Correlated changes in occlusal pattern and diet in stem Murinae during the onset of the radiation of Old World rats and mice.

Adaptive radiations in mammals are sometimes associated with the emergence of key dental innovations facilitating food processing and masticatory movements. The dietary aspects of such innovations constitute an important focus in evolutionary biology. Murine rodents, which originated during middle Miocene, currently constitute the largest extant mammalian subfamily. Their adaptive radiation combines an original chewing motion with a peculiar occlusal pattern. The fossil record clearly establishes the timing of acquisitions of those innovations, and the aim of our study was to estimate the dietary changes associated with each of them. Fossil taxa phylogenetically closest to Murinae were investigated through the use of geographic information system applied on maps obtained from first upper molars digitized by X-ray synchrotron microtomography. This methodology enables estimation of quantitative topographic descriptors of the dietary specializations of the molar crown. The peculiar forwardly directed chewing motion acquired by stem Murinae may have been a key innovation allowing the invasion of an insectivorous dietary niche. The further radiation of crown Murinae coincides with the return to the plant-dominated omnivorous dietary niche of their distant ancestors through the acquisition of new morphological traits. The retention of the forwardly directed chewing motion by the crown Murinae could have become a competitive advantage in the context of a more generalist diet.