Concentrations of rare earth (REE), U, Th, and other trace elements (TE) were measured using LA-ICP-MS along transects across five Late Eocene brontothere bones from the terrestrial Late Eocene Chadron Formation of Nebraska and four Miocene marine mammals from the Atlantic Coastal Plain. Samples were analyzed to determine REE diffusion periods, and to determine if histological factors affect post mortem uptake of REE/TE. In terrestrial fossil bones, concentrations of REE are highest at the bone surface and decrease with depth into the trabecular bone, consistent with diffusion-limited models. Histology may affect REE incorporation. An outer circumferential layer (OCL) is preserved along the outer 1 mm of the brontothere rib (F08-10) and femur (F08-09). REE concentrations in the OCL are much lower than in the underlying bone, indicating either lower incorporation or post fossilization leaching. REE concentrations are sometimes elevated in trabecular bone and Haversian systems, which may act as secondary diffusion pathways. REE concentration gradients are generally steeper in marine fossils than in terrestrial fossil bones, indicating longer periods of REE uptake in terrestrial fossils. Calculated periods of diffusion in terrestrial environments are 2.2 +/- 0.5 to 54.8 +/- 1.5 ka (based on a wetness factor of 0.5 +/- 0.1). Periods of diffusion for marine environments range from ca. 0.9 +/- 0.2 to 2.8 +/- 0.6 ka. However, within some terrestrial samples U is introduced into the bone over a much longer time span, possibly as a function of fluctuating redox conditions. If these values are representative, diffusion-fossilization periods are shorter in marine/lacustrine/spring/channel environments due to constant water saturation. Saturation of a bone during diffusion may also affect the morphology of REE signatures within the bone. In terrestrial bones, REE are strongly fractionated with depth, producing signatures varying from light-REE enriched at the surface to middle-REE depleted at depth. However, depth fractionation of REE is much less pronounced in marine bones, which may result from the introduction of fluid unreactive. These differences in REE fractionation are consistent with a greater influence of multiple secondary REE/TE diffusion paths in marine samples. Periods of diffusion for terrestrial samples differed within a single bonebed accumulation (2.2 +/- 0.5 to 54.8 +/- 1.5 ka). However, REE signatures are internally consistent with one another within the bonebed indicating that groundwater chemistry did vary during fossilization. If groundwater chemistry changes during diffusion, bone could be recording different signals, which has implications for using post-mortem REE/TE/Isotopes for paleoenvironmental reconstruction. Previous studies of soft tissue preservation in fossilized bone have inferred shorter periods of diffusion and suggested that the rate of diffusion must outpace the rate of decay. Diffusion periods in bone from well drained terrestrial settings are too long to preserve soft tissue. However, periods of diffusion in marine fossil bones are much shorter, suggesting the possibility for bio-molecule preservation. / Geology