The structure of the plant community has profound effects on rates of tooth wear in herbivorous mammals. Tooth abrasion results from three factors. The first, and in many cases the most important, is tooth-to-tooth wear. Functionally, tooth-to-tooth wear seems to produce and maintain sharp edges. The other two factors involve the abrasiveness of the diet. As a natural by-product of certain metabolic processes, plants precipitate opaline silica within their tissues as phytoliths (plant stones). During the life of a plant, phytoliths become progressively larger and more abundant in certain leaves and stems, but phytoliths are not as important components in plant reproductive parts or in the early growth of foliage (Piperno, 1988). If an animal eats mainly new growth, the abrasive effects of phytoliths on dentition can be largely avoided, but phytoliths can be significant contributors to tooth wear in species that concentrate on grazing (Piperno, 1988). More tooth wear results from the consumption of plant parts that are covered with dust (Janis, 1988), which naturally results when there are exposed land surfaces and is especially common in arid regions. In fact, the increase in seasonal aridity during the late Cenozoic may have been a major contributor to increased hypsodonty in Tertiary mammals. The abundance of dust on the surfaces of plants also depends on how close the plant parts are to the ground surface. Grasses not only incorporate large quantities of biogenic opal but also grow close to the surface, and hence grazing is an especially abrasive activity.