TRACKING SPATIAL AND TEMPORAL CHANGES IN TERRESTRIAL ECOSYSTEMS USING CARBON ISOTOPE RATIOS OF PLANT AND ANIMAL REMAINS: EXAMPLES FROM THE LATE CRETACEOUS OF NORTH AMERICA

Carbon in fossil tooth enamel of herbivorous vertebrates has an ultimate source in plant matter ingested by them. In turn, carbon isotope ratios of plants are influenced by both the photosynthetic pathways they utilize and by local environmental conditions. Thus, carbon isotope analyses of tooth enamel from herbivorous dinosaurs paired with the analysis of organic material from plants can be used to investigate terrestrial ecosystems during the Mesozoic.

This approach can be applied to the study of paleoecology in to main ways. One is to compare carbon isotope ratios of organic material and tooth enamel samples from different sites of approximately the same age in an effort to make an ecological “map” of that time period. As an example of this approach, hadrosaurian dinosaur teeth were collected from late Campanian sites in Baja, Mexico, Texas, Utah, Montana, and Alaska. Intra-site carbon isotope variations of 2 to 6 per mil were observed, and average carbon isotope ratios between sites ranged from –10 to +3 per mil. Environmental conditions, plant ecology, and animal migration can all play a role in causing these isotopic variations, and can therefore be investigated using this approach. For example, high carbon isotope ratios from coastal localities indicate that plants utilizing C4 photosynthetic pathways may have been common at this time along the Western Interior Seaway. As another example, carbon isotope ratios among some localities do not overlap, thus indicating that animal migration between areas was limited.

Alternatively, samples can be obtained from a stratigraphic series of sites located in one area in order to study ecological changes over time. In the case of the Maastrictian Hell Creek Formation of North Dakota, carbon isotope ratios of hadrosaurian tooth enamel increase gradually from ~ – 6 to –1 per mil over the last million years of the Cretaceous. This increase may reflect the impact of temperature change or saltwater incursions associated with changing sea levels on carbon isotope ratios of plants living in the Hell creek ecosystem. Carbon isotope records from tooth enamel and organic material can be compared to paleoenvironmental information obtained from plant megafossils found in the same sediments, and to marine isotope records of the same age.