Arch Form, Tooth Size, and Occlusomandibular Kinesis in the Ceboidea M. R. ZINGESER Oregon Regional Primate Research Center, 505 N.W. 185th Avenue, Beuverton, Oregon 97005 KEY WORDS Teeth . Dental occlusion . Mastication . Feeding behavior Evolution . Ceboidea. ABSTRACT Correlations between dental morphology, arch configuration, and jaw movement patterns were quantitatively investigated in 23 ceboid species to elucidate integrative aspects of occlusal functional anatomy in a n adaptive and evolutionary context. Differential maxillary-mandibular arch widths are primary in guiding lateral jaw movements. These movements are characterized according to their associated condylar shifts as either predominantly translatory or rotational. Predominantly translatory movements result from peripheral contact relationships between maxillary arches which are considerably wider posteriorly than their opposing mandibular arches. The greatest degree of mandibular movement is i n the molar region in functional association with wide “primitive” maxillary molars, narrow mandibular molars, constricted maxillary intercanine widths, and narrow maxillary incisors. In contrast, predominantly rotational masticatory jaw movements result from differential arch widths which are greatest in the maxillary canine and incisor regions. Here most jaw movement is in the anterior segment and this is reflected in small maxillary-mandibular molar width differences, a high degree of premolarization, wide-set maxillary canine teeth, and wide maxillary incisors. Possible selectional factors i n the putative evolution of rotational predominance in mastication from the more primitive translatory pattern are discussed Comparative studies of integrative aspects of the functional anatomy of teeth and jaws have considerably enhanced our understanding of the masticatory system. In this regard, the Ceboidea constitute a valuable adaptive array. These New World primates of Central and South America are a vaned group revealing many traits of common descent. Diversity is very much i n evidence i n tooth and jaw anatomy and these variations on a common theme afford interesting insights into selectional processes and evolutionary dynamics. In addition, the remarkable parallelisms between the New World and Old World primates are potential sources for elucidating general principles. A brief review of the literature and associated terminology will clarify subsequent points to be considered. The concept of centric occlusion as applied i n this paper is defined as that position of the articulating dentition which provides maximum AM. J . PHYS. ANTHROP.,45: 317-330 intercuspation. In higher primates and other mammals with fused mandibular symphyses, centric occlusion corresponds with centric relation, i.e. the mandible is simultaneously positioned symmetrically with reference to the midsaggital plane. In these isognathic forms (Crompton and Hiiemae, ’70), centric occlusion serves as a reference position separating buccal and lingual phase jaw movements. These functional jaw movements a s adduced from tooth wear facets, were first described in a series of papers by Butler (’52, ’73),Butler and Mills (’59), and Mills (’63, ’67, ’73). Additional insights into masticatory function have been obtained from cinematographic observations of living animals. Most recently, Crompton and Hiiemae (’70) used these techniques to study Didelphis, and Hiiemae and Kay (’73) and Kay and I Publication No. 869 of the Oregon Regional Primate Research Center, supported in part by Grant RR00163-17 of the National Institutes of Health. 317 318 M. R . ZINGESER IIiieniae ('74) used tooth functional analysis and cineradiography to analyze mastication in some primates. According to Hiieniae arid Kay ('73), the masticatory cycle is either puncture-crushing or chewing. In the former, food is reduced by pulping and crushing between the teeth without tooth-to-tooth contact after which the chewing cycle with its tooth-to-tooth contact occurs in two scquential phases. During Phase I (the buccal phasc o€ Butler and Mills, '59), the buccal cusps of the lower teeth on the active sidc are brought into initial contact with the buccal cusps of the uppers (figs. l A , 2A) and then move into centric occlusion (figs. l B , 2B). The lower molars then move across the upper molars duiing Phase I1 which is similar to but not identical with the lingual phase of Butler and Mills ('59). Lingual phase movements are not easily measured and are often ill-defined and quite variable. In this paper, I