LINKING LATE ORDOVICIAN MARINE EXTINCTIONS TO CLIMATIC AND OCEANOGRAPHIC EVENTS: FROM PATTERN TO PROCESS

Climate change has frequently been invoked as the cause of the Late Ordovician Mass Extinction. However, there are many possible mechanisms by which Late Ordovician climatic and oceanographic events could have led to elevated marine extinction rates and much uncertainty remains regarding which mechanisms were most important. Resolving this question requires dissecting extinction patterns across time, space, environments, and taxa. Here we use a large and taxonomically standardized global database of Rhynchonelliform brachiopod occurrences to examine extinction selectivity patterns throughout the Late Ordovician-Early Silurian and to evaluate which potential extinction mechanisms are best supported by these data. Using a combination of machine learning approaches and multinomial logistic regression we demonstrate that the first pulse of the Late Ordovician Mass Extinction, in the latest Katian Stage, has a distinctive selective fingerprint. The two most significant predictors of brachiopod genus extinction risk during this interval are paleolatitudinal range and paleobathymetric distribution: genera with absolute paleolatitudinal ranges exceeding 25° and/or paleobathymetric ranges that include shallow-water environments exhibit sharply reduced extinction rates. Neither of these patterns are prominent during other intervals, suggesting that they are indicative of extinction mechanisms unique to the latest Katian interval. The importance of paleolatitudinal range suggests that interactions between changing seawater temperature and paleogeography played a prominent role in driving extinctions. To test this hypothesis, we constructed paleolatitude-temperature transfer functions based on published sea surface temperature (SST) estimates from global circulation models with Late Ordovician paleogeographic and atmospheric boundary conditions. Models that include estimated SST range as a risk predictor are a better fit to observed extinction patterns than models that include only paleolatitudinal range. Our results suggest that integrating geographically and bathymetrically resolved paleontological datasets with climate and circulation models has great potential to shed light on marine extinction processes throughout the Phanerozoic.