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

摘要

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 δ~(13)C of all samples is -9.8 ± 1.2‰, indicating that the diet of Gomphotherium was dominated by C_3 biomass. If Gomphotherium habitually foraged on water-stressed plants in arid habitats, which would have δ~(13)C values higher than the average composition for C_3 and C_4 plants, then all but one individual in this study consumed less than 50% C_4 biomass. The maximum percentage of C_4 in the diet would be much lower if food plants had average δ~(13)C values or if Gomphotherium foraged in closed-canopy habitats. Differences in δ~(13)C 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 C_4 vegetation in the diet of Gomphotherium during the Miocene, and none of the serially sampled tusks exhibit seasonally varying δ~(13)C profiles. For most of its history in North America, Gomphotherium was a C_3 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.