DECOUPLED DIVERSITY AND ECOLOGY DURING THE END-GUADALUPIAN EXTINCTION (LATE PERMIAN)

The Permian-Triassic mass extinction has long been recognized as the most severe biotic crisis of the Phanerozoic, eliminating close to 95% of marine species and fundamentally altering the ecological structure of marine ecosystems by decimating the brachiopod-rich Paleozoic Fauna. The nature of the Permo-Triassic “brachiopod-bivalve transition” has been well documented from global diversity compilations but the ecological switch, in terms of relative abundance, has only been comprehensively studied in Early Triassic fossil assemblages. The tacit assumption that the ecological shift paralleled the taxonomic switch ignores any potential effects of the earlier end-Guadalupian extinction (c. 260 Ma) or of environmental changes during the late Paleozoic. Our quantitative counts of silicified fossil assemblages from Early (Cisuralian), Middle (Guadalupian), and Late Permian (Lopingian) tropical carbonates in the western United States, south China, and Greece confirm that Early and Middle Permian assemblages were overwhelmingly dominated by rhynchonelliform brachiopods (mean abundance 99.1%). But, in sharp contrast, Late Permian samples from similar offshore carbonate environments contained a mixed fauna with molluscs accounting for 61.2% of the individuals.

Although this substantial ecological change occurred during the Guadalupian-Lopingian transition, the end-Guadalupian extinction itself had only minor taxonomic effects. Our new Middle-Late Permian diversity compilation indicates that only 34% of genera found in the Capitanian Stage became extinct, not significantly more than in the preceding Wordian or following Wuchiapingian stages. Extinction rates during the Capitanian were also broadly similar among brachiopods (33.8%), bivalves (32.7%), and gastropods (26%). The pronounced shift in relative abundance was therefore strongly decoupled from the minimal taxonomic effects of the end-Guadalupian event, implying that severe biotic crises are not required to trigger major ecological changes. The causes of this decoupling are unknown, but it may have resulted in part from environmental changes such as a significant increase in marine nutrient levels during the Capitanian or the marked sea level lowstand at the Guadalupian-Lopingian boundary.