Towards More Accurate Paleodietary Reconstructions: The Need for Geochemical and Isotopic Analyses of Fossilized Plants

An old problem in vertebrate paleontology is determining what extinct organisms ate. Because carbon isotope ratios of tooth enamel are related to those of ingested food, isotopic studies of enamel complement those of tooth morphology and microwear. Application of carbon isotope studies is most straightforward for herbivores, and dietary inferences are made by comparing carbon isotope data from enamel with isotopic data from modern plants.

Although isotopic data from modern plants provide a basis for making broad dietary distinctions (e.g. preference for C3 versus C4 plants or for open versus closed C3 vegetation) this approach has limits. One is lack of modern studies, with the result that isotopic differences among plants observed in one setting are assumed to be appropriate across a range of environments. Another critical limitation is that modern and fossil plants are not the same, and ancient taxa that are morphologically similar to modern plants may have had different physiologies and environmental preferences. Thus it is difficult to infer specific dietary distinctions for herbivores (e.g. preference for herbaceous versus woody angiosperms or angiosperms versus gymnosperms), particularly for pre-Miocene time periods.

To overcome these problems, it is necessary to measure carbon isotope ratios of fossil plants found associated with herbivores (or that are at least the same age). The challenge for paleobotanigeochemists is making these measurements. Leaf cuticle provides an excellent substrate for analysis, but can be rare in association with plant macrofossils. More commonly, such fossils are comprised of an ‘organic' residue that is not/poorly characterized. If it is possible to determine the organic compounds present, and to identify one that is primary and that is common across taxa, then it will be possible to delineate carbon isotopic differences among ancient plants. This will in turn allow for more accurate paleodietary studies of ancient vertebrates.