Abstract

Early to Middle Ordovician carbonate rocks in western Utah and eastern Nevada preserve problematic receptaculitid fossils in solitary habitats and as common constituents of biohermal and sponge-microbialite reef buildups on a broad Ordovician shallow carbonate shelf. Both calathid and ischadid receptaculitids are widely distributed throughout the carbonate shelf. Utah and Nevada receptaculitids are geometrically aesthetic, exhibiting distinctive helicospiral surface patterns constructed by the individual merom structural elements found in all receptaculitids. An array of receptaculitid body shapes occur in the shelfal carbonates ranging from conical, cylindrical, and incomplete discoidal forms to complete, open cupshaped forms. Despite the determined efforts of paleontologists in conducting studies spanning over one hundred-ninety years, taxonomic affiliations for receptaculitids at the kingdom level remain unclear. The perforate morphology of calathid receptaculitids from western Utah, however, implies early receptaculitid functionality analogous to suspension-feeding organisms and probable relationship to the filter-feeding poriferans.