CLADAL TURNOVER: THE END-ORODVICIAN MASS EXTINCTION EVENT AS A LARGE SCALE ANALOG OF VRBA'S RELAY MODEL

The end-Ordovician mass extinction occurred in two distinct pulses, the first pulse associated with the initial onset of severe glaciation in the beginning of the Hirnantian period and the second pulse associated with the receding of the glaciers near the end of the Hirnantian. In between these two pulses there was a brief period of recovery, in which putatively cold-water taxa referred to as the Hirnantia fauna diversified and spread equatorially. These Hirnantia taxa then preferentially went extinct at the second pulse of extinction. This pattern of pulsed speciation and extinction is analogous to the patterns predicted in Vrba’s Relay Model, in which a sudden environmental shift towards cold temperatures could cause habitat fractionation and extinction in warm-water taxa as well as range expansion and speciation in cold-water taxa. Therefore, a model is proposed for the extinction/speciation patterns of the end-Ordovician which is a large scale analog of Vrba’s Relay Model, named the Cladal Turnover Model. A test is outlined which can be used to determine if the Cladal Turnover is occurring in the end-Ordovician. A case study is then performed on specific Hirnantia taxa, the trilobite genus ‘Brongniartella’. The results of this test suggest that, while the taxon ‘Brongniartella’ is derived from putatively cold-water high latitude stock (consistent with the classical definition of the Hirnantia taxa), the group does not go extinct at the end-Ordovician event and instead gives rise to warm-water low latitude descendants. This result is consistent with the Cladal Turnover Model. The extinction patterns of non-Hirnantia trilobite taxa (specifically the Deiphoninae, Sphaerexochinae, and Ceraurids) are also compared to the Cladal Turnover Model to determine if the patterns of speciation and extinction and consistent with the model.