REASSESSING THE EVOLUTIONARY FRAMEWORK FOR EARLY SPONGES: NEW LINES OF EVIDENCE FROM BURGESS SHALE-TYPE FAUNAS

Sponges are critical to basal animal evolution, but their early history is poorly understood. The modern classes show substantial differences in spicule form and structure, and soft tissue organisation, and fall into clearly definable groups. It is widely assumed that siliceous and calcareous spicules were independently derived, and that the earliest sponges would have been non-biomineralizing and fossils virtually impossible to affiliate to the stem groups of extant classes. Even among biomineralized sponges, body form and skeletal architecture are normally regarded as uninformative for high-level phylogenetic relationships, perhaps largely because this is true for modern groups.

The perceived uninformativeness of sponge fossils is challenged by re-examination of taxa from the Cambrian Burgess Shale-type faunas, and it is now necessary to reassess early sponges for potentially informative, fundamental characters that have been overlooked in traditional descriptions. Studies of spicule taphonomy show that a complex biminerallic structure was present at least in spicules of the disparate Cambrian genera Eiffelia (Calcarea/Hexactinellida: Heteractinida) and Lenica (Hexactinellida?: Protomonaxonida), and therefore probably occurred widely. This implies homology of calcarean and siliceous spicules, suggesting that the evolutionary divergence of the sponge classes, and at least some stem-group sponge lineages, should be visible in the fossil record. At least some early sponges should yield insights into relationships of Porifera to other phyla. For example, some early sponges show polyradial symmetry of definite order, particularly tetraradial and octaradial. If homologous with that of Cnidaria and/or Ctenophora, this symmetry supports a monophyletic Diploblastica, in contrast to most molecular phylogenies. This critical reassessment of primarily already known fossils constitutes a profound change in our understanding of sponges and their early evolution.