MAMMAL EVOLUTION ON AN ACTIVE LANDSCAPE: INSIGHTS FROM OREGON'S CENOZOIC RECORD OF TECTONICS AND EVOLUTIONARY CHANGE (Invited Presentation)

Oregon’s landscape has been tectonically active for tens of millions of years and preserves a terrestrial sedimentary record that spans from the mid-Eocene to the late Pleistocene. Throughout this record, we see extensive volcanism and uplift. In the early to middle Miocene, Great Basin extension added yet more landscape change to the southern part of the state. On this rapidly changing landscape, fossil vertebrates are well preserved over the last 40 million years and have been the subject of study for a series of vertebrate paleontologists over the last 160 years, demonstrating a key role for landscape change in the evolution of mammalian diversity. A critical insight gained from Shotwell’s study of the ecology of Cenozoic terrestrial faunas of Oregon was the clear ecological and taxonomic differences between Oregon’s fauna and the better-known faunas of the Great Plains.

Early work in this rich fossil record set the stage for current ongoing research on how heterogeneity over the landscape has shaped evolution of the mammalian fauna. The density and abundance of the record reveals differences across the landscape that reflect differences to local tectonic histories and habitat changes. However, the less extensive history of fossil collection in comparison with the Great Plains demands additional fieldwork to be able to understand the history of faunal heterogeneity in the Northwest. Recently-discovered vertebrate fossil sites have helped fill gaps in the temporal and taxonomic range of the Northwest record, and ongoing fieldwork continues to produce new finds that show an increasingly diverse landscape. The dense record of volcanics makes possible high-resolution dating that allows fine-scale stratigraphic correlations among localities. The well-studied tectonic history of the Northwest also allows understanding how the landscape responded to tectonic change making it possible to tie fundamental drivers of change to the changing fauna. For example, new collections from the Mascall Formation have shown how the middle Miocene fauna of central Oregon changed even over the 2-3 million years following the initial eruption of the Columbia River Basalts. Ongoing work is building Oregon into a natural laboratory for studying the relationship between the evolution of the earth’s surface and its fauna.