Functional significance of ontogenetic gradients in the enamel ridge pattern of the upper cheek dentition of the miocene hipparionin horseCormohipparion occidentale (Equidae, Perissodactyla)

Abstract

The cheek tooth dentition of hipparionine horses functions as a shearing system, masticating food in a single phase occlusal power stroke. The edges of enamel ridges, which are exposed on the occlusal surface by means of food abrasion act as shearing blades. In order to discern the function of enamel ridges, an algorithm is introduced which reduces occlusal enamel ridge patterns to their functional residual in respect to the function as a shearing system. A further algorithm discriminates actively functioning (leading) enamel edges from passive functioning (trailing) edges. The residuals of functional enamel edges are investigated in respect to their alignment in relation to the chewing direction. A cheek tooth dentiion of the Miocene hypsodont equidCormohipparion occidentale is investigated in respect to wear related ontogenetic changes in functional enamel edge alignment using a set of 8 sections per tooth roughly representing hypothetical occlusal surfaces in different individual ages.

A high degree of optimisation is found in the cheek tooth dentition ofC. occidentale in respect to the function as a shearing system of food subdivision. A decrease in function is found in the ontogeny of P2–M2, which is compensated by an increase in function by the M3 erupting last. Since this optimisation did not involve more structural investment, it is considered a highly efficient means of restricting hypsodonty to the degree absolutely deemed necessary by selection, and saving investment in unnecessary structure. This may be one of the pre-adaptations of late Miocene hipparions, making the group a diverse and successful one in large parts of the old and new world subsequently.