LATE PLEISTOCENE GASTROPODS FROM ALLUVIUM IN THE DRIFTLESS AREA OF MINNESOTA AND WISCONSIN

A recent survey of late Pleistocene alluvium beneath the Savanna Terrace in southeastern Minnesota and southwestern Wisconsin resulted in the discovery of 4 sites that contain an abundance of gastropod fossils. A total of 75 liters of sediment was collected and over 1,800 individuals were identified. At least 12 species are represented in these faunas, which are dominated by gastropods that inhabit cold, open ground habitats. Several well-represented species were found that no longer inhabit the Midwest including Discus shimeki and Columella columella alticola. These two alpine species are primarily distributed throughout the Rocky Mountains today. The fossils of D. shimeki collected in this study represent the first record of this species in both Wisconsin and Minnesota. Fossils of Vertigo modesta, Pupilla muscorum and Vallonia gracilicosta were also common at these sites. These fossils were recovered from alluvium beneath the Savanna Terrace, which was deposited between 20,000 and 13,000 BP. Geomorphic evidence such as patterned ground and ice-wedge casts indicate that permafrost was widespread throughout the northern Driftless Area during the full-glacial (21,000 to 15,000 BP). This suggests that the region was dominated by tundra during this time and the results of this survey support that interpretation. Other sites such as Elkader and Conklin Quarry in Iowa are dominated by species preferring open ground, but include some species that feed exclusively on deciduous leaves. This suggests that isolated woodlands existed in the southern part of the Driftless Area. No woodland species such as Discus macclintocki were discovered at any of the sites sampled in this survey.