Primitive Old World Monkey from the Earliest Miocene of Kenya and the Evolution of Cercopithecoid Bilophodonty

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Proceedings of the National Academy of Sciences of the United States of America





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Significance Almost nothing is known about the origin and evolution of Old World monkeys (cercopithecoids) because the first ∼12 million years of their fossil record is documented by only two isolated teeth. We describe a new primitive monkey from Kenya that dates from ∼22 million years ago and reveals a previously unknown stage of evolution. Comparisons between this monkey and other cercopithecoids offer detailed insights into the development of the novelties associated with the evolution of the cercopithecoid dentition, and particularly bilophodonty. Results suggest that this fossil monkey exhibited dental adaptations for frugivory and perhaps hard object feeding. Bilophodonty, the dental trait that unites all living cercopithecoids, evolved later, likely in response to the inclusion of leaves in the diet. Abstract Old World monkeys (Cercopithecoidea) are a highly successful primate radiation, with more than 130 living species and the broadest geographic range of any extant group except humans. Although cercopithecoids are highly variable in habitat use, social behavior, and diet, a signature dental feature unites all of its extant members: bilophodonty (bi: two, loph: crest, dont: tooth), or the presence of two cross-lophs on the molars. This feature offers an adaptable Bauplan that, with small changes to its individual components, permits its members to process vastly different kinds of food. Old World monkeys diverged from apes perhaps 30 million years ago (Ma) according to molecular estimates, and the molar lophs are sometimes incompletely developed in fossil species, suggesting a mosaic origin for this key adaptation. However, critical aspects of the group’s earliest evolution remain unknown because the cercopithecoid fossil record before ∼18 Ma consists of only two isolated teeth, one from Uganda and one from Tanzania. Here we describe a primitive Old World monkey from Nakwai, Kenya, dated at ∼22 Ma, that offers direct evidence for the initial key steps in the evolution of the cercopithecoid dentition. The simple dentition and absence of bilophodonty in the Nakwai monkey indicate that the initial radiation of Old World monkeys was first characterized by a reorganization of basic molar morphology, and a reliance on cusps rather than lophs suggests frugivorous diets and perhaps hard object feeding. Bilophodonty evolved later, likely in response to the inclusion of leaves in the diet.


Old World monkeys; Cercopithecoidea; Africa; Miocene; Bilophondonty





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