Carbon isotope ratios of mammalian tooth enamel have long been used to infer the diet of animals in the past, and hence the ecological makeup of their surroundings. In general these studies have relied on large differences in carbon isotope ratios between plants utilizing C4 and C3 photosynthetic pathways to infer changes in paleodiet and hence paleoecology over time and space. However, significant carbon isotope variations can occur within C3 plants alone as a result of differences in light, water availability, humidity, position in the forest canopy, temperature, and altitude. Therefore carbon isotope ratios of mammalian tooth enamel for pre-Miocene time periods when C4 plants were uncommon may still be used as valuable paleoecological and paleoenvironmental indicators.

This study focuses on the Eocene time period and relies on the analysis of tooth enamel carbonate from Coryphodon, a large herbivorous mammal that lived in riverine environments. Intra-tooth variations in carbon isotope ratios of 1 to 2 per mil are observed for samples from the Bighorn Basin of Wyoming, and are weakly correlated with seasonal variations in oxygen isotope ratios. These carbon isotope variations may reflect seasonal changes in habitat and/or types of C3 plants eaten by Corphodon. Alternatively they may reflect the influence of temperature or water/nutrient availability on carbon isotope ratios of a single C3 plant type. In addition to seasonal variations, carbon isotope ratios of Coryphodon vary with latitude. Samples from San Jan Basin (~36 N paleolatitude), Bighorn Basin (~45 N) and Ellesmere (~75 N) have average values of –15.0, 13.0, and –12.5 per mil respectively. The resulting negative correlation between carbon isotope ratios and temperature/latitude is different than that observed for C3 plants at the present time, and may instead reflect the influence of biological productivity, precipitation, or water availability.

These results confirm that carbon isotope ratios of tooth enamel can vary significantly in a C3 world, and that seasonal and spatial patterns can provide information regarding mammalian habitat and environmental conditions of the past.