focus upon determinants of and variations in buccal phase movements and their canine complex counterparts because associated form and functional relationships uniquely lend themselves to quantification and analysis in terms of whole arch configuration and tooth size gradients. Shared characteristics common to buccal phase and canjne shear and honing function suggest a n encompassing descriptive term p e T i p h e r d occlzi.som a n dib ul or mou erne nts since peripher a1 elements of both arches are opposed through mandibular function. These essentially lateral movements lack a propalinal component during (peripheral) incisor functioning. Transverse incisal contact shear in these edge-to-edge or underbite teeth involves the same masticatory movements that characterize the post-canine occlusion. Peripheral mastication is the p r i m a l WLOprimitively associated with shear and ingestion and considerably antedating the appearance of lingual or Phase IT function with its associated distal and lingual grinding and crushing talon and talonid clcrnents (Patterson, '56: Crompton arid Iliiemae. '70; Hiiemae and Kay, '73; hlills. ' 7 3 ; arid Butler, '73). In this connection, the canine complex with its shearing irnd honing functions is clearly a buccal phase homolog, with primitive relationships between paracone, protoconid, and talonid rim. therian dental components with built- in reciprocal honing capabilities not confined to the canine region (Zingeser, '68c, '69). The buccal phase is commonly described as associated with ipsilateral condylar rotation and contralateral distal movement as the jaw shifts from buccal to centric position (fig. 3A). Together with a slight translatory component (Bennett movement), i t describes the human condition. That this pattern is riot ubiquitous among nonhuman primates is now recognized (Mills, '63, '67; Kay and Hiiemae, '74). Our studies of ceboid teeth and jaws confirm that the rotational modality, universally present in maxillary canine tooth shear and honing, is wide-spread in mastication. In the Ceboidea, however, condylar rotation is often combined with and frequently subsidiary to translatory mandibular movements (fig. 4). The morphological basis for these variations in peripheral occlusomandibular movements, their functional arid adaptive significance, and the putative selectional factors in their evolution i n the Ceboidea are examined. MATERIALS. RATlONALE AND METHODS Skulls of mature animals with relatively unworn teeth and intact dental arches were selected. The primary- source was the U.S. National Museum of Natural History; supplementary material was obtained from the Museum of Comparative Zoology, The British Museum of Natural History, the Field Museum of Natural History, arid the Oregon Regional Primate Research Center. In all, 106 specimens representing 12 genera and 23 species were measured. These include all of the Ceboid genera except Callithrzx, Callimico, and Cricrijuo (Napier and Napier, '67). Whenever possible, I tricd to achieve a n equal sex distribution. The procedures for securing tooth measurements were those previously described by Zingeser ('67). The odontometric data are more extensive than the arch width measurements because (1) unilaterally missing teeth, which preclude accurate arch width estimates, suffice for tooth measurements, and (2) the odontometric information is supplemented by previously published data (Zingeser, '67, '73). Because of the paucity of specimens i n many genera, I used pooled samples which were heterogenous for sex and species but 319 CEBOID OCCLUSOMANDIBULAR KINESIS as far as possible balanced in numbers for both, to offset metric bias (table 1). Masticatory excursive movements are guided by contact relationships between opposing reciprocally functioning teeth. At the initiation of buccal phase contact, both maxillary and mandibular buccal cusps occlude in almost parallel fashion throughout the buccal row (figs. l A , 2A) to maximize shear. Exceptions to uniformity are most often seen in P2, a tooth which tends toward caniniform morphology (fig. lB), and the terminal molars which are morphometrically variable. In contrast with these uniform buccal phase relationships, considerable intergeneric variation is observed in the degree of movement at different sites along the tooth row which occurs as the mandibular teeth shift to centric occlusion (figs. l B , 2B). Variations in degree and direction of mandibular and associated condylar shifts needed to accommodate this buccal phase movement from initial