INFLUENCE OF BIOMECHANICAL FUNCTION AND BONE HISTOLOGY ON THE PROCESSES OF FOSSILIZATION IN LATE EOCENE BRONTOTHERES

Three associated brontothere (Perissodactyl) bones with differing biomechanical functions (metapodial, distal femur, and rib) were collected from an overbank mudstone in the Peanut Peak Member of the Late Eocene Chadron Formation (White River Group) of northwest Nebraska. The samples were analyzed for rare earth (REE) and trace elements (TE) by inductively-couple-plasma spectrometry (ICP-MS) and Laser Ablation-ICP-MS (LA-ICP-MS) to determine if biomechanical or histological factors affect post mortem uptake of REE and TE. Concentrations of REE were highest at the bone surface and decreased with depth into the trabecular bone, consistent with diffusion-limited models. However, Sr and Ba concentrations were fairly constant and U increased with depth in the metapodial and the femur. Therefore, U cannot be used to calculate rates of fossilization for these bones. The fossilization period calculated from REE profiles in the rib is on the order of 10⁴ yrs. REE signatures from the outer circumferential layer (OCL) and the outermost cortical layers were light-REE enriched whereas signatures from Haversian and trabecular bone were middle-REE depleted to heavy-REE enriched. REE signatures in trabecular bone were more LREE-enriched than surrounding cortical bone. The biomechanical function of bone does not appear to affect REE or TE uptake. However, bone histology may affect incorporation of REE and TE. In the rib, REE concentrations increased toward the center of an osteon and signatures became more LREE-enriched. The osteon and Haversian system may have enhanced fluid flow by acting as groundwater conduits during fossilization. However, in other osteons, REE concentrations were greater than surrounding bone but decreased toward the center of the osteons. REE signatures have also been used to determine fossil provenance and combat fossil poaching. Signatures from solution and LA-ICP-MS analyses are very similar, slight differences may result from different cortical depths sampled. Although solution ICP-MS is an adequate method to combat fossil poaching, LA-ICP-MS provides more precise data, and is a more reliable method.