Paleobiology of heterodont bivalves based on polymorphism of the hinge-teeth

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A study of hinge transposition in the heterodont bivalve families Carditidae, Condylocardiidae, Crassatellidae, Astartidate, Trapezidae, Cardiidae, Tellinidae, and Veneridae, showed that all these families except Condylocardiidae, Cardiidae, Tellinidae, and Veneridae rarely have transposed hinges, mainly less than 1.0%.Any valve with transposed anterior lateral teeth had transposed cardinal teeth on the same valve, and any valve with normal anterior lateral teeth always had normal cardinal teeth. The anterior lateral teeth and the cardinal teeth may be a single genetically linked single unit in these families, and the posterior lateral teeth another unit. Each unit is represented by two phenotypes, namely normal and transposed dentition. With a combination of two units by two phenotypes, therefore, four types of dentition are expected in the heterodont bivalves.In contrast to a whole transposition of gastropods with anatomical and shell asymmetry, a partial transposition is fairly common within the bivalves. The partial transposition may be caused by their symmetrical organization and may be selectively nearly neutral. The whole transposition in gastropods may lead to reproductive isolation, the partial transposition in heterodont bivalves brings high genetical diversity within a species.