contact to centric depend upon maxillary and mandibular arch peripheral width differences. These factors guide the mandible in varying degrees of condylar rotational and/or transverse translatory buccal phase movement (figs. 3, 4). In documenting peripheral arch width differences in a functionally meaningful manner, I observed the following characteristics. At the beginning of buccal phase mastication, the lingual tips of the maxillary premolar paracones (buccal cusps) and molar paracones (mesiobuccal cusps) rest against the buccal aspects of their respective mandibular articulating teeth at the talonid rim ridges distal to the protoconids (mandibular premolar buccal cusps and molar mesiobuccal cusps (figs. lA, 2A). Since this region is the hypoflexid in mandibular molars, the homologous region of the mandibular premolars and canine teeth will also be called hypoflexid i n this paper for the sake of conciseness (figs. 5B, 6B). As the mandibular teeth shift into centric occlusion, the amount of buccal phase traverse equals the distance from the lingual paracone tip to corresponding hypo- TABLE 1 Ceboid slriLll mnterinl Key 1 2 3 4 5 6 7 8 9 10 11 12 Species Crbuelln p y q m n r a Sugurn.lcs 1 Marikina geoffroyi Mystax nigrtcolis Leontocebus geofli-oyi Midus m i d n s Suguinus s p p Oedipomidas geoffroyi Oedipomidas o e d i p u s Saimiri sciureus S a i m i r i orstedti Saimiri s p p Aotu s t n ue rgcc tic 5 Callicebus ornutiis Callicebus torqiiatzts Callicebus s p p Pithrcin p i t h r c m Pithrcici monachn Chrropotea sutunus Cebus capticinus Cebus upella Ateles punisciis Ateles geoffroyi Ateles b e l z e h u t h Lagothrix lagotricha Brachytrles arachnoides Alouatta caraya M F M/ F 5 1 5 2 1 1 3 1 2 1 1 1 1 2 1 5 1 6 2 2 1 1 1 1 1 5 2 2 10 2 2 1 5 1 4 2 2 5 Total 11 4 1 1 1 1 1 1 3 2 2 8 8 2(+4) 4 2 2 8(+16) 3 3 (+2) 4(+22) 2 5 5 (+2) 12 ( 38) + The numbers i n brackets ( + n) represent specimens which yield odontometric data i n excess of those supplying both odontometric and arch width data. 1 Taxa subsumed under Saguinus after Napier and Napier ('67).The twelve listed genera are keyed to table 2 and figure 7. 320 M. R. ZINGESER Fig. 1 Aloutrttn rrcmyrr. male. A . Buccal phase. peripheral contact. B. Centric occlusion. P2 (maxillary first premolar) is caniiiiform, set out laterally and functions with the canine tooth. Greatest shift from buccal phase to centric occurs i n the posterior arch. Fig. 2 Crbrcs crrptcciwits, female. A . Buccal phase peripheral contact. B. Centric orclusion. Compare with figure 1 for relative posteroanterior shift at P3 a n d M1 i n buccal phase to centric n ~ o v e m e n t sGreatest . shift occurs anteriorly in Cebrts. flexid contact region (figs. l B , 2B). This distance is difficult to measure, but i t can he closely approximated by halving the transparacone and transhypoflexid differences: Buccal Shift = transparacone width minus transhypoflexid width 2 When maxillary widths (1/2 transparacone, figs. 5A, 6 A ) are plotted against man- dibular widths (1/2 transhypoflexid, figs. 5B, 6B), a diagrammatic representation of buccal shifts at varying sites along the tooth row is obtained together with a crude but informative approximation of relative buccal arch forms (figs. 9, 10, 11). Relative arch widths arid hence mandibular shifts can be usefully expressed by computing the posterior shift as a percentage of the anterior shift. The extreme ends of' the CEBOTD O C C L U S O M A N D I B U L A R K I N E S I S tooth row are avoided. P 3 is chosen in preference to the caniniform P2, and M (1-t) in preference to the variable Mt. In the latter designation, variations in molar number among the Ceboidea necessitate using - Maxilla __ Mandible - Centric Mandible -Peripheral Shift Fig. 3 Schematic diagram of extreme peripheral shift patterns which indicate differential arch form determinants. A. Rotational. B . Translatory. 