Lineage-Specific Responses of Tooth Shape in Murine Rodents (Murinae, Rodentia) to Late Miocene Dietary Change in the Siwaliks of Pakistan

<div><p>Past ecological responses of mammals to climate change are recognized in the fossil record by adaptive significance of morphological variations. To understand the role of dietary behavior on functional adaptations of dental morphology in rodent evolution, we examine evolutionary change of tooth shape in late Miocene Siwalik murine rodents, which experienced a dietary shift toward C₄ diets during late Miocene ecological change indicated by carbon isotopic evidence. Geometric morphometric analysis in the outline of upper first molars captures dichotomous lineages of Siwalik murines, in agreement with phylogenetic hypotheses of previous studies (two distinct clades: the <i>Karnimata</i> and <i>Progonomys</i> clades), and indicates lineage-specific functional responses to mechanical properties of their diets. Tooth shapes of the two clades are similar at their sympatric origin but deviate from each other with decreasing overlap through time. Shape change in the <i>Karnimata</i> clade is associated with greater efficiency of propalinal chewing for tough diets than in the <i>Progonomys</i> clade. Larger body mass in <i>Karnimata</i> may be related to exploitation of lower-quality food items, such as grasses, than in smaller-bodied <i>Progonomys</i>. The functional and ecophysiological aspects of <i>Karnimata</i> exploiting C₄ grasses are concordant with their isotopic dietary preference relative to <i>Progonomys</i>. Lineage-specific selection was differentially greater in <i>Karnimata</i>, and a faster rate of shape change toward derived <i>Karnimata</i> facilitated inclusion of C₄ grasses in the diet. Sympatric speciation in these clades is most plausibly explained by interspecific competition on resource utilization between the two, based on comparisons of our results with the carbon isotope data. Interspecific competition with <i>Karnimata</i> may have suppressed morphological innovation of the <i>Progonomys</i> clade. Pairwise analyses of morphological and carbon isotope data can uncover ecological causes of sympatric speciation and define functional adaptations of teeth to resources.</p>