An extinct group of mainly Lower Cambrian marine sponges which, although lacking spicules, possessed an intricate, highly porous skeleton of calcite. It was probably a monophyletic group; that is, all representatives were derived from a single ancestor. The position of the Archaeocyatha within the Porifera is uncertain, but they were probably most closely related to the class Demospongiae. Their fossil record is well known, as archaeocyaths represent the first large skeletal animals to have been associated with reefs; they were widespread in the shallow, warm waters that surrounded the many continents that occupied tropical latitudes during the Cambrian.