X-RAY DIFFRACTION ANALYSIS OF FOSSIL BONES FROM THE PALEOGENE WHITE RIVER GROUP OF SOUTH DAKOTA AND NEBRASKA: INFLUENCE OF APATITE CRYSTALLINITY ON SOFT TISSUE PRESERVATION

Controls on the preservation of cells and soft tissues in fossil bone are still unclear. We herein report the results of x-ray diffraction analyses (XRD) of fossils from the Paleogene White River Group of South Dakota and Nebraska. Our prior study of these fossils focused on demineralization of bone samples (oreodonts, brontotheres, and tortoises) to assess the quantity and quality of endogenous cell and soft tissue preservation. We have now shifted to assessments of diagenetic processes and possible correlations with the degree of soft tissue preservation.

Segregated sampling of cortical and cancellous bone mirrored sampling from earlier demineralization assays to assess internal variations in diagenetic recrystallization between cortical and cancellous bone, and to assess the relationship between apatite crystallinity and the degree of soft tissue preservation. Samples were scanned with a Bruker D8 Advance powder X-ray diffractometer from 5—70° 2θ using Cu kα radiation (40kv/40ma) and a rotating sample stage.

The Crystallinity Index (CI) of each sample was compared to our previous observations of soft tissues from these bones to delineate trends in preservation. Among the 12 samples analyzed, all cortical subsamples showed a lower CI than their cancellous counterparts (0.45-0.50 opposed to ≥0.54). These values suggest that cancellous bone exhibits greater diagenetic apatite recrystallization than denser cortical bone. We could find no apparent correlation between this pattern and the preservation of fibrous matrix or blood vessels, but osteocytes were recovered in greater abundance from samples with CI values greater than 0.50.

The cancellous subsamples from oreodont fossils exhibit anomalously high CI values compared to brontothere and tortoise subsamples. The oreodont fossils had a greater amount of sediment infill than the other fossils, but it is unknown if, or how, this might influence their apatite crystallinity. Strong calcite signatures were also recorded in all six cancellous subsamples, which suggests significantly greater permineralization than was observed in corresponding cortical bone.