Early carboniferous palynology and tetrapod evolution

Abstract

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 ofthe 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 recoveryand the changing palaeoenvironment of the Tournaisian. Assigning an accurate biozonation to the new palynomorph data enabled thetetrapods to be relatively dated and incorporated into a time resolved cladisticanalysis. 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 thebeginning of the recovery vegetation. The first dominant plant, the r-selectedly copsid 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 Stauropterisburntislandica (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 vegetationrecovered 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.

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ORCID for John Marshall: orcid.org/0000-0002-9242-3646

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