AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 94:327-337 (1994) Enamel Thickness and the Helicoidal Occlusal Plane GABRIELE A. MACHO AND MARGIT E. BERNER Hominid Palaeontology Research Group, Department of Human Anatomy and Cell Biology, The University of Liuerpool, Liuerpool L69 3BX, England (G.A.M.);Department of Anthropology, Natural History Museum Vienna, Burgring 7, A-1014 Vienna,Austria (M.E.B.) KEY WORDS Maxillary molars, Mastication, F’unctional adaptation ABSTRACT In the present study 38 unworn maxillary molars (M1 = 16, M2 = 12, M3= 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M’s and one slightly worn M3 were also used thus increasing the total sample to 44,but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uniand multivariate analyses revealed that the distribution of enamel thickness within and between maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior t o posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the paracone, i.e., the lingual slope of the paracone, than under the Phase I1 facet of the protocone, i.e., the buccal slope of that cusp. These proportional differences are most pronounced in M3, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M1 to M3 follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be “specialized in their function, i.e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly. 0 1994 Wiley-Liss, Inc. Received December 21,1992; accepted January 19,1994 Address reprint requests to Dr. Gabriele A. Macho, Hominid Palaeontology Research Group, Department of Human Anatomy and Cell Biology, The University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England. 0 1994 WILEY-LISS, INC 328 G.A.MACHO AND M.E.BERNER Differences in enamel thickness among characterized by a change of slopes of ocprimate taxa have usually been regarded as clusal surfaces in anteroposterior direction evolutionary responses to functional de- with first maxillary molars typically exhibmands through dietary specializations (An- iting more pronounced wear on lingual drews and Martin, 1991; Gantt, 1983; Jolly, cusps, whereas third upper molars are more 1970; Kay, 1981; Shellis and Hiiemae, 1986; worn buccally, while mandibular teeth show Simons, 1976). However, differences also oc- complementary patterns of wear. When cur within teeth. Molnar and colleagues viewed from the anterior, the unworn left (Molnar and Gantt, 1977; Molnar and Ward, and right dentition is aligned along an imag1977) first studied the distribution of inary curve whose concavity is facing upenamel thickness within a single tooth wards ad palatum; this curve is commonly crown and established that the areas that referred to as the curve of Monson. Once are under greater functional demands ex- wear commences this curve becomes flat hibit thicker enamel than do regions which and, eventually, reversed. In worn dentiare usually less worn. They suggested that tions one could thus define the helicoidal the unequal distribution of enamel '' . . . in- occlusal plane as exhibiting a reversed curve creases grinding efficiency and resistance to of Monson in first molars, a flat plane in wear which prolongs life" (Molnar and second molars, and a normal curve of MonWard, 1977: 562). Bearing in mind that lin- son in third molars. This definition, howgual cusps of upper molars usually interdig- ever, implies that the helicoidal occlusal itate between the lingual and buccal cusps of plane is entirely a function of wear of serilower molars it is not unexpected that lin- ally erupting molars: its expression could gual cusps of hominid maxillary molars only be enhanced by consumption of an have thicker enamel than do buccal cusps abrasive diet, thinner and less abrasion re(Macho and Thackeray, 1992). In mandibu- sistant enamel, which would reverse the lar molars, buccal cusps fulfill a complemen- curve more rapidly, and by a particularly tary function and they have relatively delayed eruption pattern of molars, as is the thicker enamel too. Also, second and third case in humans. On the other hand, it has maxillary molars have been found to have been proposed that the helicoidal occlusal substantially thicker enamel than M's, both plane is a structural orofacial feature, which absolutely and relatively (Macho and could be of functional importance (Osborn, Berner, 1993). This is probably indicative of 1982; Richards and Brown, 1986; Smith, the greater wear resistance of posterior mo- 1983, 1986). For instance, most researchers lars and may also attest to the increasing regard differences in axial inclination of bite force magnitudes from anterior to poste- teeth to account for a