Ancient latitudinal gradients of C 3 /C 4 grasses interpreted from stable isotopes of New World Pleistocene horse ( Equus ) teeth

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 C 3 and C 4 grasses. The carbon data presented here indicate a gradient (δ 13 C range of 10 parts/mil) in the relative proportion of C 3 and C 4 grasses between high latitude and equatorial Equus samples. The largest amount of change from C 3 to C 4 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 (δ 18 O range of 20 parts/mil) between high‐latitude and equatorial Equus samples. The basic pattern of latitudinal gradients of C 3 /C 4 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.