The middle Miocene from 17 to 14 Ma was a time of elevated mammalian diversity in western North America that coincided with the regional development of topographic complexity and the last global warming interval of the Neogene. Understanding the evolutionary and ecological processes that govern past diversity trends and contribute to modern diversity gradients in relation to landscape and climate requires the integration of faunal and paleoenvironmental datasets across spatio-temporal scales. Using a variety of approaches, I analyzed small-mammal and environmental data to investigate diversity and dietary-ecology responses to changes in climate across space today and through time during the middle Miocene. I additionally utilized fossil-record simulations to assess the influence of variable preservation history on estimates of diversification rates in relation to landscape change. This thesis sheds light on how interactions between tectonic activity and climate warming influenced species richness and ecology from local to regional scales.