Faunal Change, Paleoecology, and Landscape History in the Middle Miocene Dove Spring Formation, California

Abstract

The evolution and distribution of mammals have been influenced throughout earth history by tectonic processes that produce regions with varied topography and environmental gradients. Biological responses to these factors are recorded in the fossil record, but the same processes that generate suitable life habitats affect the fossil productivity of depositional environments, confounding interpretations. The goal of this dissertation is to investigate fossil assemblages of terrestrial mammals and determine whether faunal change is influenced by tectonic history. This work has focused on the fossil and geohistorical record of a continental basin at the western edge of the Basin and Range, USA. The Miocene Dove Spring Formation (12.5–8.5 Ma) has a rich vertebrate fossil record and 1800 m of sediments that hold evidence for three major tectonic episodes. Two phases of extension and a period of shearing are documented through structural and sedimentological evidence. I compiled data from museum specimens, topographic maps, and field notes from the past 60 years. Using this foundation, I integrated field work, geographic information system analysis, and stable isotope analyses to investigate the interactions between tectonic processes and changes in the species richness and faunal composition of mammal assemblages. I updated radiometric decay constants and incorporated a recent tephrochronological correlation to revise the geochronology of the Dove Spring Formation and provide context for the spatial and stratigraphic distribution of fossil localities. I conducted a diversification analysis for large mammals to estimate residence times and account for the incomplete nature of fossil preservation. Originations were prevalent early in the sequence, during an extensional tectonic episode. Extinction became the dominant process later, coinciding with the onset of shear tectonic movement. Significant changes in faunal composition correlated with these tectonic episodes. With a connection between tectonics and faunal change established, I conducted additional investigations to determine whether apparent faunal change was driven by the productivity of different lithofacies or was the result of ecological change. I sampled tooth enamel from three ungulate families (Antilocapridae, Camelidae, and Equidae) for stable isotopes of carbon and oxygen to describe their dietary ecology. The δ13C values indicate that the plant resources consumed by these ungulates remained relatively stable through the formation despite changes in the tectonic setting. The δ18O record exhibits a similar trend to a regional signal, suggesting that conditions in the basin became gradually more arid over time, with little correlation with the tectonic history. The Dove Spring Formation represents an environment that persisted for four million years and has no close modern analogue. Changes in dietary ecology were not the primary drivers of faunal change. I documented lithofacies associations and inferred depositional environments at two scales. At the basin-scale, large channels are associated with episodes of extension, while floodplains developed during the shear episode. I detected variability across the landscape at the locality scale, recognizing two types of channels, three types of floodplains, pond deposits, and crevasse splays. Changes in depositional environments over time correlate well with the basin’s tectonic history, and the shear episode exhibited the highest fossil productivity. However, the area of these environments was not a strong predictor of fossil productivity. The presence of chronically rare taxa, such as carnivores during poorly-sampled intervals indicates that not all faunal change can be attributed to sampling. Investigations of regional patterns are necessary to further our understanding of this phenomenon.