BIOGEOGRAPHY AND BIOCHRONOLOGY OF SMALL MAMMALS (CHIROPTERA, RODENTIA) FROM THE EARLY MIOCENE OF PANAMA

About 15 species of small mammals of early Miocene age were recovered by screenwashing two geologic units along the Panama Canal in central Panama at 9° North latitude. The older Las Cascadas Formation has produced a large mammal fauna indicative of the late Arikareean (~23-20 Ma) North American Land Mammal Age (NALMA). Small mammals identified from the Las Cascadas Formation include a bat possibly belonging to the extant Phyllostomidae and three rodents, the large petauristine flying squirrel Petauristodon, a smaller sciurid, and a small geomyoid similar to Texomys. A more diverse small mammal assemblage occurs in the Centenario fauna derived from the younger Cucaracha Formation. Radioisotopic dates and paleomagnetic stratigraphy constrain the age of the Centenario fauna to between 19.1 and 18.8 Ma (latest Arikareean NALMA). There are 10 species of small mammals in the Centenario fauna: a possible phyllostomid bat; the procyonid carnivore Bassaricyonoides; and eight rodents, including two species of Petauristodon; the small sciurid Nototamias; two species of Proheteromys in the extant family Heteromyidae; and three geomyoids in the extinct family Jimomyidae, Texomys stewarti and two larger undescribed taxa. A large bat from the Las Cascadas and Centenario faunas is similar to insectivorous bats in the Neotropical family Phyllostomidae. The oldest previously known member of this family is from the early Miocene of Argentina. Early Miocene phyllostomids in both North America and South America may represent an earlier unrecognized phase of the Great American Biotic Interchange. The occurrence of the related extinct family Speonycteridae in the Oligocene of Florida indicates a possible North American origin for this group. Most other small mammal genera from the early Miocene of Panama are also known from early to medial Miocene faunas in Florida and/or Texas, including the rodents Petauristodon and Texomys and the procyonid Bassaricyonoides. A tropical North American origin for these three genera is suggested by their older late Arikareean records from Panama. A mid Cenozoic Mesoamerican mammalian fauna is corroborated by the presence of the camel Aguascalientia and the protoceratid Paratoceras in early to medial Miocene faunas in Panama, Mexico, and Texas.