<i>Choromytilus</i> Soot-Ryen (Mollusca, Bivalvia) in the Miocene of Tierra del Fuego, Argentina

Abstract

The Carmen Silva Formation is a Miocene unit exposed at several localities throughout the northern half of the island of Tierra del Fuego in Argentina. It includes sandstones and conglomerates carrying a rich fauna of invertebrates -mainly mollusks- represented by at least 45 species. A survey of this fauna reveals the presence of a new species of Mytilidae that appears to belong in Choromytilus Soot-Ryen, a genus including common species living along the Pacific coast of America between California and the Straits of Magellan, and also in widely separate areas such as South Africa, Kerguelen Island, Australia and New Zealand. Despite the fact that the bearing rocks are either a conglomerate or else coarse sandstone and that the shells are thin and fragile, they are remarkably well preserved and show enough key characters to allow a correct taxonomic placement. These include the smoothness of the shell, the dysodont hinge bearing a small vestigial anterior tooth in the right valve that fits into a groove in the left valve hinge, and the compact resilial ridge. Such a character combination points towards its inclusion in Choromytilus. The significance of this record lies in the fact that it indicates that this mussel genus already inhabited the coasts of Patagonia by the early Miocene, earlier than any previous record suggests. The paleobiogeographic implications of such an extended record still have to be assessed, but it appears that Choromytilus may have been an early constituent of the Neogene faunas of the southern part of South America and from there may have spread eastward, probably aided by the circumantarctic current in a similar manner to that inferred for other groups. However, the exact mechanism of such long reaching migration remains to be solved.