| Abstract: | The relationships between the poriferan classes are currently obscure. Molecular phylogenies appear to be reaching a consensus that the hexactinellids and demosponges are closely related, despite previous attempts to separate the Hexactinellida from other sponges on cytological grounds, but the details of the transition are unknown. Similarities of spicule morphology and structure are used to infer that the transition probably occurred after the onset of silicification, and should therefore be seen in fossils. The similarity between protosponges and early demosponges has been noted previously, based largely on a thin, reticulated wall of simple spicules (monaxons in Leptomitus, stauracts in Protospongia). A close relationship is, however, unlikely, since the protospongiids possessed a precise geometric arrangement of multiple spicule size orders that is lacking in demosponges. A close morphological similarity exists between the skeletons of transitional protosponge-dictyosponge reticulosids, such as Cyathophycus, and the early hazeliid demosponges. The inner spicule layer of primitive dictyosponges consists of a cross-hatched array of fine monaxons, as seen in the wall of some simple hazeliids. Although most described species of hazeliids were morphologically complex, a simple globose species is here recorded from the Caradoc of Wales. The evolutionary link between the classes is suggested to lie within the dictyospongioid and hazeliid lineages. Although the direction of evolution cannot be certainly fixed, it is conceptually much easier to derive the demosponges from the hexactinellids, rather than vice versa. |
