Chronological correlation, paleoecology, and paleobiogeography of the late Cenozoic South American Rionegran land-mammal fauna: a review

Abstract

The rise and fall of the Rionegran land-mammal fauna of the Río Negro Formation (northern Patagonia), are related to the regression of the "Paranean" sea (Middle to Late Miocene), and to the environmental changes brought about by the Andean diastrophic Diaguita Phase (Late Pliocene). A new fission track age of 4.41 +/- 0.50 Ma has been obtained for the volcanic ash horizon in the upper part of the Río Negro Formation at the mouth of the Río Negro river. This places its mammal fauna at the beginning of the Montehermosan Land-Mammal Age (Pliocene), with a time range of 6 to 3 Ma. Geological and biological evidences show that the Rionegran sediments exposed in the western areas are older than the eastern ones, and can be assigned to the Huayquerian Land-Mammal Age (Late Miocene), ranging approximately from 9 to 6 Ma. Outside South America, the Rionegran fauna can be correlated with the Hemphillian and early Blancan North American faunas, and with the Turolian and earliest Ruscinian faunas of Europe. Rionegran mammals lived in flood-plain environments characterized by pools and swamps, alternated with woodland and grassland. Similarities of the Rionegran fauna with the apparently coeval ones of central, NE, and NW Argentina, indicate that subtropical environmental conditions spread from northern Argentina to northern Patagonia during the Late Miocene-Pliocene.