Cenozoic Terrestrial Ecosystem Evolution in Argentina: Evidence from Carbon Isotopes of Fossil Mammal Teeth

Abstract

New 813C values are presented for tooth enamel of 65 herbivorous fossil mammal specimens from lowand middle-elevation localities in Argentina ranging in age from Deseadan (ca. 27 Ma) to Lujanian (potentially as young as ca. 10,000 yrs BP). Prior to the Huayquerian (late Miocene; 96.5 Ma), all faunal levels have mean 813C values of about -11 ? loo, indicating ancient terrestrial ecosystems with predominantly C3 plants. During the late Miocene through Pleistocene (i.e., after ca. 8 Ma), there is a statistically significant carbon isotopic shift so that: (1) taken together, later faunal levels have a combined mean 813C value of 7.6%o and (2) the data set includes significantly larger ranges of 13C values (i.e., individual specimens are as positive as ca. +1%o). These post-8 Ma data indicate isotopically mixed terrestrial plant communities containing a significant component of C4 grasses. Both the magnitude and timing of this ca. 3.5%oo mean carbon isotopic shift is consistent with data reported previously from numerous late Miocene localities in the northern hemisphere and selected equatorial regions. Analysis of the 13C values for high-crowned, presumed grazing mammals from post-8 Ma localities between 21 to 35?S lat. (in Argentina and Bolivia) indicate an isotopic gradient in which mean &3C values (and hence percentage of C3 plants) are proportional to latitude. The &13C values presented here indicate that the original spread of grasslands during the middle Tertiary, and the so-called precocious hypsodonty of corresponding South American herbivores, occurred in a regime of predominantly C3 grasses. The modern ecological landscape in which grasses (and corresponding grassland biomes) are predominantly C4 in mid-latitudes, is a relatively recent global event that is first recorded in South America during the late Miocene and became relatively widespread thereafter.