Revised geochronology of the Casamayoran South American land mammal age: Climatic and biotic implications

Isotopic age determinations (Ar-40/Ar-39) and associated magnetic polarity stratigraphy for Casamayoran age fauna at Gran Barranca (Chubut, Argentina) indicate that the Barrancan "subage" of the Casamayoran South American Lan d Mammal "Age" is late Eocene, 18 to 20 million years younger than hitherto supposed. Correlations of the radioisotopically dated magnetic polarity st ratigraphy at Gran Barranca with the Cenozoic geomagnetic polarity time sca le indicate that Barrancan faunal levels at the Gran Barranca date to withi n the magnetochronologic interval from 35.34 to 36.62 megannums (Ma) or 35. 69 to 37.60 Ma. This age revision constrains the timing of an adaptive shif t in mammalian herbivores toward hypsodonty. Specifically, the appearance o f large numbers of hypsodont taxa in South America occurred sometime betwee n 36 and 32 Ma (late Eocene-early Oligocene), at approximately the same tim e that other biotic and geologic evidence has suggested the Southern high l atitudes experienced climatic cooling associated with Antarctic glaciation.