Graptolite biozonation of the Wenlock Series (Silurian) of the Builth Wells district, central Wales

The graptolite biozonation of the Wenlock Series in the Builth Wells distri ct is reassessed. The graptolite fauna is dominated by relatively few, but variable, species of Monograptus s.s., Pristiograptus and Monoclimacis. The se species are mostly long-ranging and are of limited biostratigraphical us e. Biozonation is based largely on the relatively rare cyrtograptids. The l ower Wenlock Series of the district comprises the widely recognized centrif ugus, murchisoni and riccartonensis biozones. The middle to upper Wenlock b iostratigraphy differs from that previously recognized in the UK. Above the riccartonensis Biozone, dubius, rigidus and lundgreni biozones are disting uished. Separate flexilis and ellesae biozones are not recognized: Monograp tus flexilis appears earlier than, then largely co-exists with, Cyrtograptu s rigidus s.l.; while Cyrtograptus ellesae appears later than Cyrtograptus lundgreni. The uppermost Wenlock nassa-ludensis Biozone is sparsely fossili ferous; its subdivision has not proved possible. This revised biostratigrap hical scheme for the Wenlock Series of the Builth Wells district suggests t hat the Sheinwoodian-Homerian boundary, as defined in the Welsh Borderland, might occur within, rather than at the base of, the lundgreni Biozone. How ever, difficulties in correlation with other Wenlock sequences, particularl y overseas, suggest significant biofacies control, with the Wenlock seas of the Builth Wells district appearing to have been periodically inhospitable to graptolites.