Nonpalaeocope ostracod biostratigraphy of the type Wenlock Series, Silurian, of the Welsh Borderland

Abstract. Analysis of distribution, diversity and abundance of nonpalaeocope ostracods from the type Wenlock Series demonstrates that a major faunal change occurs around the Sheinwoodian-Homerian Stage boundary and that significant increases in faunal diversity occur at that boundary and in the late Whitwell Chronozone. Low abundance and low diversity in the late Sheinwoodian is interpreted to represent maximum water depth for the type Wenlock Series whereas the high diversity fauna of the late Homerian represents shallowest water conditions for this sequence. Many late Homerian species range into the Lower Elton Formation (early Ludlow) which suggests gradual ecostratigraphic change across the Wenlock-Ludlow boundary. Ancestor-descendant relationships for several lineages in the type Wenlock Series define lineage zones which essentially coincide with assemblage zones based on nonpalaeocope ostracods.