32 1 t for the terminal molar. M 0-t) is the penultimate molar. Thus Mt is M3 in the Ceboidea except for most of the Callitrichidae which lack this tooth. Percentage buccal shift at posterior versus anterior arch sites are ranked and listed i n table 2A. The variations in buccal shift patterns expressed by posteroanterior shift percentages can be characterized as predominantly translatory when the molar shift equals or exceeds the premolar shift, and predominantly rotational when the buccal shift at the premolar site exceeds the molar shift. These designations accord with the observed condylar movements which occur concurrently with the variations in buccal shift in the articulated skull specjmens (fig. 4). These two peripheral occlusom andibular functional categories are schematized in figure 3 in terms of total arch form determinants, including incisor and canine segments as well as the post-canine dentition. Fig. 4 B a s a l view of male A . curnyu articulated skull in various functional positions. i Ipsilateral and c contralateral sides. A . Extreme buccal phase occlusion with arrows indicating approximate shift directions to and from centric. Largely translatory. B . Extreme canine honing position with arrows indicating approximate shift directions to and from centric. Mostly rotational but with a translatory component. Compare with figures 1, 3 , 5 , and 9 . 322 M . R. ZINGESER Fig. 5 Occlusal view, A . c n r o y o , male. A. Maxilla, transparacone widths at three sites identified by roman numerals a n d keyed to functionally related widths in B. Mandible, transhypoflexid widths, arabic numerals. Fig. 6 Occlusal view, C. c . u p i u . i n u s , female. A. Maxilla transparacone widths keyed to functionally related widths in B . Mandible at transhypofiexids. Compare with figure 5 for arch form a n d tooth size differences. A comparjson of figures 3A,B suggests a dichotomy of form-functional relationships. In the rotational category (fig. 3A), one would expect to find wide-set maxillary canine teeth with large honing ranges, relatively broad maxillary incisors (mesiodistally), and premolars (Buccolingually) which are compatible i n form and function with the wide anterior maxillary arch segment and associated large anterior range of jaw movement. Because of the small range of mandibular molar traverse, relatively slight differences i n buccolingual size are to be expected between opposing molars. In con- trast, in the translatory modality (fig. 3R), the greatest range of motion is in the molar region; accordingly the maxillary molars should be appreciably wider than the mandibular molars to accommodate to the large posterior translatory traverse. The relatively narrow anterior maxillary arch form and decreased anterior functional range suggest a narrower intercanine width with an associated small honing range and smaller buccolingual maxillary premolar and mesiodistal maxillary incisor sizes. To test these premises, I measured the canine honing shifts, which are computed CEBOID OCCLUSOMANDIBULAR KINESIS 323 TABLE 2 Peripheral occlusomartdibular shijt charmteristics Key Species A B C 125.0 109.0 100.0 100.0 100.0 157.1 234.8 163.6 236.4 118.1 76.7 83.6 73.9 69.4 70.8 44.4 Predominantly translatory 76.2 “Primitive” maxillary molars very wide compared with mandibular molars. 69.8 67.6 Rami often expanded. C honing 54.2 range reduced. Small maxillary D Functional categories 12 10 5 2 1 Alotiatta ctirtiyrc Lrryothrix s p p . Callicehiis s p p . Saguinirs s p p . Cebuella pygmnecc 11 4 Brachyteles a rac hnoides Aotus trivirgcitiis 96.8 92.9 117.9 138.5 69.9 73.8 53.9 97.4 9 3 7 8 6 Atcles s p p . Sainziri s p p . Clriropotrs sntanas Crbus s p p . Pithecia s p p . 80.0 80.0 64.7 63.2 62.5 293.8 225.0 581.8 366.7 450.0 88.4 69.2 86.7 83.3 81.8 90.9 Predominantly rotational 77.1 Maxillary and mandibular molars approach each other in widths. Rami 95.3 “normative.” Premolarization 88.0 marked. C honing range large. Max75.6 incisors. Posterior arch form and functional dominance. illary incisor complex well-developed. Anterior arch form and functional dominance . Species are numerically keyed to figure 7. Peripheral shift inetrics are based upon differential maxillary-mandibular arch widths. A. Shift at penultimate molar region M (1-t) as a percentage of shift at second premolar region P3. These values are ranked and serve to differentiate translatory and rotational functional categories. B. Shift at the canine region C as percentage of shift at penultimate molar region M (1-t). Large honing ranges are associated with predominantly rotational masticatory patterns. C. Penultimate mandibular molar buccolingual width as a Percentage Of Penultimate maxillary molar width, Relative maxillary-mandibular molar widths reflect differences between the two functional categories. D. Maxillary central incisur width (mesiodistal) as a percentage of maxillary penultimate molar width (buccolingual). Maxillary incisor size differences in the two categories correlate with occlusomandibular functional pattern differences. like the buccal shifts (figs. 5, 6) against their corresponding molar buccal shifts: C/M (l-t) x 100. These values are listed in table 