Evolutionary patterns of tooth shape in relation to diet inferred by stable carbon isotope of tooth enamel in murine rodents from the Miocene of northern Pakistan.

摘要

I evaluate morphological evolution of tooth shape relative to isotopic dietary inference in two sympatric clades of murine rodents (here called Karnimata and Progonomys clades) from the Miocene (13.8 to 6.5 Ma) of northern Pakistan. Murine samples from the region record the origin of the group to its diversification into the clades, beginning before and continuing through a transitional interval from C3- to C4-dominated vegetation. Carbon isotope values in enamel of first lower molars were obtained by laser-ablation GC-IRMS to infer paleodiet. Tooth shape of upper first molars was defined by morphometric distance of ecomorphological characters and 2D geometric morphometric analysis of tooth outline.;Carbon isotope data demonstrate that murine rodents experienced a remarkable C3-C4 dietary shift with the Karnimata clade consuming a greater percentage of C4 grasses than the Progonomys clade at any given time. Pairwise progressive reduction in overlap of principal component fields through time quantifies the similarity of basal members of each clade and demonstrates divergence of derived members. Change of tooth outline in the Karnimata clade is more strongly associated with reduction in spacing between anteroposteriorly positioned cusps and transverse arrangement of cusps. These features are related to increasing chewing efficiency in a shift to a more propalinal direction of mastication in murines. The results indicate that while both clades adapted to varying contributions of C4 grasses to their diets, selection pressure forcing dental adaptations was differentially greater in the Karnimata clade. Moreover, the morphological analysis and associated isotope data of these two clades of murine rodents present a fine-scale pattern of mammalian evolution that fits well with theoretical models of sympatric speciation and interspecific competition for the same food source.