MARINE BIOGEOGRAPHY AND THE RISE OF THE ISTHMUS OF PANAMA: IS THERE EVIDENCE FOR AN EARLY CLOSURE OF THE CENTRAL AMERICAN SEAWAY?

The molluscan fossil record of the Panama Isthmus, southern Caribbean and tropical eastern Pacific provides a window into the Cenozoic history of transoceanic communication through the Central American Seaway (CAS). The majority of sedimentary basins in this region accumulated in shallow shelfal paleoenvironments and preserve an excellent fossil record that ranges back to the Eocene. In this study a large dataset of molluscan occurrences was compiled from the Middle Eocene-Pleistocene fossil record as well as Recent assemblages of the Caribbean and tropical eastern Pacific. Statistical similarity among faunal assemblages throughout both ocean basins was used to define the extent of biogeographic provinces in each time interval. The number of shared species between the ocean basins, and their similarity in taxonomic composition, was also measured through the time series. Lastly, the number of paciphilic and atlantiphilic taxa (species of genera whose distribution became constricted to the Pacific and Atlantic, respectively) present in each ocean basin was also monitored in each time interval.

A single biogeographic province had extended from northern Peru into the central Caribbean during the Paleogene through early Neogene. Analysis of faunal assemblages on both sides of the isthmus indicates that shoaling of the CAS disrupted this province between the Late Miocene and Early Pliocene, ultimately forming the present-day Caribbean and Panamic provinces. Similarity in taxonomic composition in two ocean basins experienced sharp declines between the Middle Miocene and Late Miocene, and again between the Early Pliocene and Late Pliocene. The number of paciphilic taxa recorded from the western Atlantic also decline most significantly between the Late Miocene and Late Pliocene, while the decline in atlantiphilic taxa in the eastern Pacific was greatest following the Early Pliocene. Variations in the timing of change for each of these biogeographic metrics suggest that shoaling of the CAS was a prolonged event. Additionally, disruption of the once contiguous tropical eastern Pacific and Caribbean marine province took place much earlier than previous recognized.