DIFFERENTIAL RECOVERY FROM CLUSTERED ORIGINATIONS IN THE RECOVERY FROM THE LATE ORDOVICIAN MASS EXTINCTION

The ecological or evolutionary impact of mass extinctions can vary and are decoupled from the taxonomic severity of the event. Because species are unevenly distributed within the tree of life, with many species-poor and few species-rich clades, the internal branches of the tree (i.e. evolutionary history) are in some cases preserved by very few species. Therefore, extinctions that remove related species can remove more of the internal branches on the tree then predicted by species richness alone. Whether the loss of evolutionary history controls the long-term impact of extinction events has not been directly tested because of the absence of phylogenetic trees that focus on the relationships of species and genera within large clades in the geologic past.

Here, we use a phylogenetic tree of strophomenide brachiopod genera to analyze whether extinctions and originations are clustered phylogenetically during the Late Ordovician mass extinction and Early Silurian recovery. The phylogeny, which currently consists of 61 species representing 31 families/subfamilies, suggests that most major strophomenide clades contain members that survive the Late Ordovician mass extinction. We tested this result using two metrics of the phylogenetic relatedness of taxa to go extinct or originate within intervals, mean pairwise distance and mean nearest taxon distance, and focused the analysis on Upper Ordovician and Early Silurian intervals. Extinctions in the Late Ordovician are randomly distributed across the tree, with few sister taxa going extinct during this time. However, originations in the recovery interval are strongly clustered within a single clade, and a large number of closely related families originate throughout the Early Silurian and continue to diversity through the Devonian. The clades that survive the mass extinction, alternatively, are predominantly species poor and fail to recover. Therefore, while the mass extinction itself resulted in little impact to the strophomenide tree, the differential recovery following the extinction resulted in a major evolutionary shift within the group. These results illustrate the importance of incorporating phylogenetic relationships into studies of mass extinction, and reveal the impact of the recovery interval on evolution downstream of major extinction events.