When homologous cusps display non-homologous wear facets: An occlusal reorganization ensures functional continuity during dental evolution of Murinae (Rodentia, Mammalia)

ObjectiveThis study was designed to interpret the differences between the occlusal relationships in the murine rodents and those in their Miocene “cricetodont” ancestors. It aimed at understanding the functional transformations that led to the emergence of the peculiar chewing motion of the Murinae, associating forwardly directed masticatory movements to cusp interlocking, a trait unique amongst mammals.MethodsMicrowear analyses and simulations of occlusion achieved with size-increased 3D printings of teeth crafted from 3D data obtained by X-ray synchrotron microtomography at the European synchrotron radiation facility allow us to carefully interpret the occlusal relationships in Muroidea.ResultsA rotation of the direction of the chewing movements occurred from “Cricetodontinae” to Murinae. This rotation emerged without any cusp removal contrary to previous interpretations, by the way of an occlusal reorganization involving a loss of contacts between some cusps, offset by a contact with other cusps. This new organization was already present in the early and middle Miocene genus Potwarmus.ConclusionMolar tooth evolution in Murinae was characterized by the preservation and the reshaping of the primitive muroid cusps, the acquisition of supplementary cusps, and changes in the contacts between the opposite cusps. During evolution, changes of cusp patterns in upper and lower molar teeth are coordinated in order to retain a functional occlusion. Because of this functional constraint, one cusp was supposed to more likely occlude with the same opposite cusps during evolution, and therefore homologous cusps would always carry homologous attrition facets. Evolution of Murinae proves that functional continuity can also be preserved through changes in occlusal relationships independently from cusp removal.