Freshwater mollusk research in paleontology – a historical review

Freshwater mollusks have a rich and diverse fossil record. With evolutionary histories dating back into the Devonian (Bivalvia) and Carboniferous (Gastropoda) and several thousand valid species, mollusks constitute an important fossil group in the geological record of non-marine aquatic environments. In recent and ongoing studies, attention is given to the diversification history of the two groups, their colonization of fresh waters, as well as their spatiotemporal distribution. Here, we took a different angle and assessed the historical dynamics of new descriptions of freshwater mollusks, from the beginnings in the second half of the 18th century to present day. We analyzed the number of newly coined, nomenclaturally available species-group names of gastropods and bivalves over time, and illustrate their cumulative geographic distribution on maps. Our assessment focuses on Mesozoic to Cenozoic (Triassic to Pleistocene) fossil freshwater mollusks. All data were retrieved and are freely available from MolluscaBase (https://molluscabase.org/).

Taxonomic research history can be classified into six phases based on the overall numbers of original descriptions and historical events: 1) an incipient phase of Europe-centered research (1766–1820); 2) a phase of geographic expansion towards Asia, North and South America, and Africa (1821–1870); 3) a first peak of taxonomic research, with numerous taxon descriptions especially from Europe (1871–1910); 4) the period bracketed by the two world wars, which saw a decline in taxonomic research (1911–1950); 5) a rebound of new discoveries and descriptions in the second half of the 20th century (1951–1990); and finally 6) the current taxonomic depression, with a serious decline in the number of first descriptions, related to the ongoing loss of taxonomic expertise.