RAPID EARLY SILURIAN RECOVERY IN LAURENTIA FROM THE LATE ORDOVICIAN MASS EXTINCTION

We analyzed occurrences of articulate and inarticulate brachiopods, trilobites, bivalves, and anthozoans spanning the Late Ordovician and Early Silurian (Caradoc through Wenlock) in Laurentia in order to assess the effect of variation in sampling intensity on perceived diversity. Occurrences were binned into seven intervals of approximately equal duration (ca. 5 Myr). Environmental range and geographic distribution of occurrences is similar for all time intervals so that variation in diversity through time is not a result of varying environmental or geographic coverage. Analytical rarefaction and occurrence-weighted subsampling were used to account for any remaining variation in sample size among time intervals.

Sample-standardized diversity in Laurentia indicates that diversity rebounded to pre-extinction levels in the first five million years of the Silurian. This result stands in stark contrast to global studies, which suggest that diversity globally took nearly 20 Myr to rebound to pre-extinction levels. Taking the global studies at face value, our results suggest that, in Laurentia, diversity rebounded much more rapidly than global diversity, presumably by immigration of genera from other paleocontinents. Our results also suggest, however, the need for sample standardization at the global scale in order to understand the patterns of extinction and recovery associated with the Late Ordovician mass extinction.