Diversity Dynamics and Ecology of Small Mammals in Relation to the Neogene Tectonic and Climate History of Western North America.

Abstract

The middle Miocene from 17 to 14 Ma was a time of elevated mammalian diversity in western North America that coincided with regional tectonic extension, the development of topographic complexity, and the last major warming interval of the Neogene, the Miocene Climatic Optimum (MCO). Multiple processes govern species richness from local to regional scales, including community assembly, speciation, extinction, and immigration. This research centers on understanding these processes in relation to landscape and climate change and characterizing the properties of the middle Miocene diversity peak across spatial scales. I focus on rodents, which represent much of the extant and Neogene diversity in western North America. Chapter 1 presents the conceptual framework and major questions of this dissertation. In Chapter 2, I used stable isotopes in two modern heteromyid species to develop diet-isotope models across environmental and climate gradients in western North America. Rodent isotopes reliably track vegetation and climate variation and can be used for paleoecological inference. In Chapter 3, I generated a multi-proxy record from the Crowder and Cajon Valley formations in the Mojave to compare local paleoenvironmental change with faunal diversity during the MCO. These formations document the earliest evidence of C4 grass in the region, increased aridity during the MCO, and heterogeneous vegetation and moisture conditions. High species diversity within each basin and low faunal similarity across basins indicate that peak mammal diversity during the middle Miocene involved high local diversity and spatial turnover. In Chapter 4, using dental metrics and isotopic composition of fossil-rodent teeth, I evaluated corresponding changes in species dietary ecology. Within each basin, rodent dietary ecology was remarkably stable; however, over longer timescales, rodents consumed more C4-grass resources, implying a shift in their diets during the MCO. Rodents recorded finer-scale spatial and temporal variation in vegetation than did other proxies, thus providing unique paleoenvironmental information. In Chapter 5, I assessed the influence of variable preservation history through the Neogene on estimates of diversification rates in relation to tectonic activity. Simulating fossil records under several preservation scenarios, I determined that preservation alone is unlikely to produce the middle Miocene peak in mammalian diversity.