Baseline fluviatile mollusk communities are often poorly known, hampering their application in conservation purposes. Fossil/ancient faunas can become available through ex-situ dredged material. Continued collection and documentation effort is known to provide useful insights into
past communities even though their ex-situ collecting introduces bias. A very rich dredged fauna from a sand extraction near Lent (prov. Gelderland, The Netherlands) provided the opportunity to reconstruct a Holocene Rhine fauna allowing us to address the question what a natural (pre-industrial) Rhine community may have looked like. The newly reported fauna consists of common palearctic species. The dominant fluviatile component consists mostly of species adapted to flowing water, such as unionids, large sphaeriid bivalves, and valvatid and neritid snails. It includes common Unio crassus (extinct 1968?) in the Netherlands, and also yields the critically endangered river pearl mussel Pseudunio auricularius also extinct in the Netherlands, possibly since the Roman period. A terrestrial component represents possibly riparian conditions.
Radiocarbon dating indicates an Early Holocene (Boreal) age for one valve of the pearl mussels, another river pearl mussel delivered an infinite age. Reworked Cenozoic fossils confirm a Rhine provenance. The newly reported fauna, even though of an ex-situ setting, gives good insights into a Holocene fluviatile baseline fauna. Today the Rhine faunas have been drastically modified by pollution and invasions of alien species.