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Early Carboniferous Palynology and Tetrapod Evolution

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Early Carboniferous Palynology and Tetrapod Evolution University of Southampton Faculty of Environmental and Life Sciences Ocean and Earth Science Early Carboniferous Palynology and Tetrapod Evolution by Emma Jane Reeves Thesis for the degree of Doctor of Philosophy November 2019 ii University of Southampton Abstract Faculty of Environmental and Life Sciences Ocean and Earth Science Thesis for the degree of Doctor of Philosophy Early Carboniferous Palynology and Tetrapod Evolution by Emma Jane Reeves Following the End Devonian Mass Extinction, the development of complex plant communities facilitated the evolution of terrestrial vertebrate animals (tetrapods). However, a gap in the fossil record (Romer’s Gap) prevented the identification of the key anatomical features that enabled the study of tetrapod terrestriality. As part of the interdisciplinary TW:eed project, palynomorphs from the NWMF borehole and Burnmouth outcrop section revealed the pattern of plant recovery and the changing palaeoenvironment of the Tournaisian. Assigning an accurate biozonation to the new palynomorph data enabled the tetrapods to be relatively dated and incorporated into a time resolved cladistic analysis. Initially, the palynomorph distribution did not produce a clear biozonation, but applying the zonal indicator taxa from the published schemes of Higgs et al. (1988) and Turnau (1978) produced four palynozones: VI, HD, PC and Cl. The VI zone comprised simple survival flora, then the HD zone signified the beginning of the recovery vegetation. The first dominant plant, the r-selected lycopsid Oxroadia conferta, thrived in unstable conditions. Its decline marked the step-wise dominance of three K-selected inception taxa: Genomosperma kidstoni (pteridosperm), a putative medullosan pteridosperm, then Stauropteris burntislandica (fern). Concurrently, there was a continuous presence of two pteridosperms: Lyrasperma scotica (small shrub) and Stamnostoma huttonense (tree-sized) and an intermittent progymnosperm Protopitys scotica. This recovery shows how the Ballagan environment became populated by a diversity of plants that formed a structured ecosystem. As palaeosols provide a record of changes in the climate and landscape architecture, comparing CONISS dendrograms for the palaeosols from the NWMF borehole core and the palynomorphs revealed a correlation. This showed, for the first time, that fossil soils and palynomorphs can be related directly. Additionally, the fluctuating relative abundances of the miospore Schopfites claviger defined repeated drier and wetter climatic intervals. In summary, palynological evidence from the NWMF borehole and Burnmouth of the Tournaisian Ballagan Formation has demonstrated how the vegetation recovered after the End Devonian Mass Extinction event. Both the plant distribution and the soil formation were driven by changes in the climate, which generated a succession of vegetation types. iii iv LIST OF CONTENTS Abstract ................................................................................................................... iii List of Figures ....................................................................................................... xv Introduction ....................................................................................................... xv Materials and methods .................................................................................... xvii The palynomorphs ............................................................................................ xx Tournaisian biozonation .................................................................................. xxiii Plant recovery ................................................................................................. xxiii Conclusions .................................................................................................... xxvi Appendix 1 ..................................................................................................... xxvii Appendix 2 ..................................................................................................... xxvii Appendix 3 .................................................................................................... xxviii Appendix 17 ..................................................................................................... xxx List of Tables....................................................................................................... xxxi Introduction ..................................................................................................... xxxi Materials and methods ................................................................................... xxxi The palynomorphs .......................................................................................... xxxi Tournaisian biozonation ................................................................................. xxxii Plant recovery ................................................................................................ xxxii Declaration of Authorship .................................................................................. xxxiii Acknowledgements ............................................................................................ xxxv Abbreviations ................................................................................................... xxxvii 1. INTRODUCTION ............................................................................................... 1 1.1. What are mass extinctions? ........................................................................... 3 1.2. Comparing mass extinction events ................................................................. 5 1.2.1. Late Ordovician .......................................................................................... 5 1.2.2. End Permian .............................................................................................. 6 1.2.3. End Triassic ............................................................................................... 7 1.2.4. End Cretaceous ......................................................................................... 7 1.3. The End Devonian Mass Extinction (EDME)/Hangenberg Event ................... 7 1.3.1. The EDME was different to other mass extinctions .................................... 9 1.3.2. Possible drivers of the EDME ................................................................... 10 v 1.3.2.1. Glaciation ............................................................................................... 12 1.3.3. The effects of the EDME .......................................................................... 14 1.3.3.1. The effects of the EDME on terrestrial biotas ........................................ 15 1.3.4. The evolution of terrestrial vegetation ...................................................... 15 1.3.4.1. Ordovician .............................................................................................. 15 1.3.4.2. Silurian ................................................................................................... 16 1.3.4.3. Devonian ................................................................................................ 16 1.3.4.4. Devonian–Carboniferous boundary ....................................................... 17 1.3.4.5. Carboniferous ........................................................................................ 17 1.3.5. The effects of the EDME on vertebrates .................................................. 18 1.3.5.1. The fishes .............................................................................................. 18 1.3.5.1.1. Agnatha ................................................................................................ 19 1.3.5.1.2. Gnathostomata ..................................................................................... 20 1.3.5.2. The tetrapods ......................................................................................... 22 1.4. Tetrapod evolution ........................................................................................ 22 1.4.1. Summary of the skeletal adaptations of the tetrapods to life on land ....... 23 1.4.1.1. The skull and jaws ................................................................................. 24 1.4.1.2. Hearing .................................................................................................. 24 1.4.1.3. The neck ................................................................................................ 24 1.4.1.4. The fore limbs ........................................................................................ 24 1.4.1.5. The hind limbs ....................................................................................... 28 1.4.1.6. Scales .................................................................................................... 29 1.4.2. Trackways ................................................................................................ 29 1.4.3. The sequence of developments ............................................................... 31 1.5. Why the tetrapods made the transition onto land ......................................... 33 1.6. Romer’s Gap ................................................................................................ 35 1.7. Tetrapod localities in Scotland ...................................................................... 38 1.7.1. Burnmouth ...............................................................................................
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