6 - Old World monkey origins and diversification: an evolutionary study of diet and dentition

Summary

Introduction

The question of whether the earliest cercopithecoids were adapted for folivory or frugivory has implications for understanding the divergence of Old World monkeys and apes. Because the molars of all modern cercopithecoid monkeys are bilophodont, and most mammals with lophodont dentition eat leaves, the origin of Old World monkeys is commonly associated with a trend toward the inclusion of more leaves in their annual diets than in those of primitive apes and basal catarrhines (Jolly, 1970; Napier, 1970; Simons, 1970; Delson, 1975a,b, 1979; Andrews, 1981; Temerin and Cant, 1983; Andrews and Aiello, 1984).

The first suggestion that the earliest monkeys may not have been folivorous, but instead were highly frugivorous, came from an analysis of shear crest lengths (predominantly the lengths of cusp margins) on the lower second molars of the middle Miocene Victoriapithecus (Kay 1975, 1977a). Kay (1975, 1978, 1984) and Maier (1977a,b) proposed that cercopithecoid bilophodonty evolved as a consequence of selection for an efficient grinding mechanism: lophs act as guides for interlocking cusps and basins during occlusion; the size of the entoconid grinding facet is expanded; and the functional life of the crown is lengthened by increasing crown height.

Some proponents of the analogy-based scenario argued that two species existed within the Victoriapithecus sample, one interpreted to be a frugivorous cercopithecine, and the other a more folivorous colobine based on its supposedly longer shear crests (Delson, 1975a,b, 1979; Simons and Delson, 1978; Szalay and Delson, 1979). The more frugivorous species was depicted as eating leaves facultatively.