Results of this study are presented in Figures 4, 5, 6, 7, and 8 and Tables 1, 2, 3, 4, 5, and 6. These illustrate changes in some aspects of occlusal morphology with wear and identify other attributes that may not change in a consistent manner as a tooth wears. These results use teeth of different specimens to create a model of an occlusal wear sequence for the taxon. Although the scaling, alignment, and cropping procedures described above minimize differences between specimens, we must remember that individual variation can still complicate wear sequence patterns (see Discussion). Each attribute can be considered individually.

Slope is one measure of topographic relief of the occlusal surface. Although one might expect to observe a consistent decrease in mean slope as cusps become more worn, this pattern is obscured by dentin exposure, and resulting steeply walled pits (Figure 4; Table 1). There is a general decrease in slopes for all cusps between the first and the fifth wear stages. Further, four of five cusps show decreasing slope values between stages 1 and 2 and between stages 4 and 5. On the other hand, only three cusps show decreasing values between stages 2 and 3, and two show decreasing values between wear stages 3 and 4.

The rate of change of slope is an indicator of the angularity of a surface, with high numbers representing sharper edges. There is no pattern of increasing or decreasing change in slope between adjacent wear stages (Figure 5; Table 2). There is also no pattern evident between the least and the most worn specimens. Therefore, there is no consistent change in the angularity of the surface for each cusp. This may be related to enamel penetrance and steeply angled dentin island walls for each of the cusps, although more work needs to be done to confirm this relationship.

Surface area of the cusps is calculated using the two-dimensional x-y area and slope data. We would predict a general decrease in surface area as slope decreases and tooth surface is worn away. Results indicated that patterns for the buccal cusps (the protoconid, hypoconid and hypoconulid) and lingual cusps (the metaconid and entoconid) differ (Figure 6; Table 3). As expected, the surface areas of the buccal cusps show a general decrease through the wear sequence, with the lowest values seen at wear stage 5. On the other hand, there is no such pattern for the lingual cusps. This may reflect the fact that, in these primates, buccal cusps of lower molars wear much more rapidly than do lingual cusps. In fact, lingual shearing crests on gorilla lower molars can preserve much of their length well after the buccal cusp morphology has begun to wear away.

We developed a relief index to provide a more direct measure of occlusal relief than could be obtained by examining surface area alone. This is a ratio of the three-dimensional surface area to the two-dimensional x-y area. The greater the occlusal relief, the higher the relief index. As expected, all five cusps show a trend toward decreasing relief from stage 1 through stage 5 ( Figure 6; Table 4), although there is some variability. Also as expected, the decrease in relief is greatest on the buccal side of the crown.

The measurement of aspect offers intriguing possibilities for observing functional changes of cusps though the wear sequence. It may be possible to assess changes in occlusal relationships between upper and lower teeth by observing variation in modal aspect (Table 5). The colors in Figure 7 relate to modal aspect increment for each cusp, not to particular aspect direction. Perhaps the most obvious change in these data is on the buccal cusps between the fourth and fifth stages, where the prevailing aspect switches by 180 degrees! This may indicate a decreased effectiveness of the tooth as a guide for chewing motions ( Kay and Hiiemae 1974). Still, a combined study of maxillary and mandibular dentitions will be necessary to evaluate the potential of this approach.

The last variable to consider is central basin volume. We expected that as a tooth wears, the basin volume would increase gradually because the worn cusps should allow more fluid to accumulate before flowing off the molar. Instead, we saw an increase in volume between stage 1 and stage 2, followed a sharp decrease through stages three and four (Figure 8; Table 6). Due to the extreme wear seen in stage 5, the basin actually flowed right over the buccal cusps, preventing useful measurement.