SPATIAL VARIATION AND DIVERSITY IN CHONDRICHTHYAN ASSEMBLAGES WITHIN THE BURLINGTON-KEOKUK BONE BED (MISSISSIPPIAN, VISÉAN)

Although the Burlington Limestone is best known for its diverse assemblage of Lower Carboniferous (Mississippian) crinoids, it boasts a similarly diverse assemblage of Chondrichthyan remains. Chondrichthyan teeth, denticles, and spines are found throughout the Burlington Limestone, but the contact between the Burlington Limestone and the Keokuk Limestone has an especially high concentration of vertebrate remains. This layer of rock is therefore generally referred to as the Burlington-Keokuk Bone Bed. While it has been known since at least 1870, and its geographic range potentially spans more than 200 square kilometers, spatial variations in marine substrate, habitat, and the chondrichthyan community within the bone bed have not been well studied. To identify and understand these variations, we sampled the Burlington-Keokuk Bone Bed at two localities (Nelson Quarry in Mediapolis, Iowa and Biggsville Quarry in Biggsville, Illinois) and compared the alpha and beta diversities between sites as well as the bone bed lithology. Our results suggest that there is variation in the vertebrate assemblages between sites and that some taxa such as Orodus sp. and Thrinacodus sp. occupied larger geographic ranges within the Illinois Basin, whereas other taxa such as Venustodus sp., Cochliodus sp., and Chomatodus sp. may have preferred habitats with specific water depths.