DECIPHERING THE ROLE(S) OF ECOLOGICAL PATCHINESS VS. TAPHONOMIC CONTROLS ON THE FORMATION OF CONCRETIONARY ASSEMBLAGES: A CASE STUDY OF FOSSILIFEROUS CONCRETIONS FROM THE KREMMLING PALEONTOLOGICAL RESOURCE AREA IN COLORADO

Fossiliferous concretions are among the most highly sampled and important sources of paleontological information for the communities that inhabited the Cretaceous Western Interior Seaway (WIS) as well as in other settings. In spite of this importance, relatively little work has been undertaken to understand whether concretionary assemblages reflect the original ecological patchiness that is observed in modern habitats or whether there were taphonomic controls acting to produce the ‘spotty’ record found in these types of fossil concentrations. For this study, we examined the lithological, preservational, ecological, and taxonomic similarity between 79 concretions and their respective faunas collected from five different concretion-rich intervals in the Baculites compressus/B.cuneatus biozone (~73.5 Ma) in a silty-sandy lithofacies of the Pierre Shale at the Kremmling Paleontological Resource Area in Colorado. This investigation provides a comprehensive documentation of the ecological and taxonomic structure of shallow nearshore marine communities in the WIS during the Late Cretaceous. The lithology of the concretions, variation in the preservational mode of fossil specimens, taxonomic composition of samples, and the relative abundance of the different genera are interpreted to reflect paleoecological and taphonomic differences on the sea floor. We found that the lithology of the concretions and the preservational mode of most molluscan specimens found within and among concretions show little variation, which suggests similar taphonomic controls on the formation of fossiliferous concretions analyzed in this study. Furthermore, this assessment shows that there are significant differences in taxonomic composition, richness, and abundances among distinct concretions within and between concretion-rich intervals. Despite these differences, the dominance of small aragonitic and delicate fossil taxa (e.g., Drepanochilus, Anomia, Ostrea) suggests that these types of ‘spotty’ concentrations more strongly reflect the original ecological patchiness that was found on the sea floor rather than the influence of taphonomic controls.