DAZED AND CONFUSED: BIVALVE AND BRACHIOPOD OCCURRENCE TRENDS FOLLOWING THE END-PERMIAN MASS EXTINCTION

Studies suggest that survival taxa of mass extinctions go through greater attrition after extinction events than during the extinction event itself, generally during the onset of recovery. An important aspect of studying the recovery of a mass extinction is distinguishing the recovery fauna from the survival fauna. Studying the occurrence intervals of taxa aids in determining the onset of the recovery phase. This project focuses on comparing the dominant genera that occur in the Early and Middle Triassic, i.e. bivalves and brachiopods, to distinguish survival and recovery genera following the end-Permian mass extinction. For this study, we define survival genera as genera that originated prior to the mass extinction and during the Early Triassic but died out before the Middle Triassic. Recovery genera are defined as genera that disappeared during the mass extinction but reappeared during the Middle Triassic and persisted beyond (i.e., Lazarus Taxa) and genera that originated following the mass extinction and persisted beyond the Middle Triassic. Genera data were downloaded from the Paleobiology Database (PBDB) based on the following criteria: genera occurring in marine environments with carbonate, siliciclastic or mixed lithologies, environment zones ranging from marginal to shelf slope and basin, and only genera occurrences in North America. Once our genera list was compiled, we used the Strauss and Sadler (1989) method from the PBDB to plot genera range and calculate confidence intervals. This method calculates confidence intervals on the endpoints of local stratigraphic ranges using a Bayesian approach. The data set comprises 62 bivalve genera and 18 brachiopod genera. A total of 53% of the bivalve genera originated during the Triassic (31% during the Early Triassic and 22% during the Middle Triassic) while the remaining 47% of the genera originated during the Paleozoic. A total of 61% of the brachiopod genera originated during the Triassic (33% during the Early Triassic and 28% during the Middle Triassic) while the remaining 39% originated during the Paleozoic. Genera ranges reveal that 79% of bivalve and 61% of brachiopod genera are considered recovery genera. This study suggests that the majority of surviving bivalve and brachiopod genera faced relatively low attrition following the mass extinction.