SOFT TISSUE AND CELLULAR PRESERVATION IN LATE EOCENE-OLIGOCENE VERTEBRATE FOSSILS OF THE WHITE RIVER BADLANDS

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, depositional environment, diagenesis) remain poorly understood. 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) ~ 37-30 mya. We herein present the results of demineralization assays on vertebrate fossils from the WRG to explore geologic and paleoenvironmental controls on soft tissue preservation. The fossils used in this study were collected from the WRG of northwest Nebraska and southwest South Dakota, and include postcranial fragments of mammals (various taxa) and reptiles (tortoises) from both the Late Eocene (Chadron Formation) and Oligocene (Brule and Sharps Fms.). Fossils were excavated from a variety of host rock types, including siltstones, mudstones, and sandstones. Associated paleosols suggest neutral to alkaline, oxidizing paleoenvironments. Demineralization was conducted in 0.5 M EDTA at pH 8.0 for four to six 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 potentially endogenous microstructures were recovered, including numerous osteocytes and vessel fragments, with each fossil preserving microstructures regardless of geological age, depositional environment, taxon, or degree of bone crystallinity. Trends identified in trace element analyses obtained via LA-ICP-MS included higher rare earth element (REE) concentrations in cortical bone than in cancellous bone, and relatively flat to MREE depleted REE patterns with slightly positive cerium anomalies indicative of retention of early-diagenetic trace element signatures. Though our analyses identified modest variations in the chemical alterations experienced by the fossils in their respective diagenetic microenvironments, REE uptake does not appear to have influenced soft tissue and/or cellular preservation within these specimens.