Tournaisian (Early Carboniferous) recovery of terrestrial vegetation following the End Devonian Mass Extinction in the United Kingdom

Reeves, Emma J (2021) Tournaisian (Early Carboniferous) recovery of terrestrial vegetation following the End Devonian Mass Extinction in the United Kingdom. Trigger Factors of the Evolution of the Organic World, , Virtual. 16 - 17 Feb 2021.

Abstract

The pulsed End Devonian Mass Extinction (EDME) affected marine biota initially, followed by terrestrial plant life at the Devonian–Carboniferous Boundary. To investigate the recovery of the early Carboniferous vegetation after the EDME, an interval associated with the evolution of the first terrestrial vertebrate animals (tetrapods), palynomorphs from the West Mains Farm, Norham (NWMF) borehole core of the Tournaisian Ballagan Formation were examined.

Using the abundances of key spore taxa, we found that following the EDME, the depleted flora recovered to produce a stratified ecosystem with a diversity of plants. The initial simple (r-selected) survival flora was replaced by an increasingly complex (K-selected) recovery assemblage that diversified to produce a stepwise progression in the dominance of different plants. The first of these, the ground-creeping lycopod Oxroadia conferta (Anaplanisporites baccatus and Setispora pannosa), was adapted to form high-density populations in unstable conditions. It was replaced by a recovery assemblage dominated by a seed plant, represented by Genomosperma kidstoni (Prolycospora claytonii), which indicates the environment becoming more stable. The pteridosperm then declined and the fern Stauropteris burntislandica (Didymosporites scottii) became dominant. The return of O. conferta indicated a second phase of instability, until G. kidstoni once again became dominant. Additionally, the background flora of two pteridosperms, the shrubby Lyrasperma scotica (Colatisporites denticulatus) and the tree-sized Pitus primaeva (Colatisporites decorus), a sphenophyte Cheirostrobus pettycurensis (Retusotriletes incohatus) and the intermittent progymnosperm Protopitys scotica (Remysporites magnificus) indicate a diverse floral assemblage throughout this time.
To investigate the driving forces behind the recovery of the Ballagan environment, we performed a CONISS analysis of 8 outgroup taxa and 4 taxa with recognised parent plant affinities. This produced distinctive natural assemblages which were grouped together to track palaeoclimatic changes through the Tournaisian. Applying the grouped clusters to the fluctuating abundance of Schopfites claviger, revealed a repeating pattern of drier and wetter climate intervals, which suggests periodicity.

Adding the S. claviger climate information to the distribution of the borehole’s megaspores, studied for the first time from a continuous succession through the entire Ballagan Formation, also revealed their parent plants’ preference for a drier climate, except for S. pannosa which was more often present in the wetter phases. This indicates that O. conferta exhibited a broad climate tolerance alongside withstanding unstable substrates. Additionally, analyses of the abundant megaspores of O. conferta and S. burntislandica from a thin coal seam from the core showed that it represented an enclosed pool surrounded by a highly restricted, ground-level flora of lycopods and ferns in a stable, wetland community.

The common presence of roots in the palaeosols of the borehole and of intermittent Stigmaria ficoides at outcrop, indicate the presence of large, arborescent lycopsids, likely Sigillaria, growing on substrates that were stable for long enough to support more architecturally complex plants. Palaeosols provide a direct record of changes in the climate and landscape and the diversity of the borehole palaeosols, with the presence of deep vertic cracks and gypsum and anhydrite deposits, indicate a dynamic floodplain with a strongly seasonal climate with repeated cycles of wetting and drying, becoming increasingly dry towards the end of the Tournaisian. These findings agreed with the palynomorph data and a comparison of the CONISS dendrograms for the palaeosols and the palynomorphs revealed a correlation. This showed, for the first time, that fossil soils and palynomorphs can be related directly and revealed a diverse assemblage of palaeosols supporting a diverse assemblage of plant life.

Evidence from the NWMF borehole core has shown how the flora of the Ballagan environment recovered from the EDME. We found that both the soil formation and the plant distribution were driven by changes in the climate, which ultimately effected a succession of vegetation types within the Tournaisian.

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