Seven million years of North American herbivory

Melissa Pardi, Research & Collections Center, Illinois State Museum, Springfield, IL and Larisa R. G. DeSantis, Biological Sciences, Vanderbilt University, Nashville, TN

Background/Question/Methods
The fossil record provides an opportunity to place modern biodiversity issues in a broader historical context. Paleoecological interpretations of past ecosystems inferred from vertebrate fossil assemblages are largely based on our understanding of dietary and habitat preferences of fossil taxa. However, interpretations may differ depending on whether taxon- or individual-level traits are considered. Morphological traits inform interpretations of diet, and are typically described at the level of a taxon. Individuals within a taxon are approximated as being ecologically similar, and this paradigm forms the basis for many interpretations of fossil assemblages. In contrast, stable isotope proxies provide insight into the realized diets of individuals, and provide a means for studying ecological variation over time and space within a taxon. Our study presents a synthesis of the isotopic ecologies (δ13C from tooth enamel) of North American mammalian herbivores since ~7 Ma. We compare these individual-level dietary data with diets inferred from taxon tooth-crown height. We ask if morphological interpretations of dietary behavior agree with stable isotope proxy data. We also ask if grazing-adapted (high-crowned) taxa are specialists, as they are often represented in the literature, or if the ability to graze permits taxa to broaden their dietary niches.
Results/Conclusions
We demonstrate that while taxa with low-crowned teeth are specialized as browsers, high-crowned taxa often have broader diets that include more forbs and shrubs than anticipated. It has long been accepted that morphology imposes limits on diet; this synthesis supports prior work that herbivores with “grazing” adaptations at the taxon-level, such as high-crowned teeth, have the ability to consume grass but are also able to eat other foods. Individual “grazers” may in fact browse for extended periods of time. We find that taxa with grazing adaptations tend to, overall, have broad diets, but specialize at the local level (~30 to 60% of a taxon’s overall breadth). This synthesis demonstrates that “grazing-adapted” taxa are varied in their diets across space and time, and this flexibility may have reduced competition among ancient herbivores in communities that were more diverse than today. Modern systems are significantly less species diverse than those of the past, and many niches that were filled as recently as ca. 13,000 yr B.P. (such as grazing megafauna) are now absent, or nearly absent, from many ecosystems.