Porifera: Systematics

At one time, a diagnostic feature of the Porifera was the presence of spicules. As a result, certain fossil groups whose organization was consistent with that of living sponges were not placed within the phylum Porifera. In particular, groups with a solid calcareous skeleton such as the Archaeocyatha, chaetetids, sphinctozoans, stromatoporoids, and receptaculids were problematic. A great deal of insight into the phylogenetic affinities of these groups was gained with the discovery of more than 15 extant species of sponges having a solid calcareous skeleton. These species are diverse in form, and would be classified with the chaetetids, sphinctozoans and stromatoporoids if found as fossils. However, with the living material in hand, histological, cytological, and larval characteristics can be observed. This information suggests that these 15 species can readily be placed within the Calcarea and the Demospongia. This radically changes our view of poriferan phylogeny.

It is widely accepted among poriferan biologists that the Calcarea and the Demospongia are more closely related to each other than either is to the Hexactinellida. With the discovery of living chaetetids, stromatoporoids, and sphinctozoans, a fourth class was erected for these so-called sclerosponges. However, the Sclerospongia is not a natural monophyletic grouping and is thus being abandoned. The abundant fossil chaetetids, stromatoporoids, and sphinctozoans are probably part of the classes Demospongia and Calcarea, though some uncertainty still remains. The Archaeocyatha pose a special case. No living representative of this group has been discovered. Their organization is consistent with that of living sponges. The one phylogenetic analysis (carried out by Reitner) that included archaeocyaths with other sponges, grouped them as sisters to the demosponges. Therefore, although the taxonomic term Archaeocyatha is often accorded phylum status it is likely a sub-clade of the phylum Porifera, thereby violating the ranking system.


    Bergquist, P. R. 1985. Poriferan relationships. In Conway Morris, S., J. D. George, R. Gibson, and H. M. Platt (eds.), The Origins and Relationships of Lower Invertebrates. pp. 14-27. Clarendon Press, Oxford.

    Debrenne, F. and J. Vacelet. 1984. Archaeocyatha: Is the sponge model consistent with their structural organization? Palaeontographica Americana, 54: 358-369

    Kruse, P. D. 1990. Are archaeocyaths sponges, or are sponges archaeocyaths? Geological Society of Australia Special Publication, 16: 311-323.

    Reiswig, H. M. and G. O. Mackie. 1983. Studies on hexactinellid sponges. III. The taxonomic status of Hexactinellida within the Porifera. Philosophical Transactions of the Royal Society of London. B 301: 419-428.

    Reitner, J. 1990. Polyphyletic origin of the "Sphinctozoans". In Rutzler, K. (ed.), New Perspectives in Sponge Biology, Proceedings of the Third International Conference on the Biology of Sponges (Woods Hole). pp. 33-42. Smithsonian Institution Press, Washington, DC.

    Reitner, J. and D. Mehl. 1996. Monophyly of the Porifera. Verhandlungen des Naturwissenschaftlichen Vereins in Hamburg. 36: 5-32.

    Savarese, M. 1992. Functional analysis of archaeocyathan skeletal morphology and its paleobiological implications. Paleobiology 18(4): 464-480.

    Vacelet, J. 1985. Coralline sponges and the evolution of the Porifera. In Conway Morris, S., J. D. George, R. Gibson, and H. M. Platt (eds.), The Origins and Relationships of Lower Invertebrates. pp. 1-13. Clarendon Press, Oxford.