Post-extinction recovery of terrestrial vegetation following the End Devonian Mass Extinction: palynological evidence from the Tournaisian (early Carboniferous) of the UK

Abstract

As part of an investigation into the earliest Carboniferous tetrapod world, palynomorphs have been studied from a fully cored 500 m science borehole (West Mains Farm) in the Scottish Borders. Together with an outcrop section at Burnmouth, these encompass most of the Tournaisian Ballagan Formation. Both miospores and megaspores have been examined throughout, however, it is only at Burnmouth that the lowest palynological zone (VI) is present. Quantitative abundances of key spore taxa enable a robust correlation between Burnmouth and the borehole. In total, some eight distinct assemblages can be recognised through a CONISS analysis of the data. These assemblages map onto the palaeosol types present and reveal an inter-connection, with changes in climate ultimately driving a succession of vegetation types. The immediate post-extinction pattern of recovery was a simple vegetation represented by the VI palynomorphs, followed by increasing spore diversity. The vegetation then became dominated by the creeping lycopod Oxroadia (Anaplanisporites baccatus), including abundant megaspores (Lagenicula subpilosa). This was then replaced by an assemblage dominated by Prolycospora claytonii, which, although its parent plant is currently unknown, it was not necessarily a lycopod. Palaeosol evidence indicates that this may represent a wetter interval with more permanent vegetation and, hence, increased landscape stability. Further upsection, Oxroadia returned in abundance but was succeeded by larger arborescent lycopods with established Stigmaria root systems. However, this predates the first occurrence of Lycospora pusilla. In these younger parts of the succession, there are a number of inceptions of distinctive spore types that will further permit palynological sub-division of the interval. This eight-fold subdivision of the Tournaisian can now be mapped onto long Milankovitch cycles identified in shallow marine sections.

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