Article: The stratigraphical distribution and taxonomy of the trilobite Onnia in the type Onnian Stage of the uppermost Caradoc
The litho- and biostratigraphy of the type section of the Onnian Stage in the Onny River, south Shropshire, is reassessed on the basis of detailed sampling over an extended period, including years when the river level was unusually low. The base of the Onny Formation is redefined at a level within the upper part of the Onnian and thus the base of the stage lies within the Acton Scott Formation. Four biozones are defined on the basis of closely spaced samples of the trinucleid trilobite Onnia, a peri-Gondwanan immigrant. In ascending order these are: the O. superba cobboldi Local Range Zone, the O. s. creta Local Range Zone, the O. gracilis Acme Zone, and the O. s. superba Local Range Zone. The second of these is based on a new subspecies, the others on a reassessment of previously named taxa. Within the O. superba subspp. zones, fringe pit distribution of successive samples of Onnia shows considerable stasis, although early and late populations of O. s. superba can be recognized. The changes between the subspecies can be viewed as reflecting either an evolutionary lineage or subtle fluctuations in environmental controls on a cline or set of ecophenotypes within a variable species.