SELECTIVITY OF THE LATE MISSISSIPPIAN MASS EXTINCTION FOR BRACHIOPOD GENERA FROM THE CENTRAL APPALACHIAN BASIN: RESOLVING AN APPARENT PARADOX

The seventh largest mass extinction of the Phanerozoic Era occurred in late Mississippian time, coincident with the onset of the extensive late Paleozoic ice age. Analyses of brachiopods collected from Mississippian strata of the Central Appalachian basin reveal that the late Mississippian mass extinction did not selectively victimize genera based on their species diversity, relative abundance, body size, or environmental breadth. The mass extinction did, however, strongly select against genera with narrow global latitudinal ranges. This pattern of selectivity closely resembles those reported for the five largest Phanerozoic mass extinctions; traits believed to influence survival during background intervals have played little or no role in determining survival during times of mass extinction. Yet, paradoxically, these same mass extinctions have tended to intensify background extinction rates, implying that mass extinctions could not have been wholly different selective regimes from background intervals. For late Mississippian brachiopod genera, global cooling and increased seasonality when the ice age began apparently amplified the relative importance of latitudinal range size (a reflection of thermal tolerance). During background intervals, thermal tolerance would have been important for survival because the ability to withstand minor climatic perturbations must play a role in the persistence of taxa; however, its relative importance would have been tempered by the existence of other traits that also contributed to extinction resistance. The late Mississippian mass extinction intensified background rates by amplifying the role of thermal tolerance, and it was nonselective for other traits that had been important during background intervals because these traits became relatively unimportant in conferring resistance during the new climate regime. The paradox can be resolved for the late Mississippian mass extinction because this extinction was caused by a single agent, global cooling, and not by a complex of agents, as during background intervals.