GEOGRAPHIC RANGES OF MARINE GENERA FOLLOWING THE LATE ORDOVICIAN MASS EXTINCTION: DEVELOPMENT OF A STABLE DISTRIBUTION

An enduring question in the study of global diversification concerns the extent to which major increases (radiations) and decreases (mass extinctions) in diversity are accompanied by large-scale, spatial transitions among taxa. Previously, I demonstrated that the Ordovician Radiation exhibited an increase in the mean ages of marine genera, and that these genera became more widespread as they aged. This produced a significant change in the geographic-range distribution (GRD) of genera, such that, by the Late Ordovician, a larger proportion of genera were widespread (i.e., they occurred on three or more paleocontinents) than had been the case previously. Given that diversity declined appreciably at the end of the Ordovician and rebounded during the Silurian, it is worth asking whether genera originating in the Silurian rebound exhibited the same propensities, with respect to age and geographic range, as their Ordovician counterparts. To this end, I analyzed data on the global occurrences of taxa available in the Paleobiology Database (PBDB; http://paleodb.org/) to determine whether the Silurian diversification was accompanied by a renewed geographic expansion of genera.

Using Sepkoski’s global compendium, it can be demonstrated that, in parallel to the Ordovician Radiation, genera originating during the Silurian diversification aged continuously throughout the remainder of the period. However, an analysis using the PBDB of the global geographic distributions of these genera suggests that this was accompanied by little, if any, change in the GRD established previously. Assessments of individual cohorts of genera originating in several consecutive Ordovician and Silurian intervals suggest that a notable proportion of genera continued to become more widespread as they aged. Thus, the addition of new genera to the pool of widespread genera was likely counterbalanced by the extinction of widespread genera throughout the Silurian.

Therefore, once it was set during the Ordovician Radiation, it appears that the GRD of marine genera remained in a stable state through at least the end of the Silurian, apparently unaffected by the Late Ordovician mass extinction. Whether this stability continued throughout the remainder of the Paleozoic Era remains to be determined.