Ancient latitudinal gradients of C3/C4 grasses interpreted from stable isotopes of New World Pleistocene horse (Equus) teeth

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Abstract
Carbon and oxygen isotopic data are reported from 116 Pleistocene Equus teeth from sixty‐six localities in the New World ranging from 68°N (Alaska, Canada) to 35°S (Argentina). Equus species have been predominantly grazers, and as such, carbon isotopic values of their tooth enamel provide evidence of the Pleistocene distribution of C3 and C4 grasses. The carbon data presented here indicate a gradient (δ13C range of 10 parts/mil) in the relative proportion of C3 and C4 grasses between high latitude and equatorial Equus samples. The largest amount of change from C3 to C4 grasses during the Pleistocene occurred in the mid‐latitudes between about 30 to 40°. The oxygen data, which vary proportionately with temperature, indicate a latitudinal gradient (δ18O range of 20 parts/mil) between high‐latitude and equatorial Equus samples. The basic pattern of latitudinal gradients of C3/C4 grass distribution and temperature as interpreted from these Pleistocene data is similar to the modern‐day. The use of stable isotopes of fossil herbivore teeth represents a new means to interpret Pleistocene climates and terrestrial ecology.