Novel method to determine paleodiet of extinct equid Merychippus sp. using dental calculus
Within the Miocene Epoch, the emergence of grasslands within North America coincided with the incidence of higher-crowned teeth (hypsodonty) within the proto-horse Merychippus that allowed for the inclusion of these expanding grasslands as a food source. As herbivorous paleofauna consume plant matter, microscopic remains become incorporated within dental calculus and, due to their diagnostic morphology, can be used to identify dietary components. In Merychippus, the recovery of plant microfossils holds the potential to provide greater clarity on the paleodiet of these equids. In the present study, the purpose was to quantify and compare Merychippus paleodiet constituents among sample groups. Calculus samples obtained from four isolated individual Merychippus teeth contained plant microfossils that reflect dietary elements. Microfossil assemblages from each Merychippus specimen were identified to the categories of definite and possible hardwood, indeterminate grass, pooid grass, sedge, tracheid and plant fiber microfossils based upon morphological characteristics. Abundant in all assemblages were microfossils derived from hardwood plant species in the form of leaf epidermal aggregate pieces and vessel elements, indicating a possibly significant dietary component. In addition, sedge phytoliths present in three assemblages and pooid phytoliths present in all assemblages reflect the inclusion of plants within Family Cyperaceae and the Poaceae Subfamily Pooideae, respectively. The combined presence of hardwood leaf epidermal aggregate, vessel elements, and pooid grass phytoliths within all fossil assemblage groups indicated a mixed browse/grass diet that included grass of definite C3 photosynthetic character for all Merychippus specimens. Within a broader context, this study generalizes aspects of the Merychippus paleoenvironment, and stands as an evaluation for the sole use of microfossils derived from dental calculus as a means for diet determination.