Permian marine paleoecology and its implications for large-scale decoupling of brachiopod and bivalve abundance and diversity during the Lopingian (Late Permian)

The transition during the Permian–Triassic interval from brachiopod-rich benthic marine assemblages to communities with diverse molluscs has been well constrained by measures of global taxonomic richness but its ecological context has not been studied using quantitative relative abundance data. New relative abundance data from Early, Middle, and Late Permian silicified fossil assemblages from offshore shelf carbonate environments indicate that molluscs increased significantly in relative abundance, from 0.8% in the Middle Permian (Guadalupian) to 65.4% in the Late Permian (Lopingian). These numerical changes are accompanied by increases in the abundance of infaunal bivalves and motile gastropods in the Late Permian. Although this ecological change was coincident with the end-Guadalupian extinction, a new global diversity compilation indicates that the overall genus-level severity of that biotic crisis was substantially lower than previous estimates. The pronounced ecological shift was instead concurrent with the onset of deep-marine anoxic or euxinic conditions around the Guadalupian–Lopingian boundary, suggesting that increased environmental variability in proximity to the deep water mass may have given eurytopic molluscs a competitive advantage over more stenotopic brachiopods. However, the lack of global changes in relative genus richness implies dramatic decoupling of global taxonomic and local ecological processes. The pronounced increase in molluscan abundance and alpha diversity in the silicified assemblages contrasts with their unchanged global taxonomic richness and may also have resulted from increased environmental stress in offshore shelf environments. These results demonstrate that the ecological transition from the brachiopods to molluscs was more complex than inferred from global measures of taxonomic richness and imply that initial stages of Permo–Triassic environmental deterioration may have been responsible for the decoupling of local ecological from global taxonomic processes during the dramatic Late Permian rise of the Modern Fauna.