helicoidal occlusal rior (Koolstra et al., 1988; Mansour and Rey- plane, even in the absence of wear. nick, 1975a,b; Molnar and Ward, 1977; 0sThe present study on the patterning of born and Baragar, 1985; Ward and Molnar, enamel thickness within and between max1980). illary molars aims to appraise the models €or In light of the reported evolutionary re- the development of the helicoidal plane. sponse of enamel thickness to functional de- More specifically, it explores whether the mands, the present study examines whether appearance of the helicoidal occlusal plane the distribution of enamel within and may come about through a) increased absoamong molars can be reconciled with a heli- lute wear resistance of later erupting molars coidal pattern of wear. The helicoidal pat- only, or b) whether the greatest wear resistern of occlusion has been regarded as an tance of molars changes from anterior to orofacial feature typical of Homo (Tobias, posterior in such a way as to provide evi1980, 19911, but recent studies conclusively dence for the helicoidal occlusal plane being demonstrated that it also occurs in Plio- a structural feature of the orofacial skeleton. Pleistocene hominids and nonhuman pri- In a previous study, using the same set of mates, especially in chimpanzees (Osborn, data, we were mainly concerned with abso1982; Smith, 1983,1986; Ward, 1981; White lute differences in enamel thickness beand Johanson, 1982). The helicoidal plane is tween molars (Macho and Berner, 1993). ENAMEL THICKNESS AND THE HELICOIDAL PLANE 329 This investigation will examine whether systematic differences in enamel patterning can be found between molars. MATERIALS AND METHODS Thirty-eight unworn modern human maxillary molars (M‘ = 16, M2 = 12, M3 = 10) were used in the present study, as well as five slightly worn M’s and one slightly worn M3; measurements made on the worn areas of the latter teeth were coded as missing values (see also Macho and Berner, 1993).A total of 39 individuals is represented in the sample. The molars were dissected from subadult skulls which came from the Slavic necropolis near Zwentendorf, Lower Austria (10th century A.D.).They were excavated by F. Hampl between 1953 and 1958 (Ladenbauer-Orel, 19711, and are now housed in the Department of Anthropology, Natural History Museum Vienna, Austria. Following Martin (1983) the molars were cut through the tips of the mesial cusps in a plane perpendicular to the cervical margin of the crown with an ISOMET 11-1180 low speed annular saw producing a 300 pm wide cut. In mechanical sections the dentine horns can be easily visualized when they have been narrowly missed; this provides the opportunity to grind down the tooth to the optimal plane of section until the maximum dentine content is achieved. While employing this latter procedure may not be advisable when sectioning valuable fossils, because it results in loss of tooth material and, in some instances, morphology, molars used in this study could be subjected to such a procedure. Hence, in cases where the tips of the dentine horns were not cut precisely the teeth were ground and polished with aluminum oxide paste on a rotating diamond disc to the optimum plane of section. Measurements of enamel thickness derived from such sections are more accurate than those obtained from “clean” sections (Spoor et al., 19931,thus substantially reducing the confounding influence of obliquity of section on enamel thickness measurements, as pointed out by Martin (1983). Each tooth was photographed under a Wild-Photomakroskop at a magnification of 1:8, and measurements of enamel thickness were recorded in millimeters from enlarged 1 L 1 B Fig. 1. Cross-section through maxillary molar showing the seven measurements of enamel thickness studied as well as the heights of the protocone and paraeone dentine horns. photographs (1:lO) using a Vernier sliding caliper (Fig. 1). The reference plane 0-P was drawn parallel to the crown base through the lowest point of the dentinoenamel junction (DEJ) between the cusps (Macho and Berner, 1993; Macho and Thackeray, 1992). In addition to the enamel thickness measurements (Fig. l, see also Macho and Berner, 1993 for definition) the heights of the dentine horns of the protocone and the paracone were taken; they were measured as the perpendicular distance from the reference plane 0-P to the dentine horn. Since a previous study revealed the occurrence of Carabelli’s cusps to have no influence on the measurements recorded (Macho and Berner, 1993) this additional cuspule was not analyzed further. Similarly, the effects of sexual dimorphism on the measurements appear negligible (Macho and Berner, 1993). In order to test for intra- and interobserver errors five teeth were repeatedly measured by both authors on 3 consecutive days. In none of the instances did the differences exceed 1%of the dimension. Uni- and multivariate statistics were carried out using the SPSS statistical program on the IBM mainframe computer of the University of Vienna. Since the patterning of enamel between teeth of the same individuals was found to be comparable to intertooth differences in the remaining independent sample (Macho and Berner, 1993), molars derived from the same individuals were not analyzed separately. G.A. MACHO AND M.E. BERNER 330 TABLE I . Descriptive statistics of measurements (in between M2 and M3. Enamel thickness at the intercuspal fissure gave nonsignificant ANOVA results in all analyses. These results, and Measurement M’ M2 M3 F-value P those of other tests documented below, sugETl gest that the statistical significance of the 1.18 7.90 0.0010 X 1.56 1.41 ANOVA results was mainly due to the 0.31 0.17 SD 0.27 markedly smaller dimensions in first maxilN 21 12 10 Ef2 lary molars. 1.60 2.17 2.09 19.27 0.0000 X Nonetheless, inspection of mean values in 0.27 0.34 0.13 SD 16 12 10 N Table 1, as well as further statistical tests ET3 described below, indicate that the pattern1.30 1.62 1.59 13.02 0.0000 x ing of enamel thickness distribution 0.19 0.19 0.22 SD 21 12 11 N changes systematically from anterior to posEx4 terior. With regard to the cusp tip of the 1.18 1.17 1.11 0.14 0.8674 X protocone and its buccal slope the greatest 0.25 0.47 0.38 SD 21 12 11 N mean values were yielded in M2s, while the ET5 comparable measurements of the paracone 1.25 1.73 1.78 30.41 0.0000 X 0.15 0.26 0.27 SD were greatest in third upper molars (Table 21 12 11 N 1).To illustrate this observation the average ET6 amount of enamel thickness of these dimen1.04 1.62 1.64 26.82 0.0000 X 0.30 0.24 0.16 SD sions was calculated for the protocone (i.e., 17 12 11 N [ET 2 + ET 31/21 and the paracone (i.e., [ET ET7 1.34 1.01 1.08 7.92 0.0012 5 + ET 6]/2). (Enamel thickness measureX 0.17 0.30 0.32 SD ments ET 1 and ET 7 were not included in 11 21 12 N the calculations since ET 1 would correH1 X 22.79 18.20 16.65 17.09 0.0000 spond to a Phase I wear facet only after the 3.65 3.05 2.55 SD cusp has worn down considerably, whereas 21 12 11 N ET 7 is not associated with a wear facet at H2_ 1.29 0.2859 X 22.40 21.55 20.30 all.) Figure 2 shows that enamel thickness SD 3.58 3.03 3.87 increases in the anteroposterior direction N 21 12 11 but, more importantly, that the disparity beLVariablescoded with “ E T refer to enamel thickness measurements tween protocone and paracone decreases. In whereas “ H corresponds to the height dimensions of the dentine horns of the protocone and the paracone, respectively. Results of other words, while in first maxillary molars ANOVA and probability levels are also listed. the mean values are clearly separated with hardly any overlap in variation, at M3 the average paracone enamel thickness is RESULTS within one standard deviation of the enamel Descriptive statistics of all measurements thickness of the protocone (Fig. 2 ) . In order t o test further for significant difare given in Table 1. Except for two dimensions, enamel thickness a t the intercuspal ferences in enamel thickness distribution fissure (ET 4)and the height of the dentine within and among molars paired t-tests horn of the paracone (H 2 ) , all analyses of were computed between the corresponding variance (ANOVAs)yielded statistically sig- measurements of the lingual and the buccal nificant differences between maxillary mo- side for each tooth category separately. lars. Enamel thickness along both tooth Comparisons for differences in dentine horn walls (ET 1 and ET 7 ) is largest in M’s, heights are also included. Table 2 lists the whereas M2s and M3s have a considerably t-values and the significance levels, thicker enamel cover at the occlusal basin whereby negative coefficients indicate that than do first maxillary molars. Comparisons the measurement on the buccal side is larger of measurements between teeth by Stu- than that on the lingual moiety. Enamel dent’s t-test confirmed the overall similari- thickness along the lingual wall (ET 1-ET 7 ) ties of second and third molars; only ET 1 was thicker throughout the series although was significantly different at the 5% level the tests gave statistically significant remillimeters) taken’ ENAMEL THICKNESS AND THE HELICOIDAL PLANE T M' M2 Ma Fig. 2. Mean value and standard deviation of average enamel thickness of the protocone (ET 2 + ET 3112 and the paracone (ET 5 + ET 6)/2 for each tooth type separately. sults in first and second maxillary molars only. Common to all molars, protocones have statistically thicker enamel over the cusp tips than paracones (ET 2-ET 6). In contrast, proportions of enamel thickness under the grinding and the shearing surfaces of the occlusal basin (ET 3-ET 5) change considerably from anterior to posterior. First maxillary molars are endowed with statistically thicker enamel under the Phase I facet (variable ET 3) than under the Phase I1 facet (variable ET 5). In posterior molars the overall enamel coverage over the occlusal basin not only increases, but it exhibits a shift in proportions; the lingual slope of the paracone (variable ET 5) becomes much thicker than the comparable measurement on the protocone (variable ET 3; Table 2). In other words, the enamel on the buccal side of the occlusal basin exceeds that of the lingual side in posterior molars. Changes in dentine horn heights (Hl-H2) from anterior to posterior also show a reversal in proportions. While first upper molars exhibit higher protocone dentine horns, second and third molars have more prominent paracone dentine horns. Finally, discriminant analyses were car- 331 ried out to examine the relationships between enamel thickness measurements within and between teeth, and to aim for maximum separation between teeth. These analyses may elucidate biological trends although, owing to the small sample sizes, they cannot provide conclusive answers. Firstly, enamel thickness measurements that gave statistically significant results in the ANOVA were used (Tables 3,4;Fig. 3).A plot of the two functions shows that M's are separated along the first axis, whereas function I1 discriminates between second and third molars (Fig. 3). Inspection of the coefficients in each function reveals that discrimination of first maxillary molars is due to proportionally thinner enamel a t the occlusal basin with thicker enamel a t the tooth walls, when compared with posterior molars (Table 3; see also Table 1).Discrimination between second and third molars resulted from third molars exhibiting proportionally thicker enamel at the paracone and the buccal tooth wall. On the basis of these calculations 92% of all teeth were correctly assigned to their tooth category, whereby the correct prediction of M's was 100% (Table 4).It is noteworthy that there is only one clear outlier, that is, one second molar lay at the extreme end of M3s (Fig. 3). Exclusion of this specimen from the analysis resulted in the second function becoming significant at the 0.1% probability level and led to complete separation of second molars; only one third molar was misclassified. Unfortunately, owing to the young age of the outlying specimen it could not be conclusively determined whether the specimen had congenitally missing third molars. Be it as it may, both discriminant analyses agreed insofar as the variables important for discrimination between teeth remained the same, but discrimination became more complete when the extreme outlier was excluded. That this finding was not an artefact due to the distinctiveness of M's could be confirmed on a separate analysis between second and third maxillary molars only. In a Wilk's stepwise procedure on the same set of variables the results are virtually identical (Table 5 ) . Again, measurements on the buccal side contrasted with those on the lingual side and the resulting coefficients led to a G.A. MACHO AND M.E. BERNER 332 TABLE 2. Results ofpaired t-tests between corresponding measurements of the lingual and buccal side for each tooth category, separately Paired t-tests ET 2-ET 6 ET 3-ET 5 t-value P t-value P ET 1-ET 7 t-value P M' M' M3 0.00 0.00 0.35 4.82 6.57 0.98 0.00 0.00 0.00 6.49 7.07 7.49 TABLE 3. Standardized discriminant function coefficients, eigenualues, and percentage o f variance based on discriminant analysis between maxillary molars and using six measurements of enamel thickness Measurement Function I -. Function I1 0.11 -0.35 -0.02 -0.46 -0.61 0.58 4.79 92.61 1.20 0.40 0.07 -0.46 0.09 -1.13 0.38 7.39 ET 1 ET 2 ET 3 ET 5 ET 6 ET 7 Eigenvalue % variance TABLE 4. Correct assignment of maxillary molars based on SLX variables of enamel thickness recorded from sections of teeth M' MZ M3 N Predicted group membership Ml M* M3 16 12 9 16 0 0 0 11 2 0 1 7 86% correct classification of molars to their tooth category. DISCUSSION This study is the first to investigate whether the distribution of enamel thickness found within and among human maxillary molars can be reconciled with the helicoidal plane of occlusion and its associated wear pattern. Resolving this question is important for an understanding of the development of a helicoidal occlusal plane during an individual's lifetime, as well as for designing models to explain its evolutionary history. More specifically, the following questions were addressed in this study: Is the helicoidal occlusal plane entirely a function of differential wear between molars? Although the universal occurrence of the helicoidal occlusal plane in Homo sapiens 1.90 -2.25 -2.10 0.07 0.05 0.05 H 1-H 2 t-value 0.39 -3.47 -3.65 P 0.70 0.01 0.00 has not been disputed, there are detailed differences in molar wear patterns among various modern populations, notably between prehistoric and modern hunter-gatherers and agriculturalists (Smith, 1983, 1984, 1986; also for review of older literature). This observation led to the recognition that the abrasiveness of diet plays an important role in the development of the helicoidal plane, which is further influenced by enamel thickness and delayed eruption of posterior molars (Butler, 1972; Osborn, 1982; Smith, 1986). For example, thinenamelled molars are more rapidly worn than thick-enamelled teeth and would thus have developed a reversed curve of Monson by the time later erupting teeth come into occlusion. In modern humans the eruption is especially delayed, which could account for the ubiquitous occurrence of the helicodial occlusal plane. Indeed, the marked differences in overall enamel thickness between M's and M2slM3s found in this study (see also Macho and Berner, 1993) would lead to a rapid reversal of the curve of Monson in first molars and to a prolonged existence of a normal curve in posterior teeth. However, the teeth studied not only differ in their overall amount of enamel over the tooth crown, but also show a distinct pattern of enamel thickness distribution among teeth. This seems to indicate that in the course of hominid evolution enamel thickness has responded to the greater functional demands by providing greater wear resistance to lingual cusps of M's and to buccal cusps of posterior molars. While diet, overall enamel thickness between teeth, and time of eruption may contribute to establishing a pronounced helicoidal pattern of wear, it cannot be assumed that these factors could have provided sufficient selective pressure to account for the patterning of enamel thickness ENAMEL THICKNESS AND THE HELICOIDAL PLANE 333 +3 2 1 +I .5 2 F n *2 2 C 3 i 2 2 2 U 2 2 1 1 ’ 1 1 1 2 I1 2 3 # 3 0 n 1 1 1 II 3 3 3 1 1 1 -1.5 3 3 2 1 -3 -4 -2 0 +2 +4 Function I Fig. 3. Plot of discriminant functions I and I1 based on analysis of six measurements of enamel thickness. Tooth types are listed by their numbers and centroids are marked by asterisks. between maxillary molars observed in this study. Is the helicoidal occlusal plane a structural feature of the orofacial skeleton? It is commonly accepted that the expression of the helicoidal plane of occlusion need not necessarily involve a clear reversal of slope from anterior to posterior; changes in wear plane angles from first to third molars alone qualify for the dentition to be classified as helicoidal (e.g., Osborn, 1982; Smith, 1983, 1984, 1986). The present study revealed that the posterior molars analyzed are endowed to resist proportionally more wear on their buccal cusps than do anterior molars. Although enamel distribution at the cusp tips within maxillary molars follows a pattern found in other primates, i.e., lingual cusps displaying thicker enamel t h a n buccal G.A. MACHO AND M.E. BERNER 334 TABLE 5. Results of a Wilk's stepwise discriminant analysis between second and third muxcllary molars and using sir measurements of enamel thickness Variable entered ET 1 ET 2 ET 3 ET 5 ET 6 ET 7 Eigen value Significance Variable removed Wilk's lambda F-value 1 3 0.83 0.49 4.02 4.17 4 0.59 3.95 -0.68 2 0.65 4.90 -1.20 1.04 0.02 cusps (Macho and Thackeray, 1992; Molnar and Gantt, 1977; Molnar and Ward, 19771, enamel at the lingual slope of the paracone increases from anterior to posterior, whereas the comparable measurement on the protocone decreases. Thus, there is a clear change in wear resistance at the occlusal basin from lingual anteriorly to buccal posteriorly (Table 2). That this pattern of change is not paralleled by the enamel thickness at the cusp tips is not surprising given that lingual upper cusps intercuspate between the lingual and the buccal lower cusps regardless of differences in relative mandibular arch width between first and third molars (e.g., Richards and Brown, 1986; Smith, 1986). Therefore, while the lingual slope of the paracone assumes greater functional importance than the comparable