The deep-water trilobite association of the Silurian Coldwell Siltstone Formation of northern England and its wider significance

摘要

The upper Wenlock Coldwell Siltstone Formation of Cumbria and North Yorkshire, which accumulated in the Lake District Basin, contains a deep-water, trilobite-dominated fauna that is documented in detail here. The trilobites of the formation are revised on the basis of new material and existing museum collections. Delops nobilis marri Rickards and Struveria howgillensis Rickards are considered conspecific with Dalmanites mobergi Hede and Dalmanites simricus Hede respectively from the Swedish Colonus Shale. Based on the type species, Delops Rickards is recognized as a senior synonym of Lygdozoon Holloway. A new genus, Rickardsia, is erected to accommodate Dalmanites mobergi Hede, Delops nobilis (Thomas), and Delops dermolac Snajdr, which share characters that differentiate them from the type and other species of Delops. The fauna referred to previously as the Delops-Miraspis Assemblage is renamed the Rickardsia-Miraspis Association, and redefined to take account of new data. The trilobites Decoroproetus sp. nov.? and a cheirurid, and the brachiopods Leangella segmentum (Lindstrom) and Protozyga? sp., are recognized in the fauna for the first time. Comparable trilobite and brachiopod assemblages occur at the same stratigraphical level in the Welsh Basin, southern Sweden, Poland and the Czech Republic, and together with that of the Coldwell Siltstone Formation, are considered to have invaded these areas during a eustatic lowstand during the late Wenlock Epoch. Comparison with other assemblages suggests that all occupied depths that were not less than the shallowest limits of benthic assemblage (BA) 5.