STABLE BRACHIOPOD DIVERSITY PATTERNS ACROSS THE MISSISSIPPIAN-PENNSYLVANIAN GLOBAL BOUNDARY STRATOTYPE SECTION AND POINT (GSSP): FURTHER EVIDENCE AGAINST A LATE MISSISSIPPIAN MASS EXTINCTION EVENT IN NORTH AMERICA

The Late Mississippian (Serpukhovian) mass extinction is the seventh largest of the Phanerozoic with an estimated 26-28% generic loss of marine invertebrates and fifth largest in terms of ecological impact. Until recently, this extinction event received little attention, with diversity loss estimates principally derived from global databases and very few field studies having been conducted. Recent field-based studies however, have begun to delineate a dichotomy between North American and global diversity patterns during the Late Mississippian. These recent studies from the midcontinent of North America reveal evidence against an extinction event during the Late Mississippian, with observed taxonomic turnover principally attributed to extirpation and not true extinction. This study provides further insight into the Late Mississippian extinction event by documenting brachiopod diversity patterns across the mid-Carboniferous global boundary stratotype section and point (GSSP) at Arrow Canyon in southern Nevada. A combination of simple and multivariate quantitative methods were used to document diversity patterns between the upper Mississippian Indian Springs Formation and the uppermost Mississippian to lowermost Pennsylvanian section of the Bird Spring Formation. Results show an overall increase in generic diversity throughout the uppermost Mississippian and across the boundary into the lowermost Pennsylvanian where diversity peaks. Additionally, 39% of Mississippian genera sampled are not present in the Pennsylvanian samples. However, global stratigraphic ranges reveal that all Mississippian taxa sampled persisted into the Pennsylvanian, with some taxa even occurring higher in the unstudied section at Arrow Canyon. Multivariate analyses reveal similar taxonomic composition between Mississippian and Pennsylvanian samples, further suggesting a lack an extinction event. The results of this study support previous work that note a dichotomy between large-scale global events and regional/local-scale data, particularly for the Late Mississippian. Although the cause of discordance between North American regional/local and global diversity patterns is not clear, it further attests to the complexity of global extinction events.