THE mid-Cenozoic immigration of rodents and primates to South America (when it was widely isolated by oceans) represents a pre-eminent problem in the biogeographical history of placental mammals. The unexpected discovery of South America's earliest rodent in the central Chilean Andes provides information critical to resolving the source area and primitive morphology of South American caviomorphs, suggesting an African origin for the group. This rodent is part of a new fossil mammal fauna1, the first diverse assemblage known for a critical 15-25 million year gap in the fossil record. We report here that cooccurrence of numerous higher-level taxa otherwise restricted to older or younger intervals identifies this fauna as representing a new biochronological interval preceding the Deseadan (South American Land Mammal Age), previously the earliest occurrence of rodents and primates on the continent. Radioisotopic dating corroborates biostratigraphy in identifying the new Andean rodent as the earliest known from the continent.
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