Technical Note: Modeling Primate Occlusal TopographyUsing Geographic Information Systems Technology

L. FLYNN ZUCCOTTI, MALCOLM D. WILLIAMSON,W. FREDRICK LIMP, AND PETER S. UNGAR*Department of Anthropology and the Center for Advanced SpatialTechnologies, University of Arkansas, Fayetteville, Arkansas

KEY WORDS GIS; GRASS; primate dental morphology

ABSTRACT Most functional analyses of primate tooth form have beenlimited to linear or area measurements. Such studies have offered but alimited glimpse at differences in occlusal relief among taxa. Such differencesin dental topography may relate to tooth function and, so, have considerableimplications for the inference of diet from fossil teeth. In this article, wedescribe a technique to model and compare primate molars in three dimen-sions using Geographic Resources Analysis Support System (GRASS) soft-ware. We examine unworn lower second molars of three extant hominoidswith known differences in diet (Gorilla gorilla, Pan troglodytes, and Pongopygmaeus), and two fossil forms, (Afropithecus turkanesis and Dryopithecuslaietanus). First, we obtained approximately 400 landmarks on the occlusalsurfaces of each tooth using an electromagnetic digitizer. Raster ‘‘terrainmodels’’ of occlusal surfaces were then created by interpolation of thecoordinate data. We used GRASS terrain analysis automated techniques toquantify the volumes and slopes of individual cusps. We also used the GRASSwatershed technique to identify the volume of liquid that would accumulate ineach tooth’s basin (a measure of basin area), and the directions and intensityof drainage over the occlusal surface. In sum, GRASS shows considerablepotential for the characterization and comparison of tooth surfaces. Further-more, techniques described here are not limited to the study of teeth, but maybe broadly applicable to studies of skulls, joints, and other biological struc-tures. Am J Phys Anthropol 107:137–142, 1998. r 1998 Wiley-Liss, Inc.

INTRODUCTION

Numerous studies have shown that toothform can be used to predict aspects of dietand feeding behavior in living primates.Most such studies have thus far been limitedto linear or area measurements. While thesehave been very useful for functional analy-ses, many are limited in their ability toreflect occlusal relief and to depict teeth asthree-dimensional objects. This studypresents a new method to characterize andallow the comparison of primate teeth inthree dimensions using Geographic Re-sources Analysis Support System (GRASS)software.

Most quantitative studies of primate mo-lar functional anatomy have involved analy-ses of the lengths of shearing crests. Kay(1984), for example, devised a shearing quo-tient to compare shearing crest lengths and,to a degree, occlusal relief among primates.He found that folivores and insectivoreshave longer shearing crests than do frugivo-res. Further, among frugivores hard-object

Received 12 May 1998; accepted 1 June 1998.Contract grant sponsor: LSB Leakey Foundation; Contract

grant sponsor: NSF; Contract grant number SBR 9601766.*Correspondence to: Peter Ungar, Department of Anthropol-

ogy, Old Main 330, University of Arkansas, Fayetteville, AR72701.E-mail: pungar@comp.uark.edu

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 107:137–142 (1998)

r 1998 WILEY-LISS, INC.

feeders have shorter crests (and blunterteeth) than do soft-fruit eaters. This associa-tion between crest length and diet holds forall major extant primate groups: strepsir-rhines, platyrrhines, cercopithecoids, andhominoids (see Ungar [1998] for review).Such studies have important implicationsfor paleobiology, as they suggest that molarmorphology also likely reflects diet in fossilprimates.

Still, workers have begun to recognizethat even more detail about diet might begleaned from teeth given new computer-driven methods for modeling surfaces inthree dimensions (e.g., Ungar et al., 1994).Geographic Information Systems (GIS) pro-vide one promising method for modelingdental topography (e.g., Reed, 1997).

GIS were developed to apply the power ofcomputers to solving environmental and in-frastructural problems. Today, they are usedfor everything from 911 emergency vehiclerouting to assessing the habitat of endan-gered species. Many GIS tools have beencreated to examine and model the physicalsurface of the Earth. These tools offer equalpotential for examination and analysis ofother physical surfaces, such as the occlusalsurfaces of primate teeth.

GRASS is a public-domain GIS written bythe US Army Construction EngineeringLaboratory, Champaign, IL (Westervelt etal., 1987). GRASS was originally created asa tool for analysis and management of large-scale environmental issues, but its vast suiteof terrain analysis tools and programming‘‘toolbox’’ structure make it suitable for manymore tasks than originally intended. Analy-sis in GRASS is primarily performed on datain a raster format, consisting of two-dimen-sional matrices where each cell, or value,records some attribute of a surface at aparticular location. These cells might con-tain the original z value from digitizing, orthey may indicate slope, aspect (direction ofslope), or any other characteristic or indexvalue. Each theme, such as elevation orslope, is stored as an individual raster layer.GRASS supports simple three-dimensionalpoint data in what is called a ‘‘sites’’ format.

