Article: Problematical fossil cnidarians from the Upper Ordovician of the north-central USA

Sphenothallus sp. and eight species of conulariids, distributed among the genera Climacoconus Conularia, Glyptoconularia and Metaconularia, occur in the Elgin Member of the Maquoketa Formatioi (upper Ordovician) of north-eastern Iowa and south-eastern Minnesota, USA. Seven of the eight conulariid species exhibit internal test structures at their corners and/or midlines. Comparisons of these test structure: with internal thecal structures of coronatid scyphozoans corroborate the hypothesis that conulariids were mon closely related to scyphozoan cnidarians than they were to any other extant taxon of comparable rank Sphenothallus and conulariids occur in all four Elgin Member biofacies. However, the distribution of Climacoconus and Conularia is facies-dependent, with Climacoconus occurring predominantly in the brachiopod-echinoderm biofacies and the basal Maquoketa phosphorite, Conularia splendida predominantly in the trilobite-dominated biofacies, and C. trentonensis predominantly in the brachiopod-echinoderm, mixed faunas and graptolite shales biofacies. Conulariids commonly occur in monospecific clusters, possibly clonal in origin, and some specimens show orientational evidence of original attachment to Sphenothallus or nautiloid shell material. Together with previously reported data on the distribution and biostratinomy of Sphenothallus and conulariids, these results suggest that both taxa were sessile benthic organisms that inhabited all major Elgin Member bottom environments, including a shallow, oxic carbonate shelf and a deeper, dysoxic shell margin and shale basin slope. One new species, Climacoconus sinclairi, is described.