QUANTIFYING CATASTROPHE: ESTIMATING SPECIATION AND EXTINCTION RATES IN TRILOBITES DURING THE END ORDOVICIAN MASS EXTINCTION EVENT

The end Ordovician extinction event is the second largest mass extinction in the history of life. The extinction has classically been interpreted as being caused by a brief, sudden onset glacial period that occurred during the otherwise greenhouse conditions of the Late Ordovician. Previous research suggests that the extinction event had a particularly interesting effect on the evolutionary history of trilobites, strongly selecting against trilobites with nektonic or planktonic life strategies. Free-swimming trilobites went entirely extinct at the event, and trilobite groups with presumed planktonic larval stages were more likely to go extinct than trilobites with presumed benthic larvae. While this pattern has been studied broadly at the family level, there have been no detailed phylogenetically constrained estimates of speciation and extinction rates through time for any of these late Ordovician trilobite clades. Results from such an analysis are presented here in order to better understand the evolutionary effects associated with the extinction event and recovery period on several trilobite clades. Specifically, this analysis focuses on extinction and speciation rates within three different clades (Deiphoninae, Sphaerexochinae, and “ceraurids”) of cheirurid trilobites, a speciose group that is presumed to have had both a benthic larval and adult life strategy and that spans this time interval. The results from the analysis of extinction rates are used to evaluate competing theories for the cause of the End Ordovician extinction event (ie: habitat loss from lowering sea levels, gamma ray burst, etc.) to determine which cause best fits the data. Further, we consider how speciation and extinction rates covary, and compare these results with those gleaned from earlier phylogenetic biogeographic studies, to consider how speciation mode and rates of speciation are themselves related both before, during, and after the crisis interval. We find that phylogenetic information on fossil taxa can significantly enhance our understanding of tempo and mode in evolution.