IGPP Seminar Series

When Molluscs Took Over the World: Ecological Change, Environmental Stress, and the End-Permian Mass Extinction

by Dr. Matthew E. Clapham
Department of Geological Sciences, Queen's University, Kingston, ON, Canada

Abstract

The severe biotic crises during the Permian-Triassic interval had a profound impact on the composition and structure of marine ecosystems, by forcibly disassembling Paleozoic community structure and giving rise to ecological dominance by the molluscan Modern Evolutionary Fauna. Previous compilations of global biodiversity suggested that the transition to the Modern fauna was abrupt and occurred during the catastrophic end-Permian mass extinction, implying that the severity and selectivity of the biotic crisis was largely responsible for the taxonomic shift. However, the ecological shift, in terms of relative abundance within marine fossil assemblages, has not been well constrained. Quantitative counts of relative abundance from silicified Early and Middle Permian fossil assemblages from North America and Thailand confirm that members of the Paleozoic fauna, such as brachiopods, were strongly numerically dominant, consistent with inferences from global diversity compilations. However, there was a dramatic ecological shift recorded in Late Permian assemblages from south China and Greece, which were instead dominated by molluscs of the Modern fauna, including abundant burrowing bivalves that were typically rare in the older assemblages. Although a large extinction event during this Guadalupian-Lopingian interval of ecological change has been recognized in previous studies, the presence of elevated extinction rates during that interval is not supported by a new analysis that incorporates standardization techniques to correct for unequal sampling of the fossil record. They were instead coincident with the onset of anoxic conditions in deep marine basins. This relationship suggests that variable environmental stress in proximity to anoxic deep waters during the Late Permian may have favored ecological dominance by the Modern fauna. The abrupt end-Permian mass extinction, which resulted from upwelling of those anoxic waters into shallow shelf environments, may therefore represent the climax of a protracted ecological crisis beginning around the Middle-Late Permian boundary.
Tuesday, 08 May 2007
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM