BIOGEOGRAPHIC SIGNIFICANCE OF PHYLOGENETIC RELATIONSHIPS AMONG NEOGENE CORBULIDAE OF TROPICAL AMERICA

Corbulid bivalves are ubiquitous and often abundant members of late Mesozoic and Cenozoic faunas from tropical to warm temperate regions of the world. Corbulids were important members of Neogene paleocommunities in tropical America, and underwent significant morphologic evolution, speciation, and extinction in this region during the Neogene. However, phylogenetic hypotheses concerning this family are lacking. A preliminary phylogenetic analysis using maximum parsimony for shell characters from genera and subgenera occurring in the Neogene of the western Atlantic and Caribbean reveals three clades. One clade includes Caryocorbula, Bothrocorbula, and Hexacorbula, which are characterized by elongate ovate shells with pointed rostra, as well as several hinge apomorphies. A second clade includes Bicorbula, Varicorbula, Panamicorbula, and Tenuicorbula, which incorporate a variety of shell shapes but generally have rather inflated shells and relatively prominent chondrophores. A third clade includes Corbula s.s., previously reported only from the Recent of west Africa. This genus is represented in tropical America by a single species (C. hexacyma, previously placed in Hexacorbula) from the Miocene Gatun Formation of Panama. These results indicate that tropical American corbulids are not monophyletic, and represent several major corbulid clades each of which can be examined for it's independent evolutionary history in the face of the significant biotic and environmental changes occurring in tropical America during the Neogene.