Reef-building sponges

After reviewing the types of sponges within the phylum Porifera and their characteristics, the discovery during the last 20 yr is reported of about 13 living species of "coralline' sponges with solid calcareous skeletons. The types of fossil reef-building sponges are then described as they occur in the fossil record from the Lower Cambrian onwards. The discovery of "living relicts' strongly similar to these fossils has required a redistribution of taxa based on spicule and soft-tissue types. The phylogeny of calcified sponges and how their development can be extrapolated to deduce lineages in the fossil record is then discussed, and the reasons for the frequent independent evolution of calcification are explored. It is concluded that the historical development of ideas about these organisms consists of a series of misinterpretations, based on false analogies which allowed them to be pushed from one biological group to another. -J.W.Cooper