2B. Relative molar buccolingual widths were determined: and are listed in table 2C. Finally, maxillary incisor widths are measured against corresoondine molar widths: Gesiodistal 11 Buccolingual M(1 1) x 100 and these are listed in table 2D. The buccolingual dimensions of M (1-t) were plotted against the corresponding measurements of P3 on logarithmic coordinates, and the results were analyzed for features that were consistent with buccal shift ratios which derive from arch width differential characteristics (table 2A). Trend lines were then extrapolated (fig. 7). The exigencies of the sample size required that the data be pooled and balanced with regard to sex, but the need to investigate possible sex-related dimorphism ir. occlusomandibular kinetics led to a limited supplementary investigation. Sexually segregated peripheral shift data representing three genera are plotted in figure 8. Finally, relevant odontometrics are graphically presented together with arch width buccal shift diagrams for three representative genera i n figures 9, 10, and 11. FINDINGS Column A of table 2 lists the molar buccal shifts at M (l-t) as percentages of their corresponding premolar shifts at P3. These ranked percentages fall into two groups: predominantly translatory (posterior shift 2 anterior shift) and predominantly rotational (posterior shift < anterior shift). These two functional categories are associated with distinctive tooth and jaw morphological differences which will be discussed (table 2). The inclusion of Brachyteles and Aotus in the translatory group is based upon morphological concordance with this group together with a consideration of the vagaries of percentage data. The 96.8% reading for Brachyteles and the 92.9% value for Aotus both reflect a 0.1 m m difference between numerator and denominator (3.0/3.1 and 1.3/1.4), well within the error of measurement. Canine honing shift values expressed as percentages of buccal phase shifts are listed i n table 2B. As predicted, rotational predominance is associated with the greatest canine honing shift ranges. However, 324 M. R. ZINGESER the predominantly translatory Layothrix and Suguinus show exceptionally large honing ranges at 234.8 % and 236.4 % (see DISCUSSION). In connection with canine tooth excursive movements, rotation of the condyle on the ipsilateral side and concomitant antero-posterior displacement of the contralateral condyle appear to be universal among the Ceboidea (figs. 3A, 4B). These movements may entail considerable associated translatory mandibular shifts especially in animals characterized by predominantly trans1atory buccal phase functioning (fig. 4). Except for Lagotl.rrix and Snimiri (see DISCUSSION), mandibular vs. maxillary buccolingual molar ratios expressed as percentages also accord with our theoretical "model" (table 2C). They indicate that in translatory predominance maxillary molars tend to be much wider than mandibular molars, whereas i n rotational predominance the differences in opposing molar widths are considerably smaller in order to conform to the mechanical requirements of the two jaw movement patterns (cf. fig. 3). The maxillary central incisors tend to be wide relative to maxillary molars i n the rotational group compared with the translatory forms, and they thus accord with the predicted trend (table 2D). However, these ratios reflect diverse effects which are not confined to simple correlation with maxillary anterior arch widths, but include such factors as the relative differences in maxillary molar size in the two functional modalities, and dietary adaptations. An analysis of' these factors is underway. Premolar buccolingual size gradients are consistent with my theoretical expectations (fig. 7). In rotational predominance, there is a trend toward premolarization i n the maxilla, i.e., anincrease in premolar widths posteroanteriorly. This, together with a relative reduction of maxillary molar widths, is compatible with condylar rotational kinetics with its large anterior range of motion (figs. 3A, 6 A , 10). Premolarization is not equally marked in the five predominantly rotational genera. It is most evident in SuimiTi (No. 3), Chiropotoes (No. 7), and Cebus (No. 9), and less so in Pithecia (No. 6) and Ateles (No. 9) all of which are keyed to table 2 and figure 7 for cross reference. The slopes of the rotational and translatory extrapolations in figure 7 are :I 10 M!A (B-L in mm ) 2 3 I I 2 3 4 4 5 6 78910 I I I , , , $ I p3 3 2 g4' 3 2t 7 Mcl , , I l l , 5 678910 (B-L in mm) Fig. 7 Buccolingual of P3 plotted