| Summary: | The ostracod and conodont micropalaeontology of the Silurian Ludlow/Pridoli series boundary is documented from 88 localities across Wales and the Welsh Borderland. Fourteen multielement conodont species (two in open nomenclature), eight unassigned Ozarkodina elements and ten ostracod species (four in open nomenclature) are described from the Upper Whitcliffe and Downton Castle Sandstone formations and their lateral equivalents in Wales and the Welsh Borderland. A septimembrate prionodontid conodont apparatus Coryssognathus dubius (Rhodes, 1953) is reconstructed from discrete elements. Original calcareous valves of the ostracod Frostiella groenvalliana and moulds of open and partially open ostracod carapaces are described for the first time from the Welsh Basin. The Upper Whitcliffe Formation and its lateral equivalents are characterised by the ostracod Calcaribeyrichia torosa and the conodonts Ozarkodina confluens, Ozarkodina excavata, Panderodus serratus and Coryssognathus dubius. The Downton Castle Sandstone Formation and its lateral equivalents are characterised by the ostracods F. groenvalliana, Londinia arisaigensis, Londinia fissurata and Nodibeyrichia verrucosa. Conodont trends across the shelf area of the Welsh Borderland reflect an increasingly turbulent environment towards the top of the Ludlow Series. The sudden ostracod faunal change at the base of the Downton Castle Sandstone at Ludlow (shelf) contrasts with a gradual change at Long Mountain (basin) and parallels shelf-basin palynofacies changes. Variations in ostracod frequency, faunal composition and carapace preservation in the Downton Castle Sandstone Formation at Ludlow coincide with minor lithofacies variations. Local variations in the frequency of ostracods and land plant spores may be related to proximal channels delivering sediment off an irregularly prograding shoreline. Ostracod faunas correlate the base of the Downton Castle Sandstone across the Welsh Borderland to localities in E central Wales where bone beds are absent. Combined conodont and ostracod evidence suggests that the base of the Pridoli Series is concurrent with the base of the Downton Castle Sandstone Formation in Britain.
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