COMPARISON OF MARINE INVERTEBRATE BIODIVERSITY BEFORE AND AFTER THE ORDOVICIAN/SILURIAN EXTINCTION, CALUMET COUNTY PARK, STOCKBRIDGE, WISCONSIN

During the late Ordovician, extensive glaciation in the southern hemisphere resulted in a loss of marine shelf space. This had a dramatic effect on the biodiversity of continental shelves, including those in northeast Wisconsin. These environmental changes resulted in the late Ordovician Extinction. This project attempts to compare the marine invertebrate biodiversity in Upper Ordovician and Lower Silurian-aged rocks at Calumet County Park, near Stockbridge, in northeastern Wisconsin, where these rocks are well exposed near the shoreline of Lake Winnebago.

Calumet County Park contains part of the Niagara Escarpment, a major geomorphologic feature that runs through eastern Wisconsin, southern Ontario, Canada, and into northern New York. This park was ideal for this study as it contains rocks from both before and after the extinction.

Samples were collected from Upper Ordovician shales and dolomites and compared with carbonate samples from Lower Silurian rocks. Of particular interest were fossils found in dolomite samples from both systems due to their ease of visibility and extent of deposit. Close inspection of the Ordovician dolomite revealed dense deposits of marine invertebrates preserved through replacement fossilization. Organisms present include bryozoans, crinoids, gastropods, and trilobites. In the Silurian-aged dolomite, the biodiversity had changed and was much lower. Fossils present in Silurian dolomite at the site include mostly stromatolites and Thalassinoides. Also, the method of fossilization has changed. Although replacement fossilization is still present in stromatolites, dolomitization and subsequent dissolution has destroyed many suspected fossil cavities, leaving a very vuggy appearance. Close inspection of the site has yet to reveal any vugs with detailed enough impressions to identify specific organisms, save for burrows made by Thalassinoides.

Based on the fossils present, it can be suggested that the biodiversity of the Calumet County region was significantly impacted by the late Ordovician extinction. The rich biologic community present in the shallow sea was reduced and took time to recover. Further examination of the site needs to be conducted to more closely examine the Upper Ordovician and Lower Silurian shales for a more complete understanding of biologic change.