Early Cretaceous freshwater bivalves from northeastern Brazil: insights into the evolutionary history of South American freshwater mussels

We revise and describe the Early Cretaceous (Berriasian to Albian) freshwater bivalves from northeastern Brazil, including representatives of Unionida, Trigoniida, and potentially Sphaeriida. Three genera previously thought to be endemic to the Aptian to Albian Crato and Romualdo formations, Araripe Basin, Ceará State, i.e. Cratonaia, Araripenaia, and Monginellopsis, are now recorded from other interior basins. The Berriasian to Barremian Mont Serrat Conglomerate (Salvador Formation) of the Recôncavo Basin, Bahia State, hosts a diverse Unionida-dominated fauna, including Mycetopoda, Anodontites?, Cratonaia, and potentially also hyriids. Specimens tentatively assigned to the neomiodontid Musculiopsis also occur. This fauna has mixed Gondwanan and Laurasian affinities and likely thrived in ephemeral streams and/or floodplain lakes. Araripenaia is recorded from the Aptian Marizal Formation of the Tucano Basin. Supposed representatives of Sphaerium from the same strata are clam shrimps (Diplostraca). Cratonaia and Monginellopsis occur in the Aptian-Albian Itapecuru Formation of the Parnaíba Basin, Maranhão State. Articulated shells of Cratonaia, Araripenaia, and Monginellopsis are typically associated with fluvial-lacustrine deposits. Their common occurrence in Aptian-Albian strata of northeastern Brazil suggests dispersal via fluvial systems connecting the different sedimentary basins. These northeastern Brazilian assemblages are distinct from those of the Upper Cretaceous Bauru Basin in southeastern Brazil. Despite South America and North Africa being connected for most of the Early Cretaceous, no shared freshwater bivalves are known. The newly described faunas provide insight into South American Early Cretaceous freshwater mussel diversity when the proto-South Atlantic was dominated by alluvial and lacustrine settings hosting abundant but poorly known freshwater bivalves.