C ladal T urnover: the end‐ O rdovician 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. V rba's R elay M odel). 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 C ladal T urnover M odel’. The end‐ O rdovician 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 H irnantia fauna. A test is outlined that can be applied to determine whether C ladal T urnover is occurring in the end‐ O rdovician. An example case study is then performed on the H irnantia trilobite genus ‘ B rongniartella ’. The results of this test suggest that while the taxon ‘ B rongniartella’ is derived from putatively cold‐water high‐latitude stock (consistent with the classical definition of H irnantia taxa), the group does not go extinct at the end‐ O rdovician event, but instead gives rise to warm‐water low‐latitude descendants. This result is consistent with the C ladal T urnover M odel. The method proposed for testing the C ladal T urnover M odel could be applied to look at other times of major evolutionary turnover preserved in the fossil record.