Sphinctozoans

Sphinctozoans are sponges that grow as a series of chambers, one on top of the next. Most are aspiculate, including the living representative Vaceletia crypta. However, a few Mesozoic forms contain triradiate calcareous spicules embedded in the body wall. This suggests an evolutionary relationship with the Calcarea. On the other hand, soft parts of V. crypta indicate that it shares a common ancestor with the ceractinomorph demosponges. The sphinctozoan growth form has probably evolved independently within the Calcarea and the Demospongia, making the grouping polyphyletic. Thus, they cannot be thought of as a natural evolutionary group. Fossil "sphinctozoan" families for which calcarean or demosponge affinities cannot be determined are known from the Cambrian to the Cretaceous.