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Abstract
One of the long-standing concerns of paleobiologists is in identifying major faunal turnovers in the fossil record, that is, when many taxa either disappear permanently or appear for the first time, and then disentangling the biological and environmental conditions that caused them. Increments of time that are especially dense in ‘range end-points’ are generally thought to arise as a response to exceptional climatic or environmental stress that intensified the loss of incumbent taxa and/or the arrival and success of new taxa. However, prior to evaluating such possibilities, one should be confident that the stratigraphic pattern of taxonomic first and last appearances does not simply arise from hiatuses, or gaps in sediment accumulation, which can concentrate range endpoints along a single horizon. This challenge is especially pronounced for vertebrate fossil records in terrestrial settings. Fossil remains are typically scarce in terrestrial sediments, and a single locality can represent fossils collected from a wide area and stratigraphic thickness, as opposed to the finer sampling of invertebrate and micropaleontological fossils in marine successions. Yet, unlike marine settings, terrestrial sedimentary records are also subject to long phases of sediment starvation or bypass and subaerial weathering in between relatively short phases of active sediment deposition, making the formal identification of hiatuses and their paleobiological implications especially challenging. Here, I develop a framework for estimating the relative magnitude of hiatuses and test for the extent to which major hiatuses coincide with the positioning and nature of mammalian faunal turnovers in the Eocene Washakie Formation of Wyoming. After remapping all known fossil localities across the basin, and adding some new ones of my own, I evaluated the rocks for physical evidence of phases of non-deposition. I find that, while the formation contains signatures of high-magnitude, seasonally-variable flooding regimes that result in rapid deposition, nearly all range endpoints of mammalian genera are still associated with major hiatuses. In addition, some of the faunal turnovers that previously appeared to have occurred suddenly actually proceeded in a more stepwise and gradual fashion. The significance of these findings extends beyond the Washakie record and creates a model for re-evaluating fossil records in other intermontane basins in the western United States. Lastly, my final chapter addresses the potential spatial incompleteness of fossil records by using point occurrence data to estimate the occupancy of extant mammal species in five 1-degree x 1-degree quadrants in the United States. Within these quadrants, I estimate the zones of potential sediment accumulation by identifying areas with both a low slope and a high area of contributing surface flow, which is requisite for sediment delivery to a low-slope sink. I find that, among the five test areas, 82-96% of species have occurrences that overlap with a sediment sink, indicating that the lenticularity of potential deposition in terrestrial systems is unlikely to be a first-order limit on fossil preservation. Thus, other taphonomic controls notwithstanding, a local fossil record of terrestrial mammals, even in inland settings, is likely to capture the presence of most species living in a region.