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

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