against buccolingual of M(1-t), the penultimate molar, on logarithmic coordinates for maxilla (upper) a n d mandible (lower). Buccal phase trends extrapolated. The genera are numerically keyed to tables 1 a n d 2. informative. In the maxilla (upper graph) the rotational slope is 53" vs. 45" for the translatory slope, a clear indication of the trend towards premolarization in the former. In the mandible (fig. 7, lower graph), the rotational slope is 4 3 " vs. the 45" translatory slope. In the rotationally predominant mandibular arch form, the posteroanterior premolar reduction evident i n figure 7 is consistent with the narrowing of the anterior mandibular arch; this accentuates rotational movements in contact relationships with the wide anterior maxillary arch (cf. figs. 5 B and 6B). Sex-segregated peripheral functional ranges are graphed for three genera i n figure 8. Sex-related dimorphism in asso- CEBOID OCCLUSOMANDIBULAR KINESIS 32 5 I20 11.0 8.0 Alouutto coraya Suimiri snoreus Cebus ssp d=7 o o t t &k&Mlhbd3 U l l d =5 J .u CPZP3WMIMZ)IK - CPZP3WMIMZM3 Fig. 8 Sex segregated peripheral shift patterns. Significant sexual dimorphism is confined to canine honing ranges in Cebus and Saimiri. In Alouatta, premolar peripheral shift sexrelated differences correspond with caninization of male maxillary premolars. ciation with canine honing range is predictably evident. In Cebus and Saimiri, rotationally predominant forms, postcanine functional dimorphism is slight. However, in the predominantly translatory genus Alouatta, the premolars are markedly dimorphic in functional range and appear to reflect size sex-related dimorphism in the premolars as well as in the canines (Zingeser, ’67, ’68a). The dichotomy of peripheral occlusomandibular functioningin Alouatta is typical of translatory forms, since occlusomandibular rotational movements are always present in varying degrees i n association with canine shear and honing. In Alouatta males, these rotational movements also involve the large caninized maxillary premolars. Correlations between arch form, tooth size gradients, and occlusomandibular kinesis are further clarified by reference to graphs which detail relevant odontometrics together with arch width (and hence approximation of form) characteristics and associated peripheral shift patterns i n each genus. Three examples are chosen to elucidate specific facets (figs. 9, 10, 11). The Alouatta data plots (fig. 9) typically reflect translatory predominance. In conformity with the restricted honing range, the maxillary incisors are narrow and frequently in underbite relationship (Zingeser, ’67, ’68a, ’73). Maxillary molars are “primitive” with well-defined ectocingula and are very wide in comparison with the mandibular molars in adaptations for efficient transverse translatory functioning (fig. 5). The maxillary premolars decrease markedly i n width posterio-anteriorly ; the mandibular premolars (except for P2 which hones against Cl) are subequal in widths. These tooth size trends correspond roughly with arch forms as is evident in the arch widths graphed in the lower section of figure 9. They are adaptively compatible with the posterior arch form and functional dominance which characterizes this occlusomandibular kinetic category (cf. figs. 5 and 7). Cebus (fig. 10) presents quite a different picture. Tooth and arch form traits associated with the anterior functional dominance of the rotational modality are clearly evident. These include wide maxillary incisors, premolarization, and relatively slight differences between maxillary and mandibular molar widths. The rough correlation between tooth size gradients and arch forms is again evident. The role of the anterior constriction of the mandibular arch in reinforcing rotational occlusomandibular kinetics is particularly evident (fig. 9, lower section). Saguinus (fig. 11) taken as representative of the Callitrichidae, has interesting specializations, many of which are related to small body size. For example, in the foreshortened jaws of this animal, M 3 is missing and M 1 is dominant. The “primitive” maxillary first molar is very much wider than the apposing mandibular first molar in co-adaptation with the translatory modality. P4 represents a “nodal” point to the mesial of which the caniniform P2=:$ teeth shear together with the canine i n predominantly rotational movements. This 326 M. R. ZINGESER Alouatta caraya 1-1 1-2 C P-2 P-3 P-4 M-1 M-2 M-3 1-1 1-2 c 1 - 9 Mcxillc - talonid Mandible meon trigonid u) - L W W 1 c Pp ._ z 1 P' Tronsporocone