THE LATE ORDOVICIAN MASS EXTINCTION AND EARLY SILURIAN RECOVERY: COMPARISON BETWEEN LAURENTIAN AND GLOBAL DIVERSITY

Understanding diversity dynamics of mass extinctions and post-extinction recoveries requires regional studies of extinction selectivity, survival, and recovery. In a preliminary analysis of the Late Ordovician mass extinction, we examined Late Ordovician and Early Silurian diversity in Laurentia and compared it to global diversity. Our data consist of macrofaunal community lists compiled from the literature spanning the Late Ordovician and Early Silurian (Caradoc through Wenlock). Genera from 5 classes (articulate and inarticulate brachiopods, trilobites, anthozoa, and bivalves) were considered in this analysis.

Overall, raw diversity patterns in Laurentia are remarkably similar to the global patterns. Diversity is high in the Caradoc and reaches a peak in the Ashgill before dropping precipitously in the Early Llandovery (Rhuddanian). Diversity then slowly rises to reach pre-extinction levels in the Wenlock. However, the proportional drop in diversity in Laurentia is lower than the global pattern. In Laurentia, although 60% of genera to go extinct, total diversity is only reduced by about 25%. Globally, a 70% generic extinction results in a 50% reduction in diversity, far larger than in Laurentia. Rarefaction analysis of genus occurrences per time interval in Laurentia produced a different diversity curve than the raw data, suggesting that sampling biases are obscuring the true diversity trends. The rarefied diversity curve is flat from the Late Ordovician into the Early Silurian, drops slightly in the Middle Llandovery (Aeronian), and then rebounds to earlier levels. This analysis suggests that the rebound in diversity from the Late Ordovician mass extinction may have occurred much more rapidly than suggested by the raw data, at least in Laurentia. Diversity in Laurentia may have rebounded as early as the Rhuddanian, rather than later in the Wenlock. The dip in diversity in the Aeronian is believed to be due to undersampling for that time period, despite the use of rarefaction to eliminate such biases.