THE NEW WORLD TROPICS AS A CRADLE OF BIODIVERSITY DURING THE EARLY MIOCENE: CALIBRATION OF THE CENTENARIO FAUNA FROM PANAMA

New excavations along the Panama Canal have yielded a growing Miocene vertebrate assemblage referred to as the Centenario Fauna. Despite its proximity to South America, the mammals of the Centenario Fauna have entirely North American affinities. The Centenario Fauna, which is distributed throughout a ~115 m stratigraphic interval encompassing the uppermost Culebra and Cucaracha formations, mixes taxa diagnostic, or characteristic, of three successive North American Land Mammal Ages (NALMAs) as these are defined from temperate North America, i.e., the Arikareean, Hemingfordian, and Barstovian. This sympatry can neither be explained by superpositional biostratigraphy (because taxa of different NALMAs exist at the same horizons), nor by reworking (REE analyses refute this explanation), and thus we assert that these associations are biologically meaningful. Previously published age determinations using Sr-ratio and U-Pb methods constrain the age of the lower limit of the Centenario Fauna to no younger than ~19 Ma, but the upper limit has remained problematical. A fresh exposure of the Cucaracha tuff, which is demonstrably interbedded within the principal reference section measured at Centenario Bridge, has yielded two new radioisotopic ages: (1) a Ar/Ar weighted plateau age of 18.94 +/- 0.83 Ma; and (2) a U-Pb zircon age of 18.81 +/- 0.30 Ma. In addition, paleomagnetic data indicate essentially a reversed polarity at the Centenario Bridge section that correlates to chron C5Er (18.78 to 19.05 Ma). Given these geochronological and paleomagnetic constraints, the age of Centenario Fauna from the Bridge section occurs within the 0.27 myr interval of chron C5Er. This essentially correlates to the late Arikareean NALMA (Ar4) interval during the early Miocene. These results have significant ramifications for biogeographic heterochrony of Arikareean, Hemingfordian, and Barstovian mammals. The ancient New World tropics during the early Miocene apparently supported a cradle of biodiversity from which numerous taxa subsequently dispersed northward, accounting for their better-known distributions in temperate North America.