Dianulites Eichwald, 1829: An unusual Ordovician bryozoan with a high-magnesium calcite skeleton

Abstract

The ‘granular’ wall microstructure of the Ordovician stenolaemate bryozoan Dianulites Eichwald, 1829, has been studied using ultrathin sections, scanning electron microscopy (SEM), analytical SEM, and cathodoluminescence. The timing of recrystallization and the presence of microdolomite inclusions in the skeletal walls implies that the original skeleton consisted of high-magnesium calcite (HMC). Although found in some modern cheilostomes, HMC has not been recorded in living stenolaemate bryozoans, but appears to have also been present in Nicholsonella and a few other Ordovician genera traditionally assigned to the trepostomes or cystoporates. The Russian type species of Dianulites, D. fastigiatus Eichwald, 1829, is revised and recorded for the first time in North America from the Fillmore Formation (Lower Ordovician) of Utah. Unusually among bryozoans, D. fastigiatus has turbinate, cone- or horn-shaped colonies, straight to slightly curved, with zooids opening on the flat, broad end of the cone; the sides of the cone comprise calcified exterior walls. This growth-form resembles some solitary rugose corals and other benthic animals thought to have lived with all but their tops buried in soft sediment. Such an interpretation is supported in Dianulites by the scarcity of epibionts on the exterior walls of the cone and by the occurrence of specimens comprising stacks of subcolonies, suggesting periods of partial burial of the living tissues by sediment.