SEM-EDS ANALYSES OF SOFT TISSUES PRESERVED IN FOSSIL BONES FROM THE PALEOGENE WHITE RIVER GROUP OF NEBRASKA AND SOUTH DAKOTA

Factors controlling the preservation of endogenous cells and soft tissues in fossil bone remain poorly constrained. Building on previous studies which investigated the influences of biomechanics, bone tissue density, and diagenetic recrystallization, this study sought to address possible geochemical controls on the preservation of endogenous cells and soft tissues via scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). Samples for this analysis were taken from the same six bones from the White River Group as examined in our previous work, namely representatives of three common taxa – a brontothere, oreodont, and tortoise. New subsamples of cortical and trabecular bone were taken from each fossil and demineralized in 0.5M EDTA pH 8.0. After several weeks, the resulting demineralization products were pipetted onto silicon wafers and allowed to dry for SEM-EDS analysis.

Despite a low yield of material from the tortoise samples, microstructures morphologically consistent with blood vessels and fibrous matrix were identified and examined via EDS. The oreodont samples yielded abundant, well-preserved vessel casts, as well as fragments of fibrous matrix, whereas the brontothere samples yielded primarily fibrous matrix fragments and a single hollow vessel fragment. Osteocytes were not observed in our SEM analyses. EDS analyses revealed vessel casts to be composed primarily of silicon and oxygen, likely indicative of permineralization by quartz and/or zeolites. Fibrous matrix fragments exhibited considerable variation in the presence and quantity of other major elements, including sodium, aluminum, iron, calcium, chlorine, and carbon – the last of which may indicate partial organic composition. The recovered vascular microstructures also exhibited variations in both surface texture and preservation style (e.g., as solid casts vs hollow, potentially organic features). These contrasts imply possible differences in mineral incorporation and diagenetic preservation pathways between soft tissue types and among vertebrate fossils of the White River Group, but further analyses are required to confirm these preliminary trends. For example, most elemental signals were found to vary more between fossil specimens than between soft tissue microstructure types.