Novel approaches to the environments and ecosystems of the fish-tetrapod transition

Sammanfattning: The fish-tetrapod transition is one of the most important evolutionary events in Earth’s history, giving rise to terrestrial vertebrates around 390 million years ago. It set the stage for a series of evolutionary events that ultimately resulted in modern-day terrestrial vertebrates including ourselves. The fish-tetrapod transition occurred during the Middle Palaeozoic and although it has been the subject of intense study over the last century, many questions remain unanswered. In this thesis, novel techniques were used to help elucidate certain aspects of the fish-tetrapod transition. The first project sought to use numerical tidal simulations to test the premise of a hypothesis that large tides occurred during the Middle Palaeozoic and acted as a driver for the evolution of lungs and limbs. The simulations produced for the Late Silurian-Late Devonian revealed unusually large tides during the Late Silurian, thus the origin of lungs, supporting the hypothesis that deoxygenated tidal pools could have been the setting for this evolutionary step. The following three projects used propagation phase-contrast synchrotron microtomography (PPC-SRμCT) to analyse new tetrapod material from the terminal Famennian (latest Devonian) and coprolite material from the earliest Tournaisian of Greenland (earliest Carboniferous), spanning a mass extinction event (the Hangenberg crisis) believed to have impacted early tetrapod diversity. Spectacular data sets were generated using this technique, with analysis of the tetrapod material revealing the presence of new taxa, making East Greenland home to the greatest known diversity of tetrapods in the world during the Devonian. Synchrotron scanning allowed for the accurate determination of coprolite morphotypes from a post-Hangenberg crisis lake deposit, revealing greater diversity among the coprolites compared with vertebrate body fossil taxa and thus demonstrating that the fauna contained additional taxa not captured by the body fossil record. Most of the large coprolites are non-spiral and were probably produced by a large aquatic tetrapod. One large coprolite is spiral and is postulated to have been produced by a chondrichthyan. Virtual reconstructions of several coprolites were generated using the scan data. The largest coprolites were full of actinopterygian and acanthodian remains, showing that the probable tetrapod was a proficient aquatic predator. Another large coprolite contained remains of two new body fossil taxa; an actinopterygian and small tetrapod. The coprolite data challenge our initial interpretation of a low-diversity lake fauna, revealing instead a complex ecosystem immediately after a major mass extinction event. Tetrapods and chondrichthyans appear to have been the apex predators in this ecosystem. This thesis demonstrates the capabilities of two novel analytical techniques, tidal simulation and synchrotron microtomography, to uncover previously inaccessible information about the fish-tetrapod transition and its environmental-ecological context.