Palaeoecology and systematics of Ordovician biotas from Welsh volcaniclastic deposits
The effects of explosive volcanism on local ecosystems are investigated in Middle Ordovician siliciclastics from the Welsh Basin. Bulk sampling analysis has provided quantitative data, regarding population proportions and abundance, following ash deposition in nearshore, shallow dysaerobic basin, and deeper basinal facies. Consistent ecological effects include the destruction of small sessile benthos by rapid burial, followed by re-establishment of mobile and opportunistic taxa, and a bimodal, planktic-benthic bloom in dysaerobic facies. The results are explained through vertical circulation initiated by turbid surface waters following ash deposition. Upwelling of subsurface, nutrient-rich waters of stratified basins is accompanied by downwelling of oxygenated surface waters, entrained into broadly spaced columns. The duration and nature of the events are investigated by ecological, sedimentological, and mechanical approaches, and high sedimentation rate invoked, resulting from seismicity associated with local volcanism. Systematic studies are included on Porifera, Echinodermata and Palaeoscolecida, the unusual preservation of each resulting from volcanism-related processes. The poriferan fauna provides significant information on non-lithistid demosponges and hexactinellids, including the earliest representatives of several groups. Rapid silicification of the proteinaceous skeleton of two species indicates a new source of soft-tissue preservation. Echinoderms comprise the most diverse pre-Caradoc fauna known from Britain, including six crinoids, three asteroids, and a cystoid.