littleabout function, the two principal variables indental studies today. As demonstrated here,however, metric analyses can illustrate importanttrends in the masticatory apparatus and in somecases serve as taxonomic indicators.

The correlation between premolar (P34) and

molar (M1-2) area is strong for platyrrhines as awhole (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 bydifferent premolar areas. Hypotheses of a closeAotus - Callicebus phylogenetic relationship(Rosenberger 1981) are strengthened by theoverlapping ranges and similar regressions ofthese 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 theactual occlusal surface area. This increases the r-value for platyrrhine-wide comparisons, but theslope of the regression for callitrichines would bethe same in either case because all callitrichineswould lose roughly the same amount of relativeocclusal area. One conclusion to draw from themetric indices as they are presented here is thatdental reduction in callitichines has been primar-ily nonmetric in distinction (Plavcan and Gomez1990). In premolar comparisons (Figures 3 and4), which are free of the problems associatedwith hypocone loss, callitrichines, including Ce-buella, Callimico and Leontopithecus, fallconsistently along the platyrrhine regression. AsFigures 5 and 6 suggest, there may in fact be no

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64

metrically diagnostic postcanine features of thecallitrichine radiation.

The radiation of large-bodied atelines, suchas Alouatta and Ateles, is characterized by thevery trend that is lacking in callitrichines. Fig-ures 5 and 6 indicate that ateline premolar andmolar area are tightly related to the length of thepostcanine toothrow. More importantly, from aproportional point of view, strict thresholds of.2-.3 apply to the toothrow length/molar arearatio (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 ofthe y-axis index. As tooth area increases in large-bodied platyrrhines, the denominator of the y-axis variable increases and so the difference in theresulting fraction is logarithmically expressed.Nonetheless, for individuals with similar postca-nine toothrow lengths, the difference in variationbetween callitrichines and atelines reflects the na-ture of selection on body size decrease in theformer and tooth size increase in the latter.

Figure 7 represents a combination of Figures5 and 6, in which the vertical axis now indicatesthe summed p3-M2 area. The four principal post-canine teeth as a battery logarithmically increasein width as length increases, rather than one van-able (length) being directionally dependent upon acomponent of the other (premolars or molars).

CONCLUSION

This analysis is part of a larger treatment ofplatyrrhine morphological variation. No attemptsto infer dental function or possible phylogeneticrelationships are made here because the compari-sons are exclusively size-based. The value ofsuch comparisons lies in their usefulness fordiscerning trends in adaptive radiations and fortracking patterns within and across taxa. The re-gressions illustrated in this study show that anumber of taxonomic markers can be demonstra-ted through the use of metric indices, and theyunderscore the need for further data collectionand analyses. A better understanding of the richdiversity of fossil platyrrhines from the Miocenedepends upon an understanding of the metric re-lationships among the living forms.

ACKNOWLEDGMENTS

This work was supported financially by aThomas J. Dee Fellowship from the Field Mu-seum of Natural History, a Robert H. Lowie

Fellowship from the University of California atBerkeley, and a National Science FoundationGraduate Fellowship. I thank Dr. Bruce Patter-son for access to specimens under his care, Dr.A.L. Rosenberger for helpful comments, andGary Richards for technical assistance.

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