FUNDAMENTAL DIFFERENCES IN TECTONICALLY-DRIVEN DIVERSITY DYNAMICS AND FAUNAL CONNECTIVITY BETWEEN SMALL AND LARGE MAMMAL FAUNAS DURING THE NORTH AMERICAN MIOCENE

Studies of the role of geologic change in driving evolution have benefitted from the contrast between high levels of tectonic activity in the North American Intermontane West and the relative quiescence of the Great Plains through most of the Cenozoic . While we have a good sense of the changes in alpha and beta diversity in these regions through the Miocene, it remains difficult to distinguish between two possible mechanisms by which tectonics could drive diversity change: (1) in situ diversification driven by breaking up geographic ranges and (2) ecological diversification through the maintenance of greater landscape-scale habitat diversity. It is likely that these mechanisms operate differently between large and small mammals, so it is necessary to consider patterns partitioned by body size. The difference between these two mechanisms can be understood by tracking the faunal connections between active and passive regions during periods of tectonic activity. We compare the Miocene faunas of the Northwest (NW) and the Northern Great Plains (NGP) and find that rodent faunas are quite dissimilar between the NW and the NGP, sharing almost no species in common. This contrast is particularly striking given that large mammal faunas share a majority of species between these two regions. However, across land mammal ages, the North American rodent fauna is well-mixed. Many taxa arrived through immigration from other regions, as indicated by their phylogenetic relationships. When new species arise through in situ evolution, few of these species are partitioned geographically within the region, suggesting that the first proposed mechanism (in situ species range fragmentation) is not a major factor in tectonics-driven diversity maintenance. Hence, the dissimilarity between regional rodent faunas seems to reflect the differences in ecological structure between the regions, suggesting that tectonic activity acts on species richness by maintaining environmental diversity.