SOUTH-AMERICAN FOSSIL MAMMALS AND CARBON ISOTOPES - A 25-MILLION-YEARSEQUENCE FROM THE BOLIVIAN ANDES

Cenozoic mammals of South America include an array of endemic herbivor es with adaptations for browsing or grazing that evolved in parallel w ith separate herbivore radiations in the northern hemisphere. In one S outh American order, the Notoungulata, the advent of high-crowned teet h, which is classically interpreted as an adaptation for feeding on gr asses (grazing), occurred at least 10 m.y. earlier than in northern he misphere mammals. In order to understand these ancient diets and coevo lutionary dietary shifts (from C-3 to C-4 plants), stable carbon isoto pes (delta(13)C) from primary structural carbonate were analyzed from 41 tooth enamel samples of South American herbivores and northern hemi sphere immigrants. These samples were taken from 10 localities spannin g from about 25 m.y. to 7500 yr ago, i.e., during Tertiary isolation o f South American followed by the Great American Interchange. Eight of these localities are currently situated at high-elevations (between 32 00 and 4000 m) but are estimated to have been at much lower elevations prior to Andean uplift. Although there is a predominance of high-crow ned, presumably grazing herbivores at Salla, the oldest locality sampl ed, delta(13)C values between -10.7 to -8.4 parts per thousand indicat e feeding on C-3 plants. During the middle Miocene (15-12.5 m.y.) delt a(13)C values between -10.7 and -7.5 parts per thousand also suggest C -3 plant communities, whereas during the late Miocene delta(13)C value s between -8.2 to -5.5 parts per thousand 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 su ch as horses and camels, whereas mastodons were more generalized feede rs. There is considerable overlap of delta(13)C values between endemic and immigrant taxa, suggesting no discernable resource partitioning b etween them. The delta(13)C values of South American endemic mammals s uggest that the low-crowned litoptern Macrauchenia fed on C-3 plants w hereas the high-crowned notoungulate Toxodon fed on C-4 plants, presum ably grasses. Although high-elevation (> 3000 m) sites during the Plio -Pleistocene indicate C-3-dominated communities, similar-aged localiti es at lower elevations (< 2000 m) have mixed C-3/C-4 plant communities . The present-day biotic communities of the central Andes were establi shed during the late Miocene and early Pliocene prior to about 3.5 Ma.