Observations on the Genus Dimya (Mollusca: Bivalvia) in the Cenozoic Faunas of the Western Atlantic Region

Abstract

The bivalved molluscan genus Dimya, heretofore known to be represented in the western Atlantic faunas by one Miocene species and variety from the Dominican Republic (D. grandis and var. divaricata, both of Dall, 1896) and two Recent species from the Caribbean Sea and adjacent areas (D. argentea Dall, 1886, and D. tigrina Bayer, 1971), proves to be more widely distributed in the paleontological record. Three new species are described: D. alieni, from the Cook Mountain Formation, middle Eocene of Louisiana; D. rufaripa, from the lower Oligocene Red Bluff Formation of Mississippi; and D. fimbricostata, from the lower Pleistocene Moin Formation of Costa Rica. In addition, other specimens from the Moin Formation are referred to D. tigrina Bayer, and, three valves from the well-known Bowden, Jamaica, locality serve to document the occurrence of the genus in that fauna. The geologic and the geographic distribution of the known fossil and Recent species of Dimya and Dimyodon is discussed. It is concluded that Dimya probably is descended from the Upper Triassic to middle Oligocene genus Dimyodon, possibly first appearing in the late Cretaceous, but certainly present in the Paleocene and achieving widespread distribution in the middle and upper Eocene of the Old World Tethyan faunas before becoming relatively rare and apparently confined to the deeper water facies of Oligocene and later faunas. The Pleistocene forms here described constitute the first known records of its occurrence in deposits of that epoch. A number of Recent species are known from the western Atlantic and the Pacific faunas; the average depth for the 47 available dredging records for these species is 311 meters, the few available bottom temperature records average slightly over 59°F (15°C). Annotated lists of those species that have been referred, at one time or another, to Dimya and Dimyodon are appended.