Palaeontology of sponges — a review

The fossil record of sponges is a very old one, and begins in the Precambrian, but is rather incomplete. Only those sponges with a rigid skeleton, i.e., Hexactinosida and Lychniscosida (both hexactinellids), lithistids (demosponges with desmas), and sponges with a massive calcareous skeleton (polyphyletic demosponges and Calcarea) have a more or less continuous fossil record that is, however, inadequately studied, especially from the Tertiary. The beginning of sponge diversification during the Cambrian is relatively well known thanks to their very good preservation, from the Chengjiang fauna in China and Burgess Shale in Canada, where even sponges with unfused spicules occur. The majority of palaeontologic studies are concerned with taxonomic aspects of fossil sponge faunas, but investigations of the microstructure of the calcareous skeleton, of phylogeny (especially of the early forms), and of various aspects of their ecology have produced important results. Future research still has to fill gaps in the knowledge of fossil faunas, especially "soft" sponges, and in the phylogeny of sponges (especially for the polyphyletic groups like lithistids), but new approaches such as studies of biomarkers are already emerging.