A COMPARATIVE STUDY OF REE SIGNATURES AND AUTHIGENIC CEMENTS IN DINOSAUR TEETH AND GAR SCALES FROM THE UPPER CRETACEOUS TWO MEDICINE AND JUDITH RIVER FORMATIONS OF MONTANA

The Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana preserve coeval deposits of the semi-arid alluvial uplands (TMF) and the coastal plain lowlands (JRF). Previous work has documented the abundance and quality of vertebrate fossils in both formations. In this study we explore the diagenesis of dinosaur teeth, crocodile teeth, and gar scales using REE content and authigenic cements. The fossils under investigation were collected from several well-documented sites of fluvial and lacustrine origin.

Authigenic cements filling cracks and dentine tubules were examined in 23 theropod teeth, 19 hadrosaur teeth, 6 crocodile teeth, 4 ceratopsian, ankylosaur and myledaphus teeth, and 8 gar scales using polarized light microscopy and SEM-EDS. Fills include calcite, sulfates (Ba,Sr), pyrite, bornite, authigenic and detrital clays, iron oxides, and clastic particles. Taxonomic differences in tooth architecture (e.g., enamel thickness, susceptibility to cracking, etc.) had no apparent effect on cementation processes. Overall patterns of authigenic cementation serve to describe the general process of permineralization, and also characterize pore water chemistry in the burial environment.

Rare earth element (REE) concentrations were determined for the same sample of teeth and scales using LA-ICP-MS. Preliminary results show that TMF teeth have higher REE concentrations than JRF teeth and are more enriched in light REE. TMF teeth also tend to show more variability in REE concentrations. These REE patterns suggest that TMF teeth likely (1) spent the early stages of diagenesis in or upon alluvial soils (which tend to be depleted in HREE), and (2) experienced greater small-scale variability in the factors controlling REE uptake. These findings are consistent with independent sedimentologic and taphonomic data, and are also in line with previous studies of other workers that focused on REE patterns in TMF and JRF bone.

Finally, the overall REE patterns in enamel and dentine are similar within a given element, but normalized total REE concentrations vary in these tissues from relatively low (enamel) to relatively high (dentine). This pattern is consistent with porosity and crystallite density contrasts in enamel and dentine, and illustrates the importance of these factors in diagenetic processes.