Taxonomic revision of the unionid bivalves from the Lower Cretaceous Tetori Group in Japan with reference to their habitats in meandering river systems

Abstract

Unio ogamigoensis, the first unionid bivalve reported from the Tetori Group, is redescribed as Margaritifera ogamigoensis on the basis of the hinge teeth and muscle attachment scars of the holotype and newly added specimens from the Kuwajima Formation of the Tetori Group, central Japan. This species occurs within crevasse splay sandstones that are intercalated with the fluvial floodplain deposits of the meandering river system. Nakamuranaia kagaensis, which is commonly found in muddy fine-grained sandstone and mudstone in the overbank deposits, is also redescribed from Archaeounio kagaensis, i.e., Archaeounio becomes a synonym of Nakamuranaia. The mode of occurrence of the two species indicates that M. ogamigoensis was a rapidly flowing stream dweller, whereas N. kagaensis inhabited stagnant shallow oxbow lakes.