PALEOECOLOGY OF THE PECATONICA AND MCGREGOR MEMBERS OF THE PLATTEVILLE FORMATION, EASTERN DANE COUNTY, WISCONSIN

Upper Ordovician marine carbonate facies are well-known through the Upper Midwest and Great Lakes region, and typically dominated by classic Paleozoic taxa like brachiopods and bryozoans. Mollusk-dominated paleocommunities are less frequently documented. In this research project, we examined the paleoecological and stratigraphic characteristics of the Platteville Formation in eastern Dane County, Wisconsin, focusing on the Pecatonica and McGregor Members exposed at a privately owned quarry. Through systematic field sampling and multivariate paleoecological analyses, distinct trends in faunal distribution and diversity were identified. Gastropods, monoplacophorans, and nautiloid cephalopods are the dominant taxa, with variations in assemblages reflecting environmental gradients and post-depositional alterations. The findings suggest a deepening upward sequence or shifts in turbidity and nutrient levels, as evidenced by changes in bedding composition, faunal assemblages, and the presence of pyrite. These results contribute to a more refined understanding of Late Ordovician mollusk-dominated biofacies and environmental transitions in eastern Dane County, Wisconsin.