THE EFFECTS OF TAPHONOMIC VARIABILITY ON THE PRESERVATION OF CELLS AND SOFT TISSUES IN FOSSIL BONES FROM BADLANDS NATIONAL PARK, SOUTH DAKOTA

Cells and flexible soft tissues, many containing traces of original biologic molecules, have frequently been discovered in fossil bones of dinosaurs and many other extinct vertebrates. These discoveries indicate that paleontologists should reexamine the long-held presumption that most organic matter always decays or is completely replaced by mineral precipitation during fossilization. However, this topic is often considered controversial because it remains unclear how such remarkable preservation takes place. We examined six vertebrate fossils from the Oligocene Sharps Formation in Badlands National Park in order to address the potential influence of lithology (volcaniclastic siltstone, indurated siltstone, sandstone), depositional environment (fluvial vs. eolian), and intensity of pre-fossilization weathering on their preservation of cellular and soft tissue microstructures. The tortoise shells are badly weathered, regardless of lithology, with trabecular material absent and infilled with dark, reddish siltstone. The postcranial bones of mammals are comparatively well preserved. Fossil shells of tortoises (n=4) and postcranial bones of mammals (n=2) were demineralized in 0.5 M EDTA pH 8.0 over 6-8 weeks. Subsets of each specimen’s demineralized residues were mounted on glass slides and examined under a petrographic microscope using transmitted light. Structures resembling vertebrate osteocytes, blood vessel fragments, and fibrous matrix were recovered from badly-weathered tortoise shells and well-preserved mammal bones, which suggests that the degree of weathering prior to fossilization, as well as lithology, may be minor influences on soft tissue preservation. Our next steps include measurements of apatite crystallinity and rare earth element uptake in order to assess the relationship between the degree of bone apatite alteration during fossilization and quality of soft tissue preservation.