Mi P' MZ M" I 19.0 r 18.0 17.016.0 - / p 3lmm 150 - 14.013.012.0 11.010.0 - Lf 2.8 0 28 m d e-P, 9 ' '< ', /O' P,-e e-M, i M, M, M, Tronshypoflexld Fig. 9 Aloziatta carayn odontometrics (above) and buccal phase shift diagram (below). Differential widths of maxillary and mandibular arches determine the buccal shift pattern. The approximation to arch shapes (lower diagram) should be compared with tooth size gradients (upper diagram and figs. 5, 7). Tooth size and arch form characteristics are typical of translatory predominance. pattern is common to both sexes and is also evident i n Cebuella. DISCUSSION The translatory jaw excursive modality is primitive, appearing early i n therian evolution (Crompton and Hiiemae, '70; Mills, '67, '73). It is associated with molar (as opposed to both molar and premolar) mastication and correlates with primitive molar morphology in which buccal phase features dominate (Kay and Hiiemae, '74). Buccal phase translatory predominance is typical of' most Ceboidea (table 2) and is not correlated per se with dietary predilections. In common with most primates, the Ceboidea exhibit much plasticity i n feeding behavior. However, feeding adaptations have considerable selectional advantage i n competitive situations such as those that accrue to Alouatta which can digest mature leaves when more desirable fruit is lacking (Hladik and Hladik, '69). The ramus expansion i n many predominantly translatory ceboids is a herbivorous-follivorous adaptation associated with highly developed superficial masseter and internal pterygoid muscles, which parallel similar morphology in ungulates and other herbivores (Radinsky, '66; Turnbull, '70). It is functionally associated with the "universally specialized" temporomandibular joint-occlusal region orientation pattern (Biegert, '63) which allows simultaneous CEBOID OCCLUSOMANDIB ULAR KINESIS 32 7 Cebus spp 1-1 1-2 c tngonid ~~ _ _ ~ _ _ _ u) I I al 9.0 6 I I I 5-5 5-5 10 Transporocone P,-M, I M, L M, M, Tronshyp:flexid Fig. 10 Cebits s p p as in figure 9. Premolarization is marked. Tooth size and arch form traits are typical of rotational predominance. contact upon closure coupled with great functional mobility (Zingeser, ’73). However, this form-functional configuration is not confined to the predominantly translatory category but is also found i n the Pithecinae. On the other hand, ramus expansion is not seen in such predominantly translatory forms as the largely insectivorous Callitrichidae. The inconsistencies in table 2 need to be examined. Among the translatory group, Lagothrix and Saguinus are exceptional in having high values for the canine honing range whereas in the rotational group Saimiri shows a relatively small honing range (column B). It is to be noted that Lagothrix is a “misfit” with regard to the other values i n table 2. This genus is intermediate among the Atelinae in many masticatory traits, falling between the primitive Brachyteles (translatory) and Ateles (rotational) (Zingeser, ’73). It appears to be actively evolving variations in sex-related canine tooth dimorphism according to the evidence put forth by Fooden (’63). Large canine teeth in S a p i n u s (the Tamarins) together with the apparent anomolous retention of translatory chewing may have evolved because of jaw foreshortening in this genus, thus necessitating this combination for maximum efficiency. The status of Saimiri in this scheme clearly points to translatory functional affinities. For example, the relative canine honing range (column B) is the smallest of the rotational group and the mandibular/maxillary molar width ratio suggests translatory function. This ratio reflects the primitive morphology of Saimiri molars. Further evidence of translator): buccal phase tendencies is seen i n figure 8 i n comparisons with the peripheral occlusomandibular patterns of’ Alouattn and Cebus. These characteristics suggest that rotational buccal phase masticatory functioning in Saimiri is a relatively recent evolutionary development. Whereas the maxillary incisor/molar ratios (table ZD) generally come up to paradigm expecta- 32 8 M. R. ZINGESEH Saguinus spp - Maxilla tolonid 'I 4 10.0 4.0 11 P' - e-P, P' P4 M' I I 5-c 4 M2 I MI MI M, Transhypoflexid Fig. 11 S n g t t i n u s s p p as i n figures 8 a n d 9. The size dominance of M1 is typical of the Callitrichidae. MI is appreciably wider than M , i n adaptation to transverse translatory masticatory kinesis. P2=3 are caniniform and jaw- movements involving these teeth and the maxillarv canines are largely