NEW FRESHWATER MOLLUSCS FROM THE PANAMA CANAL MIOCENE CUCARACHA FORMATION AND PALEOENVIRONMENTAL RECONSTRUCTION IMPLICATIONS

The Cucaracha Formation (early Miocene, 19-18 Ma) of the Panama Canal Basin preserves a rare record of terrestrial neotropical paleoenvironments and is important in understanding the history of biotic interchange between North and South America in the geologic past. We describe a new assemblage of freshwater molluscs from the Cucaracha Fm., the paleoenvironment in which they were preserved, and potential paleobiogeographic implications. The mollusc assemblage consists of 1 taxon of freshwater mussels (Superfamily Unionoidea) and 4 taxa of freshwater gastropods (Superfamily Cerithioidea). The mussels superficially resemble unionid bivalves, but until specimens are discovered having muscle scars and dentition, further identification will be reserved. Three of the gastropod taxa are tentatively assigned to the family Thiaridae due to the following characteristics: spindle shaped shells, distinctive spiral ribs, an anteriorly truncated columella, and separate inner/outer lips of the aperture. Two of these thiarid morphotypes may belong to the genus Aylacostoma, based on a distinctive ornamented shoulder on later whorls, whereas the third likely represents a species of Hemisinus. The fourth gastropod morphotype possesses distinctive axial ribbing, distinctive spiral ribs only on the last whorl, and connected inner/outer lips of the aperture. The mollusc assemblage is preserved in the horizontally laminated litharenite component of an interbedded sandstone/organic rich mudstone horizon. Mudstone intraclasts within the sandstone are consistently oriented and occasionally imbricated. The interbedded sandstone/mudstone has an erosional lower contact with a well developed red paleosol, which was previously interpreted as representing a dry forest soil. These sedimentological characteristics suggest the mollusc fauna was preserved in a small, high flow velocity fluvial system within a seasonally dry tropical forest. Prior to this study, the gastropod genera Aylacostoma and Hemisinus are only known from South American deposits during the Miocene. If our preliminary identifications are correct, the Cucaracha mollusc fauna provides evidence of an early dispersal event of freshwater invertebrates between Central and South America, well before the mammalian Great American Biotic Interchange.