Abstract
The wear facets and the microwear formed on the teeth should be considered as phenomena that are related mechanically with jaw movement and phylogenetically with the other kinds of primates, not with the direct kinds of food.
The upper first molar in 108 individuals of Saimiri were observed by using scanning electron microscopy. The frequency of cavitation on the four cusps, the frequency of appearance of every 10 facets, the dominant kinds of microwear and the other feature of the facets were observed in relation to their function during mastication. The frequency of cusp cavitation was higher on the protocone and the hypocone than the other cusps. Thus cusp cavitation appears to be influenced not only by the puncture-crushing cycle but also by the chewing cycle. In addition, this extent of the puncture-crushing cycle is one of the characteristic features of Saimiri in Platyrrhine primates.
Wear facets 1, 2, 3, 4 and 6 are divided into two parts (ridge and lingual) according to the differences of the frequency of the appearance and the feature of the microwears. In facets 3 and 4, these two parts tend to merge together. The trend is more conspicuous in facet 3 than in facet 4. Facet 3 is prominently well formed among facets 1, 2, 3 and 4. Facet 5 and the ridge part of facet 6 have numerous pits. Facet 8 is well formed and has heavy pits and microflakes dominantly. The lingual part of facet 6, which forms the mesial wall of entoflexus, has numerous striations. Facets 9 and 10 appear in the specimens that have more than moderate tooth wear.
These features of the facets suggest the micro movement of the lower molar during chewing cycle as below : At the beginning of phase I the horizontal direction of movement of the lower molar is mesiolingual. However, at the end of phase I, the direction might transiently change distolingually. Next, the lower molar moves mesiolingually again for phase II . Thus, facet 8 and the lingual part of facet 6 appear to guide the detailed movement of the lower molar.
