THE LAS CASCADAS FOSSIL ASSEMBLAGE: BIOSTRATIGRAPHIC AND PALEOBIOGEOGRAPHIC IMPLICATIONS OF THE OLDEST MAMMALS FROM THE PANAMA CANAL AREA, CENTRAL AMERICA

Recently discovered early Miocene mammals from the Panama Canal offer a unique opportunity to understand the paleobiogeographic evolution of southern Central America prior to the formation of the Isthmus of Panama. The new Las Cascadas fossil assemblage is distributed throughout the uppermost part of the Las Cascadas Formation, a volcaniclastic and tuffaceous sequence that includes the first evidence of land exposure in the Panama Canal basin. The new fossil association includes the oldest record of North American mammalian taxa in Central America such as floridatraguline camelids, anthracotheres, parahippine equids, amphicyonids, mustelids, sciurid and geomyoid rodents, bats, protoceratids, moschids, and peccaries. While interpreted biochronological relationships of the small camelids and the presence of the European immigrant amphicyonid Cynelos sp. constrained the age of the upper portion of the Las Cascadas Formation to ~20-23 Ma, a new age determination using U-Pb in magmatic zircons (20.93 +/- 0.17 Ma) of an interbedded tuff places it at the beginning of the late Arikareean North American Land Mammal Age (NALMA). This new age also supports a tropical origin for the floridatraguline camels based on primitive characters shared by the Las Cascadas camels and a small camel from the earliest Miocene Buda Local Fauna (L. F.) from Florida. The faunal composition of the mammalian fossil assemblages from the Panama Canal area, including the younger (~19 Ma) Centenario Fauna, support the presence of an early Miocene (Arikareean) paleobiogeographic province connected with Mexico, the Texas Gulf Coast, and Florida. The inferred older appearance of terrestrial mammals in southern Central America suggests that ungulates appearing in the Miocene of North America may have had their origins in Central America. The remarkable absence of floridatraguline camelids and primitive parahippine equids in the Centenario Fauna suggests that terrestrial mammals entered Central America by the earliest Miocene (Arikareean NALMA) before dispersing northward from the New World tropics during the Hemingfordian NALMA, reaching temperate areas of the Gulf Coast and Florida. Discovery of more complete fossils from the Panama Canal would be critical for further evaluating this hypothesis.