The biota of Upper Silurian submarine channel deposits, Welsh Borderland
The Upper Silurian submarine channel deposits at Leintwardine in the Welsh Borderland provide a unique palaeoenvironmental setting in the fossil record, that of an indigenous biota within shelf-edge channel heads. Along with typical Silurian fossils such as brachiopods and trilobites, the deposits contain an exceptionally preserved fauna including predominantly fully articulated echinoderms and disarticulated to articulated arthropods. The biota is interpreted to comprise indigenous and exotic elements, with variable transportation, both within and between species. The echinoderms are stelleroid-dominated although less abundant crinoids, echinoids and ophiocistioids also occur. The echinoderms occur within discrete horizons, so-called starfish beds, whereas other fossils occur sporadically through the remainder of the channel fill. Oxygen levels within the Church Hill Channel do not appear to have been restricted, and periodically high sedimentation rates, likely to be storm-triggered, are interpreted to be central to the preservation of much of the unusual biota. The stelleroids are diverse, comprising 15 species; asteroids and ophiuroids are represented. One new ophiuroid species is erected, Loriolaster calceatus sp. nov. and the subspecies Urasterella ruthveni var. leintwardinensis Spencer is not maintained. A previously undescribed specimen, likely to be a juvenile, is described as Coccaster? sp. Specimens previously described in the literature as Bdellacoma vermiformis Salter and Palasterina antiqua (Hisinger) are reassigned to Klasmura? sp. and Palasterina sp., respectively. The eurypterids comprise mainly carcinosomatids, with a lesser number of pterygotids. The pronounced serration with a deep notch on the distal podomere of prosomal appendage VI is identified as being diagnostic of Carcinosoma punctatum Salter. Newly collected and previously described material is assigned to Carcinosoma sp. And Carcinosoma? sp. Previously undescribed material is assigned to Erettopterus? sp. A distal portion of a free ramus is reassigned to Pterygotus? sp.