SOUTH-AMERICA EARLIEST RODENT AND RECOGNITION OF A NEW INTERVAL OF MAMMALIAN EVOLUTION

THE mid-Cenozoic immigration of rodents and primates to South America (when it was widely isolated by oceans) represents a pre-eminent probl em in the biogeographical history of placental mammals. The unexpected discovery of South America's earliest rodent in the central Chilean A ndes provides information critical to resolving the source area and pr imitive morphology of South American caviomorphs, suggesting an Africa n origin for the group. This rodent is part of a new fossil mammal fau na1, the first diverse assemblage known for a critical 15-25 million y ear gap in the fossil record. We report here that co-occurrence of num erous higher-level taxa otherwise restricted to older or younger inter vals identifies this fauna as representing a new biochronological inte rval preceding the Deseadan (South American Land Mammal Age), previous ly the earliest occurrence of rodents and primates on the continent. R adioisotopic dating corroborates biostratigraphy in identifying the ne w Andean rodent as the earliest known from the continent.