Pliocene–Pleistocene Unionida from Rhodes (Dodecanese, Greece): insights into the evolution of Eastern Mediterranean freshwater mussels

The island of Rhodes (eastern Greece) is devoid of large freshwater mussels today, but during the Pliocene and earliest Pleistocene, while still connected to Anatolia, it hosted a diverse fauna of Unionida, comprising 6 species in total. Two species, Unio pseudatavus Bukowski, 1896, which has its type locality on Rhodes, and Unio sp., co-occur in the Pliocene Salakos Formation; both are assigned to the Unio pictorum group here. Potomida semirugata (Lamarck, 1819) (Lamprotulini) occurs in fluvial deposits of the Late Pliocene Damatria Formation. The overlying lower Kritika Formation, dated as close to the Pliocene-Pleistocene boundary, has yielded Unio bruguierianus Bourguignat, 1853 (Unio crassus group), Leguminaia hedenborgi n. sp. (Gonideini), the margaritiferid Pseudunio auricularius (Spengler, 1793) and potentially additional specimens of P. semirugata. The new species Leguminaia hedenborgi is described, and lectotypes for Unio pseudatavus Bukowski, 1896 and Unio prusii Bourguignat, 1856 (a junior synonym of U. bruguierianus) are designated. The fossil records of the 4 genera involved reliably date back to the Eocene, Oligocene or Miocene of the Mediterranean and Paratethys regions. At tribe level, European Unionidae likely have their origins in Asia and migrated westward during several Cenozoic dispersal events. Mesozoic Unionidae in Europe thus may represent extinct clades.