Global catastrophe or random decline? Evolutionary perspectives on the fall and rise of tetrapods across the Permo-Triassic boundary

Around 251 million years ago, at the end of the Palaeozoic Era, a major crisis affected marine and terrestrial organisms, resulting in a catastrophic drop in animal and plant diversity. This crisis is known as the Permian mass extinction event. While the factors underlying this crisis are still unclear, one aspect of it stands out: life at large suffered a remarkable collapse. This extinction is the most dramatic of all large-scale biological crises that have taken place over the last 550 million years. The few survivors of the end Permian event are held to have undergone a slow recovery and were responsible for carrying out a 'critical mass' of diversity that allowed expansion of life at the beginning of the Mesozoic Era. This recovery was accompanied by a major remodelling of animal and plant communities, resulting in the dominance of certain groups that were poorly represented before the extinction, the emergence of new groups, and the establishment of modern-looking ecosystems and food chains. But just how dramatic was the end-Permian event? This project seeks to assess the impact of the Permian extinction on terrestrial vertebrate communities. Some groups of vertebrates appear to have carried on through the event seemingly unaffected; others showed a sudden or steady decrese in abundance and went extinct forever before the end Permian; yet others experienced an explosive radiation in the lowermost Triassic. It is clear that different vertebrates responded differently to the crisis, but the patterns and dynamics of the extinction require detailed scrutiny. This project therefore looks at ways in which data from the geological and fossil records can be amalgamated to offer a more precise characterization of a critical episode in the history of life, and to make sense of the patchy fossil documentation in terms of biological signal (evolution). There is much scope for clarifying whether an intense post-extinction 'rebound' is the sole or the main mechanism accounting for the elevated taxonomic richness (high diversity) of some groups in the lowermost Triassic (especially when compared to diversity values during the mid to late Permian), following a period of slow and steady recovery. But how slow was this recovery? So far, the analysis of evolutionary relationships of groups and studies of the organization and/or level of complexity of ecosystems have given contrasting answers and there are reasons to suspect that, once again, different types of vertebrates recovered at different rates. Furthermore, I aim to disentangle the complex interplay between originations and extinctions immediately before and after the extinction event and in order to assess the influence of secondary, small-scale (background) extinctions. This is important because it is known that some vertebrates disappeared or declined progressively some time before the Permian event. In addition, some other vertebrates went through a series of sudden diversification 'bursts' before the extinction proper. The project will attempt to tease out geographical or ecological factors that might drive patterns of declining diversity. In other words, the extinction may have selectively operated on groups that lived in certain regions and/or habitats. Finally, the project will determine whether observed variations in diversity before and after the Permo-Triassic extinction are strongly correlated with outcrop availability (extent and number of fossil-bearing localities) and whether abundance levels in some geological sections do in fact reflect real taxon richness. Corrections for missing portions of phylogeny are possible, and these provide a clearer picture of the extent to which groups are represented even in the absence of direct fossil evidence. Ultimately, this research aims to realise the potential of fossil vertebrates to uncover, refine or challenge evolutionary models during periods of major biotic disaster and large-scale faunal transformations.