SAMPLING STANDARDIZED REGIONAL DIVERSITY AND TURNOVER PATTERNS DURING THE LATE ORDOVICIAN MASS EXTINCTION

Understanding the biogeographic structure of mass extinctions and post-extinction recoveries can shed light on the evolutionary impact of mass extinction events. Raw diversity trends are correlated with variations in rock volume, however, and attempts to understand regional diversity dynamics must account for this bias. Here, we analyzed occurrences of articulate and inarticulate brachiopods, trilobites, bivalves, and anthozoans for three paleocontinents (Laurentia, Baltica, and Avalonia) spanning the Late Ordovician mass extinction and Early Silurian recovery. Data were compiled from a survey of the literature and supplemented by the Paleobiology Database (http://www.paleodb.org/). The structure of this database allows us to assess regional diversity trends while removing the biasing effects of variations in sample size.

Baltic and Avalonian diversity trends change little after sample standardization. Standardized diversity curves for both continents show large drops in diversity at the extinction boundary and protracted recoveries, with diversity beginning to increase 10-15 myr after the extinction. The Laurentian diversity curve, however, changes dramatically once the effects of sample size are removed. Raw diversity trends are similar to those of Baltica and Laurentia. Standardized diversity, however, remains flat or increases through the time interval considered, despite high extinction levels in the Late Ordovician. This implies a complete rebound in diversity to pre-extinction levels within 5 myr of the extinction, 10 myr before the recovery in Baltica and Avalonia. Several processes could drive a rapid rebound in Laurentia without replenishing Baltic and Avalonian diversity. These include immigration of genera into Laurentia in the Early Silurian, higher speciation rates in Laurentia due to its position in lower latitudes, or supressed origination rates in high latitude continents due to more severe and longer lasting environmental perturbations. Whatever the process, the resulting spatial complexity in the recovery process must have played an important role in the degree of ecosystem restructuring following the Late Ordovician mass extinction.