Lower Cambrian halkieriids from Oxfordshire, UK

Abstract

Lower Cambrian sediments from the Withycombe Farm borehole (Oxfordshire) yield isolated halkieriid sclerites. All bear prominent ridges, and most appear to be either cultrates or palmates, although distinction between the two in the available material is generally difficult. Two other sclerites are more elongate: one may represent a spinose sclerite, while the other is possibly derived from the siculate zone of the original scleritome. Adjacent horizons of the Withycombe Formation yield disc-like fossils, superficially similar to hyolith opercula or monoplacophorans. These are tentatively identified as deriving from the same halkieriid scleritome. Two main types are identified, and these may correspond to the anterior and posterior shells of Halkieria evangelista from the lower Cambrian Sirius Passet fauna of North Greenland. The age of the Withycombe Formation within the Cambrian is not entirely resolved, but faunal evidence suggests an equivalence to the Cuslett Formation of southeast Newfoundland, which corresponds to either the upper Tommotian or lower Atdabanian of the standard Siberian section.