Article: The first recorded trigonioidoidean bivalve from Europe
Michael J. Barker, Martin C. Munt and Jon D. Radley
The non-marine bivalve superfamily Trigonioidoidea has been considered to be restricted to the Cretaceous of east Asia. It is a distinctive taxon of unionoids, characterized by separated anterior adductor and anterior pedal retractor muscle scars, and an ornament which typically comprises chevron-forming ribs. Well preserved bivalves (with chevron-forming ribs) from the Wealden Group (Barremian and ? Upper Hauterivian, Lower Cretaceous) of the Isle of Wight (southern England), which had previously been assigned to the unionacean unionoids, were re-examined and separated anterior adductor and anterior pedal musculature recognized. The combined character states of musculature, ornament and hinge teeth indicate affinity to the trigonioidid genus Nippononaia. However, placement in the established subgenera N. (Nippononaia) and N. (Eonippononaid) is precluded by the distinctively larger angle produced by the specimens' chevron ornament. Therefore, a new subgenus, N. (Subnippononaia), and a new species, N. (S.) fordi, are proposed to accommodate them. This constitutes the first confirmed record of the superfamily Trigonioidoidea outside of east Asia.