To reconstruct the paleodiet and habitat preference of gomphotheres, we measured the carbon and oxygen isotope composition of 68 bone and tooth samples for three species of Gomphotheriidae from 24 different localities (Argentina, Bolivia, Ecuador, Chile, and Brazil). Additionally, we measured the isotopic oxygen in the phosphate of 30 samples to control diagenetic alteration. We calculated the correlation between pairs of δ18Op-δ18Oc values for enamel, dentine, and bone, taken from the same individual in order to verify whether the oxygen of structural apatite carbonate was in equilibrium with body water. Because of the good correlation obtained among pairs of the three skeletal components, we considered the δ13C results of all components to be equally representative of both gomphothere groups, and we used them collectively in the analysis of the data.
To compare the different groups of specimens, we divided the samples into six groups, taking into account their taxonomy as well as their geographic and stratigraphic distribution. Cuvieronius specimens from Chile were exclusively C3 plants eaters, whereas specimens from Bolivia and Ecuador had a mixed C3-C4 diet. Stegomastodon showed a wider range of dietary adaptations. Specimens from Quequén Salado in Buenos Aires Province were entirely C3 feeders, whereas the diet of specimens from La Carolina Peninsula (Ecuador) was exclusively C4. The remaining South American groups analyzed were C3-C4 mixed-feeders. Carbon isotope composition of bone and teeth decreased as latitude increased. We found evidence of an exclusively C3-dominated diet at approximately 35–41°S. This result confirms that ancient feeding ecology cannot always be inferred from dental morphology or extant relatives. Data from middle and late Pleistocene indicated that, over time, there was an adaptive change in paleodiet from predominantly mixed-feeders to more specialized feeders. We propose that this dietary evolution was one of the causes that forced gomphotheres to extinction in South America. In addition, the data presented in this paper suggest that because of the different feeding preferences among mastodons, mammoths, and gomphotheres, only the bunodont gomphotheres reached South America.