area on the protocone in anteroposterior direction, the cusp tip of the paracone is nonetheless not directly involved in the chewing cycle. In light of these considerations the distribution of enamel thickness found in this study closely matches a pattern of greatest wear along the occlusal surface defined as helicoid. It is interesting that chimpanzees, which have also been reported to develop a helicoidal occlusal plane, albeit to a lesser extent (Smith, 1986), might show a comparable pattern of enamel thickness distribution within and among teeth. Out of three first maxillary molars of Pan sectioned by Martin (19831, the buccal slope of the protocone (his measurement 3'" had thicker enamel than the lingual slope of the paracone (his measurement "h) in two cases, and they were equal in one. On the other hand, all second upper molars had thicker enamel under the Phase I wear facet of the paracone than under the Phase I1 Standardized discriminant coefficients I 1.39 0.70 facet of the protocone, although the only third molar sectioned by Martin did not show any disparity between the two sides. The measurements provided by Martin (1983) for Pongo and Gorilla do not show such a pattern of enamel thickness distribution among molars. Hence, the data on enamel thicknesses available give credence to Smith's proposal that Homo and Pan show similarities in their masticatory apparatus and that differences in wear plane angles from first to third molars between humans and chimpanzees are of degree rather than kind. What is the evolutionary and functional importance of the helicoidal occlusal plane? Neumann (1922) was the first to draw at- tention to the inclination of tooth crowns for the development of the occlusal plane. Subsequently, this interpretation not only gained support from most anthropologists (Osborn, 1982; Richards and Brown, 1986; Smith 1983,1984,1986; Ward, 1981),but its general acceptance provided a sensible explanation for the more frequent occurrence of the helicoidal occlusal plane in Homo as compared to other hominids (White and Johanson, 1982). Since the Plio-Pleistocene, the foreshortening of the dental arcade in hominids resulted in molars coming to lie mostly posterior to the root of the zygomatic arch and medially to the masseterlpterygoid complex; and both factors appear to be important for the development of a helicoidal plane (Smith, 1986). Moreover, the reduction of the dental arches and their retraction under the skull necessitated axial inclination of the molar roots. Although axial tilt of molars has also been reported for early hom- ENAMEL THICKNESS AND THE HELICOIDAL PLANE inids (Ward et al., 1982) it is less pronounced than it is in humans (Dempster et al., 1963). The present findings indirectly provide evidence for the evolutionary history of axial inclination of molars in the coronal plane in Homo. The height of the protocone dentine horn decreases substantially in an anteroposterior direction whereas the height of the paracone dentine horn remains virtually the same from the first t o the third upper molar (Table 1). Bearing in mind that a) the alveolae into which the teeth are implanted are relatively straight, and that b) the occlusal planes form a functional masticatory unit, such disparities in cusp heights can only be counteracted by tilting the axes of posterior teeth in order for both cusps to reach the occlusal plane; even so, the occlusal plane of posterior molars is still somewhat oblique (Smith, 1986). Therefore, it seems probable that a trend towards pronounced axial inclination of the teeth in the course of evolution has been paralleled by differential changes in cusp heights in order for the masticatory complex to remain functional. However, posterior molars of hominids are not only tilted in a coronal plane, as discussed above, but also in a sagittal one. Human lower third molars, in particular, have undergone a forward tilt during evolution as a result of the displacement of the temporomandibular joint in relation to the occlusal plane (Osborn, 1987). These changes not only facilitated the generally more pronounced development of the curve of Spee in humans when compared to other hominoids, but it also rendered third molars functional despite their disadvantageous position (Osborn, 1987). The curve of Spee refers to the curved occlusion of upper and lower teeth in lateral view, with the concavity pointing upwards. Robinson (1956) appears to be the first to make mention of this curve when discussing taxonomic differences between Plio-Pleistocene hominids. Nonetheless, the importance of the curve of Spee for the development of the