Understanding the paleoecology and niche dynamics of mammals in the Mascall Fauna (Middle Miocene), Oregon

Understanding species-environment interactions through time is important for studying evolutionary processes such as taxonomic turnover. Studying this ecological interaction in the past sets a baseline for expected changes to occur given future environmental changes. Here I study species-environment interactions at the local and continental scale to understand how faunal composition, dietary niches, and climatic niches of species shift during periods of climatic and environmental change. A thorough revision of the Barstovian Mascall Fauna in Oregon provides an up-to-date faunal list and new ages for tuffs in order to examine faunal change at a local scale across a major climatic event, the mid-Miocene Climatic Optimum. Fifteen species are added and six synonymies are incorporated into the original faunal list. The Mascall Tuff, the most fossiliferous layer of the Mascall Formation, is dated at 15.3 Ma and the Kangaroo Tuff in the upper unit of the formation is dated at 13.6 Ma. Faunal comparisons to other Barstovian localities in North America reveal patterns of provinciality during this time. Faunal composition is consistent through the formation except for a turnover from browsing to grazing equids. To examine this turnover in more detail, the dietary niches of four equid genera are reconstructed using stable carbon isotopes. Dietary niche partitioning is evident based on 13C values and tooth morphology: Archaeohippus was a small brachyodont equid with a narrow dietary niche most likely consuming crown leaf vegetation in the clearings of a woodland habitat; Parahipppus and Desmatippus were medium sized browsing equids with a broad dietary niche eating C3 browse in an open environment; and aff. Acritohippus was a larger grazer also with a broad dietary niche eating C3 grasses in an open environment. The dietary niches of these four genera did not change through time across the mid-Miocene Climatic Optimum. Extinction of the browsing genera, especially Archaeohippus with its narrow niche breadth, is most likely to do opening of habitats as C3 grasses spread after the mid-Miocene Climatic Optimum. There is no evidence of C4 vegetation in the diet of aff. Acritohippus suggesting it was not a dominant part of the vegetation in the Pacific Northwest during the Barstovian, in contrast to other regions of North America during that time. Although the most common method of reconstructing niche space of extinct mammals is using stable isotopes to reconstruct their dietary niche, given other potential environmental proxy data, a more complete niche space of a species can be reconstructed in the fossil record. It is often difficult to obtain sufficient data to reconstruct entire environmental layers to study niche dynamics through time but proxy data can be obtained to examine the species niche in multivariate space. I test the potential for using this data to test for niche instability through time by studying the climatic niches of 15 mammalian species from the Last Glacial Maximum to the Recent. I find evidence supporting species niche shifts, primarily along the temperature axis, however the shifts are not greater than the shift in climate from the Last Glacial Maximum to the Recent. Therefore, species are shifting their realized niche space and are not evolving their fundamental niche space. When studying niche dynamics in multivariate space in the fossil record, tests for realized niche stability are possible, however tests for niche evolution require background environmental layers and information on the size and shape of the fundamental niche space.