Abstract

The Builth Inlier of central Wales exposes a highly fossiliferous Middle to Late Ordovician (Darriwilian to basal Sandbian) siliciclastic succession in a volcanic, back-arc basin setting. Articulated echinoderm faunas are preserved in a range of paleoenvironments, together with widespread, dissociated ossicles. These have enabled a reconstruction of the distributions of echinoderm groups across a range of noncarbonate facies. The oldest echinoderm assemblages are from sandstone and siltstone deposits, and have yielded only the crinoid Iocrinus pauli and rare asteroids. The most diverse echinoderm assemblages are from a nearshore siliciclastic facies at Llandegley Rocks and equivalent sites. These are dominated by crinoids, together with rare asteroids, caryocystitid rhombiferans, echinozoans, and possible mitrate stylophorans. Deeper-water environments were dominated by the mitrate stylophoran Anatifopsis, associated with variable numbers of holothurians, cornutes, solutans, and rare cystoids (rhombiferans and undetermined blastozoans). The ecological distribution of echinoderms in the area can be summarized as follows: (1) high-diversity faunas in the shallowest environments, composed primarily of large crinoids, with asterozoans and other groups; (2) low-diversity faunas in intermediate water depths, with crinoids (one species) and asterozoans; (3) moderately diverse faunas in deep-water communities with generally small taxa, dominated by stylophorans. These results agree with previous interpretations of environmental preferences among the different groups of echinoderms, but also indicate a separation of dominantly Cambrian-type classes in offshore sediments and Ordovician-type taxa in shallow water. Based on assessments of functional morphology and previously published data, the environmental distributions of these taxa are likely to have been controlled by a complex of factors including sediment type, flow regime, nutrient availability, and temperature. Shallow-water siliciclastic settings may have been crucial environments for echinoderm diversification during the Great Ordovician Biodiversification Event.

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