ABSTRACT The Pierre Shale Formation is a well-studied fossiliferous marine shale unit that spans a large portion of western North America. In South Dakota, the Pierre Shale Formation is divided by the Missouri River into two geographic regions: the Black Hills Region to the west and the Central/Eastern Region to the east. Both regions have extensive outcrops, including exposures of the Sharon Springs Member. The Sharon Springs Member preserves an abundance of invertebrate and vertebrate fossils, including mosasaurs. Given that fossil-rich outcrops of the Sharon Springs occur in both geographic regions, this study aimed to analyze trends in preservation of mosasaur fossils to determine if there are significant differences in the degree of weathering and geochemistry of fossils. The degree of subaerial and submarine weathering, as indicated by gypsum encrustation and abrasion, was characterized based on physical evidence of alteration. X-ray fluorescence was used to determine the geochemical composition of fossil specimens, including phosphorus (P), sulfur (S), strontium (Sr), and iron (Fe). The mosasaur fossils displayed statistically significant differences in the degrees of weathering across the two regions (p = 0.005) and fossils recovered from the Black Hills Region were more highly weathered. Fossils examined from the Central/Eastern region contained significantly higher Fe concentrations (mean: 62578 ppm) compared to the Black Hills Region (mean: 26438 ppm). The average P, S, and Sr content of fossils preserved in the two regions were not significantly different. Fossils from both regions display high variability in chemical composition, likely reflecting the dynamic nature of early diagenesis and taphonomy. These results suggest that Fe in vertebrate fossils may be a useful indicator of diagenetic conditions in the Western Interior Seaway.
Rich et al. (Tue,) studied this question.