A COMPARATIVE INVESTIGATION OF DIAGENESIS IN FOSSIL TEETH: A CASE STUDY FROM THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS OF MONTANA

Previous work has documented large-scale patterns in the abundance and quality of vertebrate fossils in the Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana. These two richly fossiliferous and coeval units preserve deposits of the semi-arid alluvial uplands (TMF) and the coastal plain lowlands (JRF). In this study we delve deeper into the taphonomy of both formations and characterize the diagenetic signatures of dinosaur and crocodile teeth from several localities (mostly vertebrate microsites) that represent a variety of paleoenvironments.

Authigenic cements filling cracks and dentine tubules were examined using polarized light microscopy and SEM-EDS. Fills include calcite, microcrystalline silica, barite, chlorite, pyrite, and iron oxides. Based on our preliminary results, authigenic cements serve to distinguish the two formations and also appear to track more localized depositional environments. For example, barite is only found in TMF teeth and usually fills pores in dentine. Authigenic chlorite is ubiquitous within one TMF site and is unique to this site. Pyrite is most common in the JRF, as are 10-20 µm spherical aggregates of iron oxides.

Rare earth element (REE) concentrations in bones and teeth were determined using LA-ICP-MS. REE are incorporated into the apatite structure shortly after death and the ratio of heavy to light REE can be used as an indicator of the degree of terrestrial weathering. Variability in REE concentration within bones from a single site can serve as a measure of mixing and reworking. In our sample, the TMF teeth have higher REE concentrations than JRF teeth and are more enriched in light REE. In addition, samples from individual TMF sites tend to show more variability in REE concentrations. These findings are consistent with previous studies that focused on REE patterns in TMF and JRF bone. With regard to individual teeth in our sample, enamel is always less REE enriched than corresponding dentine, but still mirrors the heavy to light REE ratio in the dentine.