Abstract

Differences in the mineralogy of hinge teeth and inner shell layers in the family Dimyidae form the basis for a revision of genera. The stem genus Atreta (Late Triassic to Late Cretaceous) has aragonitic denticulate hinge teeth on the right valve articulating with pitted sockets on the left valve. The same arrangement is present in Neoatreta n. gen. (Paleocene? Miocene to Recent) but with the appearance of extensive calcitic overarching of the resilifer. In Dimyella (Eocene to Recent), aragonitic teeth and sockets are still present but are modified into hook shaped denticulate teeth with corresponding sockets. All three of these genera have inner aragonitic crossed-lamellar shell layers that extend well outside the pallial line. In contrast, Dimya (Eocene to Recent) and Basiliomya (Pliocene to Recent) comprise a second clade in which aragonitic hinge teeth are absent and hinge articulation is calcitic, derived from the calcitic rim. Dimya has only weak hinge articulation and has an aragonitic inner shell layer delimited by the pallial line; in Basiliomya calcitic hinge teeth are more prominent and the entire inner shell layer inside the pallial line is foliated calcite. Diploschiza (Cretaceous, Albian to Maastrichtian), here reinstated from synonymy with Atreta, is probably a precursor of the Dimya-Basiliomya clade based on incipient calcitic hinge teeth. Predation pressures probably drove the evolution of this cemented family from its original habitat on hardgrounds in moderately deep water into much greater depths or into cryptic habitats, including submarine caves.

New combinations are Dimyella malnatrensis (Corselli and Bernocchi), D. molokaia (Dall, Bartsch, and Rehder), D. similis (v. Koenen), Neoatreta dissimilis (Tate), N. filipina (Bartsch), N. kaiparaensis (Laws), N. phaidra (Woodring), and N. plana (Martin).

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