rotational. tions? they show much variation which can be attributed to complex factors. The data in table 2 appear to substantiate a direct correlation between the range of maxillary canine tooth honing excursion and the presence of the rotational modality in mastication. As I have mentioned, rotational jaw movements are associated in all genera with canine tooth honing and shear; the relationship between the two seems clear. To effectively hone the length of long maxillary canine teeth against their functionally complementary mandibular honing notches at C, and P2 (Zingeser, '69), the maxillary teeih m u s t splay laterally in proportion to their length. If they did not, the degree of jaw opening and associated orthal honing movements required would probably be inadaptive. The extension of rotational jaw movements to include the entire premolar-molar row can be explained in terms of evolutionary dynamics. With the evolution of larger, and hence more widely splayed maxillary canine teeth through whatever selectional pressures (see Zingeser, '68b), the cervical bases of these teeth also extend farther laterally. For functional efficiency in the maintaining arch continuity, p2 first and then others i n the premolar row tend to align with the canine teeth. The resulting single rotational modality is assumed to have functional and hence selectional advantages. Secondary molar and premolar changes probably followed these primary arch form and related mandibular excursive evolu- CEBOID OCCLUSOMANDIBULAR KINESIS tionary developments, and a somewhat different course is discerned among the various Ceboid lines. In the Cebinae, especially Ccbus, premolarization is marked, and the premolars clearly assume and even dominate masticatory function (figs. 6, 9). The situation is more diverse i n the Atelinae. Brachyteles, a facultative leaf eater with small canines, is probably closest to the atelinine-alouattine ancestral form (Zingeser, ’73). Jaw functioning is largely translatory, maxillary molars are very large and replete with “prkitive” features, the rami are expanded, and maxillary incisors are small and in underbite relationship. At the other end of the atelinine spectrum, Ateles with its large, splayed-out maxillary cuspids, exclusively rotational jaw function, moderate degree of premolarization, large edge-to-edge incisors, and rim-ridged molars and premolars stands in marked and informative contrast, with Lugothrix intermediate. When metric and morphological comparisons are made between the dentitioris of these three Atelinae, important trends emerge. In odoiitometric comparisons, differences in buccolingual size among the molars are appreciably greater in the maxilla than i n the mandible. Thus in comparing the predominantly translatory Brnchyfeles with the rotationally predominant Ateles, it is the maxillary molars that are reduced in size as a consequence of the loss of the ectocingula; these are prominent in Brtichyteles but absent i n Ateles. That maxillary molars, primitively wide i n comparison with mandibular molars i n order to be compatible with translatory function, should show reduction in assuming rotational modality is completely i n accord with “mechanical” considerations (figs. 3A,B). Less diversity is seen among the Pithecinae. All show rotational predominance. Chiropotes (fig. 7, No. 7) exhibits extreme premolarization and concomitant reduction i n maxillary molar widths, while these trends are not as obvious in Pithecia (fig. 7, No. 6). Ctrcajao, appears to be intermediate, but the data are incomplete. The interrelationships between arch form, tooth morphology, and occlusomandibular kinesis shown in this study of the Ceboidea provide promise of elucidating form and functional correlations and evolutionary trends in other groups. In view of 329 the many striking parallelisms between New and Old World primates, the extension of these investigations to the catarrhines seems eminently worthwhile. ACKNOWLEDGMENTS I gratefully acknowledge the generous assistance of Dr. Richard W. Thorington, Jr. of the United States National Museum who made available the material upon which this paper is largely based. I am indebted to Ms. Louise Zingeser for assisting in data processing and to the staff of the Oregon Regional Primate Research Center who cooperated in the preparation of the manuscript. I am especially grateful to Mr. Joel Ito for his skilled rendition of the diagrams and graphs and to Mr. Harry Wohlscin for the photography. 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