Fossil freshwater sponges: Taxonomy, geographic distribution, and critical review

Sponges are one of the most ancient animal phyla with about 8850 living species and about 5000 described fossil taxa. Most sponges are marine and live at all depths of all oceans. Freshwater bodies (lakes, rivers) are inhabited only by a small minority of species, ca. 240 (< 3%) comprising the order Spongillida (Demospongiae) most of which are able to produce specialized resting bodies to survive harsh terrestrial environmental conditions. This highly disproportionate ratio of marine and freshwater sponges is even more accentuated in the field of palaeontology with rare records reported up to the Miocene (< 0.4% of all known fossil sponges). Only a few fossil taxa were correctly supported by strong and convincing taxonomic morphotraits at genus and species level, thus we provide here an overview of fossil freshwater sponges focusing on their morphotraits and distribution in time and space. Each recorded taxon is described in detail following the modern taxonomy and nomenclature. All fossil data suggest a clear trend of long term conservative morphology in the evolutionary history of Spongillida, although some traits of Recent gemmules evolved in a wide array of adaptive morpho-functional novelties. The majority of accepted fossil species belongs to the cosmopolitan family Spongillidae. The genera Oncosclera and Potamophloios of the family Potamolepidae seem to have had, in the past, a much larger geographic range than today. A synthesis of fossil taxa morphotraits is also provided in an Appendix 1.