PALEOENVIRONMENTAL SIGNIFICANCE OF LATE PLEISTOCENE TERRESTRIAL GASTROPODS FROM ALLUVIUM AND COLLUVIUM IN THE DRIFTLESS AREA OF WISCONSIN AND MINNESOTA

This report describes seven newly discovered late Pleistocene fossil localities with abundant gastropod shells from southwestern Wisconsin and southeastern Minnesota. Fossils were recovered from exposures of colluvial material along hillslopes and from alluvial sediments beneath late Pleistocene high terrace surfaces. A total of 234 kg of sediment was collected and more than 18,000 individuals were identified. At least 12 species of terrestrial gastropods are represented in these fossil assemblages, which are dominated by northern, alpine, and arctic taxa. Shells of Columella columella alticola, Discus shimeki, Vertigo modesta modesta, and Vertigo alpestris oughtoni were common fossils, but these species no longer inhabit the Upper Mississippi River Valley. In contrast, shells of deciduous forest taxa such as Discus whitneyi, Euconulus fulvus, and Hendersonia occulta occurred as rare fossils, but can be found in this region today. Widespread geomorphic evidence such as ice-wedge casts and patterned ground indicate permafrost was widespread throughout southwestern Wisconsin between 24,000 and 14,000 BP. Previous studies at Elkader and Conklin Quarry in Iowa described gastropod faunas dominated by cold, open ground species as well as those that feed exclusively upon deciduous leaves. The results of this study support the interpretation that tundra vegetation existed throughout the Driftless Area during the last glacial period, but also suggest that isolated woodland vegetation was present as far north as Trempealeau County in west-central Wisconsin. The Driftless Area was likely an important refuge for temperate species during glacial conditions, much as it is for cold-adapted species today.