Dietary reconstruction of Miocene Gomphotherium (Mammalia, Proboscidea) from the Great Plains region, USA, based on the carbon isotope composition of tusk and molar enamel

The Miocene of the Great Plains of North America has long been recognized as an interval of major ecological reorganization. To reconstruct the dietary response of the proboscidean Gomphotherium to Miocene ecosystem change in the Great Plains, we analyzed the carbon isotope composition of 185 serial samples of tusk enamel from 17 individuals and bulk samples of posterior molars from 15 individuals of Gomphotherium from localities in the Great Plains ranging in age from the Early Barstovian land mammal age (ca. 15 Ma) to the Early Hemphillian land mammal age (ca. 8 Ma). Sets of samples from each tusk were designed to encompass about 1 year of tusk growth. Based on cheek tooth morphology, Gomphotherium is thought to be a browser with a diet primarily of dicots. The mean δ¹³C of all samples is -9.8±1.2‰, indicating that the diet of Gomphotherium was dominated by C₃ biomass. If Gomphotherium habitually foraged on water-stressed plants in arid habitats, which would have δ¹³C values higher than the average composition for C₃ and C₄ plants, then all but one individual in this study consumed less than 50% C₄ biomass. The maximum percentage of C₄ in the diet would be much lower if food plants had average δ¹³C values or if Gomphotherium foraged in closed-canopy habitats. Differences in δ¹³C values between specimens from the southwestern US and the Great Plains, as well as between some coeval specimens from Nebraska, suggest geographic differences in either diet or typical foraging habitat. The data do not indicate a trend toward inclusion of more C₄ vegetation in the diet of Gomphotherium during the Miocene, and none of the serially sampled tusks exhibit seasonally varying δ¹³C profiles. For most of its history in North America, Gomphotherium was a C₃ browser or a mixed feeder with a preference for browse. Our results indicate that areas of wooded habitat sufficient to support herds of large-bodied herbivores remained available in the Great Plains through the Late Miocene.