MASS EXTINCTIONS AS LARGE-SCALE ANALOGUES OF SPECIES TURNOVER: A STUDY OF THE END ORDOVICIAN MASS EXTINCTION EVENT

Cyclic phenomena play a large role in motivating evolutionary patterns at many hierarchical levels. Modern studies of individual populations, such as the Grant’s work on Darwin’s Finches, have shown cyclic shifts in phenotype/genotype that correlate with climatic variations. At the level of species, similar patterns can be observed in the climatically mediated origination and extinction of species as described as part of Vrba’s Turnover Pulse Hypothesis. Notably, species-level patterns are analogous but not homologous to patterns at the population-level, because they cannot be explained through simple extrapolation of population-level variation to the species level. Mass extinction events, which may or may not be cyclical, rewrite the evolutionary status quo, often pruning successful clades while allowing less successful clades to thrive. Following on her Turnover Pulse hypothesis, Vrba developed the Relay Model that more thoroughly emphasized the clade context of species-lineages and makes predictions about how species turnover varies across different clades. This study focuses on the nature of species turnover during the end Ordovician mass extinction and considers the extent to which the patterns matched those predicted by Vrba’s Relay Model. The analysis is based on phylogenetic and biogeographic studies of three diverse Ordovician-Silurian trilobite groups: the Sphaerexochinae and Deiphoninae (both cheirurids); and the Homalonotidae. The Deiphoninae and Homalonotidae show patterns of origination and extinction predicted by the Relay Model while the Sphaerexochinae passed through the mass extinction event relatively unscathed (which is compatible with certain types of clades in the Relay Model). Again, hierarchical patterns at one level, the species, cannot be simply extrapolated up the hierarchy. This points out the importance of the hierarchical perspective for our understanding of macroevolutionary theory.