Problems of recognition and implications of Dinantian conodont biofacies in the British Isles

Extensive new collections from the Dinantian rocks of the British Isles confirm previously published opinions concerning the relationships between conodont distribution and facies. Conodonts from time-equivalent rocks, representative of numerous environments from the supratidal through the shelf into the open basin, indicate that Cavusgnathus and related genera are typical of near-shore environments whereas Siphonodella and Gnathodus characterise deeper water environments. This relationship is also demonstrated within vertical rock sequences through the Dinantian Subsystem, although the change is commonly accentuated by unconformable contacts. A limited number of Dinantian biofacies are recognised. Comparisons are made between conodont associations from the south and north of Britain through the Courceyan-Brigantian interval of time. Conodonts from South Wales and the Mendip region are documented including a new discovery of Scaliognathus anchoralis. The absence, or near-absence, of conodonts from a significant portion of the Chadian to Asbian sequence is attributed to extreme water shallowness over very broad areas of the Dinantian shelf. It is suggested that lack of competition in unfavourable shallow-water environments resulted in the success of long-ranging species. Correlations based on these species are only of local relevance. Only conodont zones based on evolutionary lineages are likely to be of international application. The boundary stratotypes for the six stages of the Dinantian Subsystem recognised in the British Isles have been selected at outcrops in different parts of the region. The rocks at the stratotype sections represent different lithofacies. Where present in the stratotypes, the conodonts are a reflection of the local lithofacies. They provide only limited information concerning the evolutionary sequence of Dinantian conodonts.