Article: Cladal Turnover: the end-Ordovician as a large-scale analogue of species turnover

Modern studies of individual populations have shown large cyclical shifts in phenotype/genotype that correlate with climatic variations. At the hierarchical level of the species, similar patterns can be observed in the climatically mediated turnover of species (i.e. Vrba's Relay Model). In turn, mass extinction events may have a similar analogous relationship with species turnover, while operating at the clade level. In this paper, such an analogous process is described and named ‘the Cladal Turnover Model’. The end-Ordovician mass extinction event is used as a test case to investigate the existence of such a phenomenon, with specific focus on the origin and extinction of the Hirnantiafauna. A test is outlined that can be applied to determine whether Cladal Turnover is occurring in the end-Ordovician. An example case study is then performed on the Hirnantia 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 Hirnantia taxa), the group does not go extinct at the end-Ordovician event, but instead gives rise to warm-water low-latitude descendants. This result is consistent with the Cladal Turnover Model. The method proposed for testing the Cladal Turnover Model could be applied to look at other times of major evolutionary turnover preserved in the fossil record.