TAPHONOMY OF TERRESTRIAL GASTROPODS AT THE BIG PLATTE LOCALITY, SOUTHWESTERN WISCONSIN

This study demonstrates the differences in Amino Acid racemization within selected terrestrial gastropod genera at the Big Platte site in southwestern Wisconsin. Big Platte is a mass wasting exposure of a 1.2 meter tall by 3 meter wide lens of late Pleistocene colluvium containing numerous gastropod fossils. Gastropods present include several species from arctic or near-arctic habitats including Discus shimeki, Columella columella alticola, and Vertigo modsta modesta. Boreal species such as Discus whitneyi and Euconulus fulvus are also present. Radiocarbon dating and Amino Acid Racemization analysis of Succineid gastropod shells from this deposit indicate that most snails within this unit share a similar age history. However, the shell condition and Amino Acid racemization results from Helicodiscus parallelus and Glyphalinia indentata suggest that these species infiltrated the assemblage recently. Abundance measures and Amino Acid Racemization ratios suggest that the presence of these modern species is partially depth dependent. These results could a have significant impact on other studies of "non-analog" gastropod assemblages based solely on the presence or absence of particular species.