Examination of small bioherms in the Early Pennsylvanian Morrow Series of northeastern Oklahoma makes possible better understanding of the changes in structure and composition of organic mounds through time during the Carboniferous of North America. Three phases of biohermal development are described: initiation and stabilization of the substrate by blue-green algae, diversification and habitation of the bioherms by a variety of organisms, and domination by encrusting bryozoans or blue-green algae with termination of mound growth. Three assemblages of organisms may be recognised and include an unusual cavity association. Comparison of the bioherms with modern patch reefs enables greater understanding of the significance of modern and fossil assemblages developed on a carbonate mud substrate.