Recovery of soft tissues, such as original cells, blood vessels, and proteinaceous fibers, from fossil bone is becoming more frequent, but the factors that control such exceptional preservation are not well understood. This study assesses the influence(s) exerted on soft tissue preservation by several possible controlling factors. Specifically, this study assesses biomechanical function, apatite recrystallization, bone tissue density, taxonomic identity, and depositional environment as possible controls on the quantity and quality of preservation of endogenous microstructures. Six bones derived from three taxa from the Paleogene White River Group of South Dakota and Nebraska – namely an oreodont from the Oligocene Brule Formation and a brontothere and tortoise from the Eocene Chadron Formation – were subsampled for cortical and trabecular bone, which were then assessed via demineralization, thin sectioning, transmitted light and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). Bones belonging to each clade were acquired from bonebeds to minimize intraspecific variation in taphonomic history, and (where possible) similar skeletal elements were selected from each skeleton to similarly minimize differences in biomechanical function between corresponding samples. Initial demineralization analyses showed high yields of potentially endogenous cells and soft tissues in all three taxa: only two subsamples lacked structures morphologically consistent with osteocytes, and microstructures visually consistent with vertebrate blood vessels and fibrous matrix were recovered from all 12 subsamples pf the six fossils. Variation in the dominant tissue type corresponded with taxonomic identity and bone type and was independent of biomechanical function, as defined in this study, but this assertion requires a larger dataset to be conclusive. Apatite crystallinity loosely correlated with osteocyte preservation, but the association was less robust than the taxonomic identity. Transmitted light microscopy of histological thin sections revealed varying levels of histological alteration among the bones. SEM-EDS analyses of demineralized microstructures identified apparent zeolite mineralization and zeolite crystals within the majority of blood vessel fragments, although some examples of hollow vessels were found which were identified to be composed primarily of silicon, oxygen, and carbon. Elemental mapping of thin sections via SEM-EDS revealed evidence of double-medium diffusion through Haversian canals and trabecular voids, as well as a dominance of enlarged, recrystallized bioapatite crystals. Observed variations in thin section and SEM also suggest different taphonomic histories for the three clades, particularly the oreodont samples, as certain features observed in thin sections were not observed in SEM. The demineralization data indicate a potential correlation between taxonomic identity and soft tissue preservation, but geochemical and thin section analyses suggest geochemical processes controlling mineralization may have a greater influence on the abundance of microstructures recovered through demineralization assays. / Geology

Understanding local and regional reactions to the global Eocene-Oligocene climate transition is a continuing challenge. The White River Group in the North American midcontinent preserves dynamic fluvial, volcaniclastic and lacustrine facies that yield to aeolianites. To test whether this shift in sedimentation style was driven by climate change, 20 paleosols from 8 profiles were analyzed from the fluvial-aeolian Orella Member through the aeolian-dominated Whitney Member of the earliest Oligocene Brule Formation at Toadstool Geologic Park, NE. Paleosol morphology and geochemistry were used to assess the balance of aeolian vs. alluvial sedimentation at key stratigraphic intervals and lithologic transitions. Significant loess deposition began at least as early as the lower Orella Member but is masked in most settings by concomitant fluvial deposition. As fluvial influence on landscapes waned across the Orella-Whitney Member boundary, loess deposits predominated and became more recognizable. Paleosols follow different pedogenic pathways in direct response to depositional setting. Whereas all paleosols formed through top-down pedogenesis in alluvial settings, paleosols in aeolian-dominated settings formed though pedogenic upbuilding during aggradational phases and through top-down pedogenesis during depositional hiatuses. The disparity between each style of pedogenic development creates fundamentally different pedogenic associations that must first be understood before climatic interpretations can be drawn from macroscopic paleosol morphology alone. Microscopic analysis of loessic and alluvial paleosols indicates that pedogenic features do not greatly change across the Orella-Whitney Member boundary. Furthermore, results of climofunction calculations from five paleosol Bw and Btk horizons show mean annual temperature (ca. 9.0-10.5 °C) and precipitation (ca. 650-800 mm/y) do not significantly vary across the Orella-Whitney Member transition. Clay mineralogy and the presence of pedogenic carbonate and translocated clay corroborate paleoclimate estimates. However, geochemical paleosol profiles are uniform and do not reflect observed vertical associations of pedogenic features. Constant additions of aeolian sediment, which replenishes base losses through leaching, explain this phenomenon. Interpretations of paleovegetation from root trace morphology and paleosol taxonomy indicate that predominantly open canopy to savanna habitats were in place in the lower Orella Member and continued into the Whitney Member. Evidence for riparian partitioning exists in the lower Orella Member but disappears as fluvial deposits wane in the Whitney Member. Lacking evidence of climate change from paleosol analysis, changes in sedimentation style and vegetative biomes are most likely a reaction to increased aeolian deposition. / Geology