South American fossil mammals and carbon isotopes: a 25 million-year sequence from the Bolivian Andes

Abstract

Cenozoic mammals of South America include an array of endemic herbivores with adaptations for browsing or grazing that evolved in parallel with separate herbivore radiations in the northern hemisphere. In one South American order, the Notoungulata, the advent of high-crowned teeth, which is classically interpreted as an adaptation for feeding on grasses (grazing), occurred at least 10 m.y. earlier than in northern hemisphere mammals. In order to understand these ancient diets and coevolutionary dietary shifts (from C 3 to C 4 plants), stable carbon isotopes ( δ 13C) from primary structural carbonate were analyzed from 41 tooth enamel samples of South American herbivores and northern hemisphere immigrants. These samples were taken from 10 localities spanning from about 25 m.y. to 7500 yr ago, i.e., during Tertiary isolation of South American followed by the Great American Interchange. Eight of these localities are currently situated at high-elevations (between 3200 and 4000 m) but are estimated to have been at much lower elevations prior to Andean uplift. Although there is a predominance of high-crowned, presumably grazing herbivores at Salla, the oldest locality sampled, δ 13C values between −10.7 to −8.4% 0 indicate feeding on C 3 plants. During the middle Miocene (12.5–15 m.y.) δ 13C values between −10.7 and −7.5% 0 also suggest C 3 plant communities, whereas during the late Miocene δ 13C values between − 8.2 to −5.5% 0 suggest mixed C 3/C 4 plant communities. Clear evidence of C 4 plants (i.e., principally grasses) does not exist in the carbon isotopic signature of fossil mammal teeth until the Plio-Pleistocene, although it is less widespread because of elevational effects of the uplifting Andes. During the height of the Great American Interchange at 1 Ma, carbon isotopic data suggest food resource partitioning and specialization among immigrant herbivores such as horses and camels, whereas mastodons were more generalized feeders. There is considerable overlap of δ 1C values between endemic and immigrant taxa, suggesting no discernable resource partitioning between them. The δ 13C values of South American endemic mammals suggest that the low-crowned litoptern Macrauchenia fed on C 3 plants whereas the high-crowned notoungulate Toxodon fed on C 4 plants, presumably grasses. Although high-elevation (> 3000 m) sites during the Plio-Pleistocene indicate C 3-dominated communities, similar-aged localities at lower elevations (< 2000 m) have mixed C 3/C 4 plant communities. The present-day biotic communities of the central Andes were established during the late Miocene and early Pliocene prior to about 3.5 Ma.