TRACKING BONE TAPHONOMY AT THE MICROSTRUCTURAL SCALE: A CRETACEOUS CASE STUDY

Vertebrate hard parts are subject to a host of physical, chemical, and biological processes that culminate in either destruction or fossilization. Macroscopic taphonomic effects yield data relevant to interpretation of the vertebrate fossil record. Recent work on microbially mediated bioerosion is beginning to provide insights into biotic controls on bone preservation and on microorganisms inhabiting ancient ecosystems. This study focused on marine vertebrate bones (n= 56) derived from a conglomerate at the base of the Turonian Carlile Shale exposed in eastern South Dakota. Two key questions were addressed: 1) do spongy trabecular bone and dense compact bone from the same depositional setting exhibit comparable levels of microstructural degradation? and 2) does the frequency of bioerosion increase with increased physical taphonomic modification in compact bone tissue (e.g., brecciation)?

We characterized several types of microstructural degradation in the sample, which includes 44 bones comprised of compact tissue, 13 bones comprised of trabecular tissue, and one bone with both tissue types. Several different tunnel types of biogenic origin were documented that ranged in size from less than 10 to 40 microns in diameter. The frequency and distribution of physical taphonomic signals including brecciation and discoloration were also recorded.

Trabecular bone and compact bone tissues both exhibit significant evidence of microstructural degradation, but trabecular bone contains more evidence of bioerosion. For example, 69% of trabecular bones contain small-scale tunnels of biogenic origin, while only 32% of compact bones document comparable modifications. In contrast, secondary fractures and discoloration are, on average, 20% more common in compact bone tissue than in trabecular bone samples. These data suggest that there is little relationship between bioerosion and abiotically mediated modifications. Application of the Oxford Histological Index (OHI), a measure of degree of microstructural degradation, indicates that compact bones with the highest frequency of bioerosion typically have lower values for abiotic degradation. Similarly, highly fractured bones exhibit fewer bioerosional tunnels. There is no clear correlation between abiotic and biotic modification in this sample.