Microfossil distribution across the base of the Wenlock series in the type area

Rich microfossil assemblages have been recovered from closely spaced samples collected from the uppermost Llandovery and lowermost Wenlock strata of the Wenlock Edge area. Large numbers of acritarchs, chitinozoans, conodonts, foraminiferans, ostracods, and other microfossils occur through the sampled sequence and provide a much better basis for correlation of the base of the Wenlock Series than can currently be achieved with macrofossil groups. The boundary cannot be shown to be coincident with the base of any biozone, but lies within the amorphognathoides conodont interval, between the base of acritarch zone 5 and the last occurrence of Pterospathodus amorphognathoides. At the stratotype locality for the base of the Wenlock Series, these horizons are separated by about 40 cm of strata. Elsewhere, the range of P. amorphognathoides encompasses much greater thicknesses of strata and, even using several fossil groups, the position of the boundary can only be broadly delimited.