Barbatia mytiloides and the evolution of shell torsion in arcid pelecypods

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Abstract
B. mytiloides (Brocchi) (Arcidae, Arcinae) from the Neogene of northwestern Italy shows a variable but significant amount of dextral shell torsion about the hinge axis. This represents a morphological and functional convergence with the sinistrally torted ark Trisidos. Polyphyletic evolution of shell torsion in mud-dwelling, semi-infaunal descendants of epibyssate Barbatia is probably due to ventilation problems that forced these forms to lie with the posterior commissure horizontal. Depending on which valve happened to lie lowermost in the non-torted reclining or semi-infaunal ancestors, shell torsion became established in a dextral or sinistral direction.