Scleritome construction, biofacies, biostratigraphy and systematics of the tommotiid 'Eccentrotheca helenia' sp. nov. from the Early Cambrian of South Australia

Abstract

Large collections of 'Eccentrotheca helenia' sp. nov. from the lower Cambrian Wilkawillina and Ajax limestones in the Arrowie Basin, South Australia, contain abundant low, cap-shaped and high, laterally compressed isolated sclerites in addition to partially articulated tubular specimens. The scleritome of 'Eccentrotheca helenia' sp. nov. is fully described for the first time and shown to be formed by ontogenetic fusion of sclerites into successively stacked sclerite rings, forming a larger, tubular structure. The apical termination of the tube is highly variable, but is primarily constructed by low, cap-shaped sclerites and characterised by a central aperture of variable inclination. The adapical portion of the tube is predominantly constructed by high, laterally compressed sclerites, but individual sclerite rings can contain both cap-shaped and laterally compressed sclerites along with sclerites of intermediate morphology. The apical aperture presumably housed organic structures for attachment to a hard substrate, but the scleritome also occasionally preserves small lateral perforations between fused sclerites, which may have served to stabilise the scleritome by providing additional points of anchorage. In the Arrowie Basin, 'E. helenia' is found in association with archaeocyath-microbial-spongiomorph-dominated bioherms and most likely inhabited pendant or cryptic habitats within these bioherms. 'Eccentrotheca'-like sclerites form an integral part of the scleritomes of many tommotiids which may confuse taxonomic analysis. Sclerites previously assigned to 'E.'guano, consistently occur together with sclerites of 'Kulparina rostrata' in stratigraphic intervals consistently older than strata hosting 'E. helenia'. Rare fused specimens indicate that the sclerites of 'K. rostrata' and 'E.'guano belong to the same scleritome.