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.
Bibliographical noteFunding Information:
This research would not have been possible without the generous access to specimens provided by Richard Tedford (AMNH), Meng Jin (AMNH), and Mike Voorhies (UNSM). Lora Wingate performed the stable isotope analyses of tusk enamel samples in the University of Michigan Stable Isotope Laboratory. Some of the ideas in this paper were developed through discussions with P. Koch while D.L. Fox was a post-doc at UC, Santa Cruz. We thank Kathryn Hoppe and S. David Webb for thorough reviews. This research was supported by two University of Michigan Scott Turner Awards in Earth Sciences, a Geological Society of America Student Research Award, and a Paleontological Society Grant-in-Aid to D.L. Fox while a graduate student at the University of Michigan and NSF grant SBR-9211984 to D.C. Fisher.
- Carbon isotopes