helicoidal plane is equivocal and has been explained in two different ways. Pleasure and Friedman (1938:1614) suggested that the ". . . reversal of pitch in the last molar is employed because it endows the dentures with balancing contact on 335 the non-working side." Subsequent researchers occasionally took up this idea to explain the helicoidal occlusal plane in the dentition (e.g., Hall, 1976), but recently Richards and Brown (1986) refuted this hypothesis and maintained that nonfood side contacts decrease as wear progresses; this results in wear to occur predominantly on the chewing side. Alternatively, Osborn (1982) proffered a mechanism that placed functional importance on the curve of Spee, suggesting that molars on the working side function in a smooth grinding movement because of this curve. Moreover, an important consequence of this complex interrelationship between the helicoidal occlusal plane and the curve of Spee is that molars function in series rather than simultaneously (Osborn, 19821, and third molars retain their functional importance (Osborn, 19871, while the crusWshear ratio of the force increases on posterior molars (Osborn, 1993). The thick enamel of posterior molars together with results yielded in studies in related fields lend further support t o such a complex model of mastication. Microwear as well as enamel thickness studies in Pan suggest that the amount of compression and shearing loads applied to food items differs between molars (Gordon, 1982, 1984; see also Macho and Berner, 1993). For example, the increase in pit frequencies on crushing1 grinding surfaces from 30% in first molars to over 50% in third molars implies that compression is highest in third molars, whereas shearing is lowest. Gordon (1982) also noticed that shearing facets are inclined more relative t o the occlusal plane and concluded that the flatter occlusal morphology in third molars in Pan relates to the qualitatively different wear processes occurring in posterior molars. It is relevant that changes of patterning of enamel thickness from M1 to M3 in this protohominid and those reported for Homo in the present study are comparable (see also Macho and Berner, 1993; Martin, 1983). The thicker enamel cover over the occlusal basin reported here together with the decrease in dentine horn heights evince that cusps of posterior molars are less pronounced than those of anterior teeth. Moreover, it should be borne in mind that enamel deposition re- G.A. MACHO AND M.E. BERNER 336 sults in progressive outward expansion of the cusps (Butler, 1956; Corruccini, 1987a,b; Kraus, 1952; Macho and Thackeray, 19931, which results in a proportionally greater occlusal basin for crushing and grinding purposes. In other words, owing to their thicker enamel, cusp tips of posterior molars are more displaced in relation to the underlying dentine horns than are cusp tips of M’s. Taken together, these differences in morphology as well as the changes in amount and patterning of enamel thickness from anterior to posterior suggest proportionally more crushing and grinding posteriorly, while first maxillary molars would be better suited to resist shear forces. It is therefore probable that human molars are “specialized in their function as are those of the chimpanzee (Gordon, 1982, 1984; Osborn, 1982). Since the hominid palate has shortened and retracted during evolution, this could only have been achieved by a complex restructuring of the orofacial skeleton involving axial tilt of the molars in two planes; this would lead t o a more pronounced curve of Spee and helicoidal occlusal plane. Ultimate clarification of this hypothesis will only come from studies that compare surface patterns of microwear with structural features of enamel. CONCLUSION This study is the first to demonstrate that the distribution of enamel thickness within and between teeth as well as the changes in dentine horn heights of the protocone and the paracone from M1to M3 accord with the helicoidal pattern of occlusion. While the findings of this investigation provide evidence for a long evolutionary history of this trait, they also lend support to the suggestion that some factors, such as axial inclination of teeth, could be particularly important for its development. Moreover, the helicoidal occlusal plane has usually been viewed as a by-product of evolutionary changes that have occurred in the orofacial skeleton. In light of the conclusions reached in this study, however, the possibility that the helicoidal plane could also represent a functional adaptation in itself must be considered. 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