Selectivity in the evolution of Palaeozoic arthropod groups, with focus on mass extinctions and radiations: a phylogenetic approach

Abstract

Mass extinctions are known to be extraordinary events during which the normal rules of natural selection do not apply. Evidence points to the operation of a different selective regime, one where clade-level properties are selected upon; however, it is still unclear whether survivorship rules apply across different extinction events, the consequences of differential responses in diversity and disparity during extinction events, and the factors governing subsequent recoveries. This dissertation explores these issues by studying three clades of arthropods that experience multiple extinction events, allowing for the effect of different mass extinctions on the same group to be compared utilizing phylogenetic methodology. Three phylogenies where generated, one each for Stylonurina, Eurypterina, and Aulacopleuroidea. Variations in diversity, disparity, and volatility, are compared across these clades for two extinction events: the end-Ordovician (443 Ma) and the late Devonian (385- 359 Ma). Consistent differences in how morphospace occupation changes across the end- Ordovician and late Devonian mass extinction events reveal that the underlying driving factors of individual extinction events can result in very different selective pressures, suggesting that it may not be possible to identify general survivorship rules for all mass extinctions. Ecology is revealed to be a major factor behind the responses of individual species to the late Devonian biodiversity crisis. The somewhat contradictory selective signal exhibited by the studied clades reveals how different aspects of ecology can influence diversity, disparity and volatility in different ways, each influencing aspects of clade survival and recovery in different ways. The results also demonstrate the bearing of the evolutionary history of a clade on such studies. Contingency is an important factor in determining the response of clades to mass extinctions, and only through having a firm understanding of the history of a group can such factors be appropriately accounted for.