Katharine Loughney Dissertation Defense
Congratulations to Katharine Loughney who defended her dissertation on January 4, 2018
Advisor: Catherine Badgley
The fossil record of the Great Basin of western North America formed during an interval of intense tectonic activity and climate change. Geographically restricted basins formed during regional extension in the early and middle Miocene. The Middle Miocene Climatic Optimum (MMCO) caused global climate to warm 2-4°C between 17 and 14 Ma. The Barstow Formation, Mojave Desert, California, was deposited in an extensional basin and encompasses the MMCO; the sequence has a robust geochronologic framework and a well-studied mammal-fossil record, making it an ideal sequence to study the influence of tectonics, climate, and depositional settings on fossil preservation through time. I characterized the facies, environments, and taphonomic histories of fossil localities in the Barstow Formation. I integrated data gathered from field work, museum visits, and laboratory analyses in order to understand the influence of changing tectonic and climatic regimes on the preservation of mammal fossils.
In Chapter II, I characterized the major facies of the formation to understand the depositional settings and stratigraphic distribution of fossil localities. Depositional environments in the basin changed over time from alluvial fans and playa lakes to floodplains and wetlands as subsidence decreased; with this change, depositional environments became more suitable for mammal habitation and more conducive to the preservation of vertebrate remains. In Chapter III, I used carbon and hydrogen (D) isotopes from plant-derived biomarkers and soil organic matter to reconstruct environmental and hydrologic changes, and I counted phytolith (plant silica) assemblages to reconstruct vegetation composition and habitats through time. Negative shifts in δ13C and δD indicate that precipitation increased during the MMCO, and riparian environments with abundant palms formed at this time. After the MMCO, enrichment in δ13C and δD values indicate decreased precipitation and changes in biomass; grass morphotypes dominate phytolith assemblages in the upper part of the formation as habitats became more open.
In Chapters IV and V, I reconstructed the taphonomic histories of fossil localities based on fossils in museum collections and the facies settings of field sites. Fossil assemblages accumulated through biological activity at long-term sites of mortality and through fluvial processes. Localities representing long-term sites of mortality formed in channel-margin settings and around waterholes, where mammals congregated and where their remains were buried over hundreds of years. Fluvial processes concentrated fossils in channel lags and crevasse-splay deposits and preserve fewer specimens than biological accumulations. Most fossil material is from medium-size mammals (50-200 kg), as the remains of these animals were able to withstand carnivore activity, weathering, and burial better than remains of smaller and larger mammals. Faunal diversity increases through the formation, and turnover is high in stratigraphic intervals with localities that had high preservation potential. In Chapter VI, I synthesize the previous chapters and evaluate the fossil record of the Great Basin in the context of its tectonic history during the Miocene. As tectonic activity declined through the Miocene, the number of mammal genera and sediment thickness of fossiliferous formations increased. The factors that influence the fossil record can be better understood with the integration of its depositional, environmental, and preservational contexts.