In this article, we demonstrate howGRASS can be used to interpolate an occlu-sal surface, and to gather data on volume,

slope, and aspect of individual cusps. Fur-ther, we show how GRASS tools, such as ther.watershed command, can be used to gener-ate important new categories of data, includ-ing the volume of liquid that would accumu-late in each tooth’s basin (a measure of basinarea), and the directions and intensity ofdrainage over the occlusal surface.

METHODS AND RESULTS

High-resolution replicas were taken oforiginal unworn lower second molars of theextant hominoids Gorilla gorilla, Pan tro-glodytes, and Pongo pygmaeus, and the fos-sil Miocene catarrhines Afropithecus tur-kanesis and Dryopithecus laietanus. Theliving taxa cover the spectrum of dietaryadaptations of extant great apes, includingthe more folivorous Gorilla and the frugivo-res Pan and Pongo.

High-resolution molds of each specimenwere prepared using a polyvinylsiloxane den-tal impression putty (Coltene, Inc., Mawath,NJ). Casts were poured with an epoxy resinand catalyst (Tap Plastics, Inc., Dublin, CA).

Data collection

Data were collected using a Polhemus3Draw Pro (Polhemus Inc., Colchester, VT)electromagnetic digitizer. This consists of atablet and stylus that allow the user to tracethe occlusal surface of a specimen, recordingx, y, and z coordinates. Specimens wereplaced at the center of the tablet for consis-tency, with the buccal side facing the x axis.Approximately 400 coordinates were col-

TABLE 1. Slope and volume values* for specimensincluded in this study

Afro-pithecus

Dryo-pithecus Gorilla Pan Pongo

Average slopeHypoconulid 42.21 23.75 47.33 41.73 40.1Hypoconid 40.49 20.71 51.5 42.21 33.91Protoconid 32.7 33.23 38.72 34.71 27.89Metaconid 33.53 23.25 38.08 30.78 43.2

Entoconid 40.36 37.73 41.36 42.73 41.2Cusp Volume

Hypoconulid 3.54 0.5 16.94 1.75 3.58Hypoconid 3.06 0.8 6.74 1 0.8Protoconid 0.3 0.54 2.03 0.55 0.95Metaconid 0.29 1.02 1.66 0.7 2.53Entoconid 2.33 6.15 5.32 4.14 4.04

Basin Volume 0.04 0.27 0.23 0.76 8.02

* Cusp slope values are in degrees, and volumes are in mm3.

138 L.F. ZUCOTTI ET AL.

lected for each surface at a rate of 70 pointsper minute to a resolution of 0.13 mm. Thedigitizer was attached to a PC microcom-puter, and coordinates of each point werestored as ASCII files using the 3Draw soft-ware (Polhemus Inc., Colchester, VT).

The resulting ASCII files were importedinto GRASS 4.1, running on a Sun Sparc-Server 690. Where single x,y coordinateswere recorded more than once, resultingz values were averaged through a UNIXshell script specifically created for the task.Duplicate records were also parsed to asecond file, so that they could be readilyinspected for possible data collection errors.

Interpolation

Once the unique records were convertedto a GRASS sites file, they were interpolatedto create a continuous raster layer of z values.We tested several interpolation algorithms,including inverse-distance weighting andkriging (a geostatistical method) for theirability to accurately fit the known datapoints. The most satisfactory results were

achieved using a GRASS program calleds.surf.tps, which is based on a thin-platesplining model (Mitasova and Hofierka,1993). This program simulates draping amembrane over the known points. By chang-ing the spline tension, the behavior of themembrane may be altered from that of aflexible rubber sheet to a thin metal plate.Other parameters used in this study includesmoothing, minimum interpolation distance,and minimum number of points for interpo-lation. It was found that the best resultswere achieved with a fairly low spline ten-sion (tension 5 20) and a small amount ofsmoothing (smooth 5 2) (see Fig. 1).

Individual cusp characterizations

First, contour lines and slope data werecomputed for the entire surface of eachtooth. We identified individual cusps by se-lecting the lowest elevation contour linesthat fully surrounded those cusps. The areasdefined by the contour lines were then usedto isolate slope and elevation raster cells foreach cusp (Fig. 2). Volume and average slope

Fig. 1. Surface models of teeth examined in this study (upper left to lower right): Afropithecus,Dryopithecus, Gorilla, Pan, Pongo.

