RARE EARTH ELEMENTS AND RATES OF FOSSILIZATION IN TETRAPOD BONES

The process of fossilization is a complex and poorly understood suite of events transforming biogenic apatite into fossil apatite. Advances in molecular paleontology have allowed for investigations into the nature of bones, from archaeological to fossil material, including crystallinity, trace element composition, collagen content, and histology. The incorporation of trace elements, specifically rare earth elements (REEs), into bone during diagenesis and the resulting profiles across a transect from the outer cortex inwards provide an indication of relative rates of pore closure indicating the point of fossilization. This study aimed to address the process of fossilization, specifically the rate at which this occurs, and whether the rate is dependent on the host sediment. A total of 30 dinosaur bones collected from the Hell Creek, Judith River and Two Medicine Formations of Montana, in addition to one Pleistocene mammal bone from the North Sea, were examined using a wide range of analytical techniques, including Fourier transform infrared spectrometry (FT-IR) and laser ablation-inductively-coupled plasma mass spectrometry (LA-ICP-MS). The bones were collected from well-constrained depositional environments, focusing on mudstone and sandstone end-member sediments. Preliminary data indicate a difference in rate of fossilization, using the REE profile, between bones hosted in sandstone and mudstone. From the outer cortex towards the center of the sandstone-hosted bones, a steep decline in the REE abundance was observed, possibly indicating relatively rapid pore closure during recrystallization. Bones from mudstone units displayed a shallower, more gradual decline in REE abundance, indicating a slower rate of pore closure allowing ions to move farther into the bone. These results illuminate the potential store of information held within the bones themselves that may shed light on the processes in operation during early diagenesis and fossilization. The difference in rate of pore closure between different sediment types has far-reaching implications for future interpretations of vertebrate accumulations in terms of identifying reworked material, as well as addressing potential biases in the fossil record.