DOES SAMPLING ARTIFACT PLAY AN INFLUENTIAL ROLE IN THE FRASNIAN-FAMENNIAN MASS EXTINCTION EVENT ON THE NORTH AMERICAN MID-CONTINENT?

One recent debate regarding mass extinctions is whether such rapid declines in global diversity result primarily from factors causing actual extinction, or are drastically exaggerated by sampling artifact. Eustatic sea-level change can bias observed diversity by decreasing the marine rock volume available for sampling and by removing environmental conditions necessary for preservation of certain taxa. This study examines the degree to which such sampling biases are influencing the apparent decrease in global diversity at the Frasnian-Famennian (Upper Devonian) mass extinction event on the mid-continent. The Iowa Basin was chosen for study because the Devonian-Mississippian units are well studied, highly fossiliferous, and exhibits great variation in marine depositional environments. Preliminary examination focused on the influence of surface area, maximum thickness, rock volume, and percent carbonate facies of six Devonian-Mississippian lithostratigraphic units on brachiopod species diversity estimates within the same units. Surface area of the lithostratigraphic units is derived from digitized pixels from a geologic map produced by the Iowa Geological Survey. Maximum thickness of these units is taken from Iowa Geological Survey estimates and rock volume is determined from both above-mentioned sources. Percent carbonate facies and brachiopod species diversity is estimated from the literature, noting that previous workers maintained a constant sampling method. The affects these four variables have on brachiopod species diversity through the Frasnian-Famennian boundary are significant. Strong positive correlations exist between species diversity and estimated surface area (r=0.81, p<0.05), maximum thickness (r=0.74, p<0.10), rock volume (r=0.86, p<0.05), and percent carbonate (r=0.82, p<0.05) across the different lithostratigraphic units. While preliminary, these strong correlations across the Frasnian-Famennian boundary likely reflect sampling biases and warrant a deeper look. These results will be further tested through field examination of the Spiriferoidea clade of brachiopods.