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

Carbon and oxygen isotopic data are reported from 116 Pleistocene Equus tee th from sixty-six localities in the New World ranging from 68 degrees N (Al aska, Canada) to 35 degrees S (Argentina). Equus species have been predomin antly grazers, and as such, carbon isotopic values of their tooth enamel pr ovide evidence of the Pleistocene distribution of C-3 and C-4 grasses. The carbon data presented here indicate a gradient (delta(13)C range of 10 part s/mil) in the relative proportion of C-3 and C-4 grasses between high latit ude 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 ab out 30 to 40 degrees. The oxygen data, which vary proportionately with temp erature, indicate a latitudinal gradient (delta(18)O range of 20 parts/mil) between high-latitude and equatorial Equus samples. The basic pattern of l atitudinal gradients of C-3/C-4 grass distribution and temperature as inter preted from these Pleistocene data is similar to the modern-day. The use of stable isotopes of fossil herbivore teeth represents a new means to interp ret Pleistocene climates and terrestrial ecology.