Integrated approaches to the reconstruction of early land vegetation and environments from lower Devonian Strata, Central-South Wales

• Main Author: Morris, Jennifer Louise
• Published: Cardiff University 2009
• Subjects: 560; QE Geology : QK Botany
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.567103

Integrated approaches to the reconstruction of Lower Devonian vegetation and environments are presented, combining palaeobotanical, palynological and sedimentological evidence from Old Red Sandstone strata of the Anglo-Welsh Basin. A new lower Lochkovian plant assemblage from central-south Wales is similar in diversity to contemporaneous assemblages along the southern margins of Laurussia. Coalified megafossils of rhyniophytes and rhyniophytoids e.g. Cooksonia hemisphaerica, represent basal embryophytes. Geometric morphometric analysis of sporangial morphology revealed a strong taphonomic control on shape. Newly discovered highly-branched mesofossils are synonymous with published charcoalified specimens from lower Prídolí and middle Lochkovian localities, and represent stem-group embryophytes with bryophytic characters. The non-embryophytes, with the largest biomass, include the fungal-like Prototaxites and associated mycelia, Pachytheca, and evidence for microbial biofilms. Several new dispersed palynomorph taxa are described, assemblages dominated by cryptospores. With additional published palynomorph and sedimentological data, broad palynofacies are constructed to reveal some information regarding lower Lochkovian habitats. Using core data from this locality, lower strata are correlated to the Raglan Mudstone Formation, and a two-stage, ephemeral, mud-dominated, dryland river system is envisaged. The appearance of sandier, meandering channel deposits in upper strata are correlated to the St. Maughans Formation, which suggests either a change in fluvial morphology or the switching-on of trunk channels, the causes for which are discussed. By combining palaeobotanical and sedimentological data, several plant taphofacies are recognised and a taphofacies model envisaged, the most significant taphonomic constraint on palaeoecological studies being the stratinomic partitioning of vegetation prior to burial by fluvial hydraulic sorting. Plant material is restricted to channel elements with low preservational potential, therefore the extent of phytoterrestrialisation and soil productivity may have previously been underestimated. Indirect evidence for significant soil productivity, which may have increased chemical weathering, potentially altering atmospheric CO2 levels, is calculated from the stable carbon isotopic values of pedogenic carbonate nodules.