CONTROLS ON SOFT TISSUE PRESERVATION IN EOCENE-OLIGOCENE VERTEBRATE FOSSILS OF THE WHITE RIVER GROUP

Multiple studies have identified soft tissue and cellular structures in fossil bone, such as osteocytes, blood vessels, and fibrous/proteinaceous matrix, which closely resemble those of extant vertebrates. However, controls on soft tissue preservation in vertebrate fossils (e.g. paleoclimate, paleoenvironment, depositional environment, diagenesis) remain poorly understood. The strata of the Eocene-Oligocene White River Group (WRG) of the northern Great Plains provide a perfect setting for evaluating these potential influences across the Eocene-Oligocene Transition (EOT), during which global temperatures cooled and terrestrial landscapes became drier, with the extensive forests and fluvial environments of the Eocene giving way to eolian-dominated savannas during the Oligocene (~ 37-30 mya). We herein present the results of initial demineralization assays on vertebrate fossils from the WRG to explore the geologic and paleoenvironmental controls on soft tissue preservation. The four vertebrate fossils used in this preliminary study were collected from the WRG of northwest Nebraska and southwest South Dakota, and include postcranial fragments of a brontothere (late Eocene, channel sandstone), oreodont (late Eocene, floodplain mudstone), and two tortoise shells (late Eocene floodplain mudstone, and Oligocene eolian siltstone). Demineralization was conducted in 0.5 M Ethylenediaminetetraacetic acid (EDTA) at pH 8.0 for four weeks, with exchanges of fresh EDTA performed every 48 hrs. Resulting demineralization products were loaded onto standard glass slides, cover-slipped, and imaged by optical microscopy. Many potential endogenous microstructures were identified, including numerous osteocytes and vessel fragments. As we expand our dataset with additional fossils from the Middle Oligocene of the South Dakota Badlands, we will be able to compare the results from older fossils to that of the newly collected fossils. Additionally, we will be able to address the influence of depositional environments and paleopedology on the rate and degree of fossilization as recorded by REE uptake and apatite crystallinity.