139MODELING PRIMATE TOOTH FORM USING GIS

for individual cusps were then calculated bysumming elevation cells and averaging slopecells, respectively. GRASS also calculatesdescriptive statistics to characterize the spa-tial distribution of slope and aspect valuesfor the cusps. Such data may prove to bevaluable for comparisons of cusp form withinand between individuals and species.

Basin drainage and volume

GRASS also provides other tools thatmight be valuable for the characterizationand comparison of occlusal surfaces. Forexample, the way food or liquid might accu-

mulate or drain over a tooth’s surface isreadily modeled by GRASS commands de-signed to simulate the flow of water andsediment across a terrain. Such routinespromise to shed considerable light on toothfunction.

We used the GRASS command r.water-shed to delineate drainage patterns acrossthe occlusal surface, assigning each cell avalue that reflects how many other cellsdrain into it. By reclassing the resultingraster layer to show only those cells with avalue of 100 or higher, the primary drainageroutes can be displayed (Fig. 3). Basins can

Fig. 2. Contour lines and slopes of cusps for each tooth (upper left to lower right): Afropithecus,Dryopithecus, Gorilla, Pan, Pongo. Steeper slopes are represented by darker shading.

140 L.F. ZUCOTTI ET AL.

be defined as those areas where fluid wouldaccumulate. When r.watershed encounterssuch an area, it determines the lowest pointof overflow and continues delineating thedrainage. GRASS can then ‘‘fill’’ the basinand measure its volume (Fig. 3).

DISCUSSION

Methods described here can be used tomodel and compare tooth occlusal surfacesin three dimensions. GRASS can interpolate

dental surfaces from three-dimensional pointcoordinates and determine numerous at-tributes of those surfaces, including cusparea, volume, slope, and aspect. Modelingdental topography using GIS also allows usto examine surface characteristics such asdrainage and accumulation of fluid. Result-ing data can be used to accurately character-ize teeth in three dimensions, and to com-pare taxa in both functional and phylogeneticanalyses.

Fig. 3. Combined shaded relief maps with drainage and basin information (upper left to lower right):Afropithecus, Dryopithecus, Gorilla, Pan, Pongo. Lines represent drainage routes, and enclosed shapesrepresent fluid accumulations or basins.

141MODELING PRIMATE TOOTH FORM USING GIS

Because of limitations to the resolution ofthe digitizer and the geometry of the stylusused in this study, thin-plate splining wasnecessary to create a model of each surface.We expect that a device capable of collectingpoints with greater precision, such as ahigh-resolution laser digitizer (Rosenbergerand Calvo, 1998), would eliminate the needfor interpolation.

Methods described in this article also haveimportant applications beyond dental mor-phology. GRASS holds the potential to modeland compare a broad range of complex sur-faces, including skulls, joints, and otherbiological structures.

ACKNOWLEDGMENTS

We thank Drs. William Stanley and LaszloKordos for allowing us to cast specimens intheir care. We thank Dr. Mark Teaford forproviding us with the Afropithecus turkane-sis cast. Computer systems were provided by

Intergraph Corporation and SUN Microsys-tems.

LITERATURE CITED

Kay RF (1984) On the use of anatomical features to inferforaging behavior in extinct primates. In PS Rodmanand JGH Cant (eds): Adaptations for Foraging inNonhuman Primates: Contributions to an Organis-mal Biology of Prosimians, Monkeys and Apes. NewYork: Columbia University, pp 21–53.

Mitasova H, and Hofierka J (1993) Interpolation byregularized spline with tension. II. Application toterrain modeling and surface geometry analysis. Math.Geol. 25:657–669.

Reed DNO (1997) Contour mapping as a new method forinterpreting diet from tooth morphology. Am. J. Phys.Anthropol. Suppl. 24:194.

Rosenberger AL, and Calvo CM (1998) Digital morphol-ogy: The Smithsonian’s BioVisualization Lab. Evol.Anthropol. 6:77–78.

Ungar PS (1998) Dental allometry, morphology andwear as evidence for diet in fossil primates. Evol.Anthropol. 6:205–217.

Ungar PS, Walker A, and Coffing KE (1994) A reanalysisof the Lukeino Molar (KNM-LU 335). Am. J. Phys.Anthropol. 94:165–173.

Westervelt J, Goran W, and Shapiro M (1987) Develop-ment and applications of GRASS: The geographicresources analysis support system. In BK Opitz (ed):Geographic Information Systems in Government, Vol.2. Hampton, VA: A. Deepak Publishing, pp 605–624.

142 L.F. ZUCOTTI ET AL.