LATE CARADOC GRAPTOLITIC FAUNAL GRADIENTS ACROSS THE IAPETUS OCEAN

Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales and Scotland became progressively more disparate despite the narrowing of the ocean. We compare faunal distributions in continuous sections from opposite sides of Iapetus, at Whitland in South Wales and Hartfel l Score in the Southern Uplands of Scotland. A comprehensive graptolit e range-chart is given for each. The graptolite assemblages from the c lingani Biozone are subdivided into a lower Ensigraptus caudatus Subzo ne and upper Dicellograptus morrisi Subzone at both localities, though the faunas differ in detail. Higher in the sequence, the distinctive Scottish linearis Biozone fauna is not recognizable at Whitland, its p resumed equivalent being a fauna dominated by variable morphotypes of the genus Normalograptus. This suggests that environmental gradients ( depth and/or temperature) were changing more rapidly than the geograph y. Significantly, the subsequent inception of limestone deposition at Whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic, bioturbated sediments elsewhere in the We lsh Basin and in Scotland. This was possibly a response to an early ph ase of cooling prior to the end-Ordovician glaciation. In Wales the cl aimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofac ies variation.