CELL-SEARCHING: DID HABITAT OPENNESS PARALLEL GRASS DOMINANCE IN THE CENOZOIC ASSEMBLY OF GREAT PLAINS GRASSLANDS?

The Great Plains Region accounts for approximately one million square miles of the continental interior of North America. The grassland ecosystems that dominate the region are ecologically, economically, and culturally important. Microscopic plant silica (phytolith) evidence suggests that open, grass-dominated habitats began to spread in the region approximately 25 million years ago, and became the dominant vegetation type by the end of the Miocene. This interpretation is largely based on the relative proportion of morphotypes diagnostic of open habitat grasses in fossil phytolith assemblages. However, whereas these data reliably indicate when grasses became ecologically important, they are more ambiguous with regards to canopy structure of early grass-dominated vegetation and the habitats that they replaced. In particular, it is unclear whether openness preceded or coincided with the rise to ecological dominance of open habitat grasses. Although several proxies have been used to address this question (e.g., fossil mammal functional morphology) their signals are mixed and they provide indirect evidence.

To address these outstanding questions, we use a recent method for estimating ancient canopy cover (reconstructed Leaf Area Index; rLAI) from the morphology of epidermal phytoliths. These phytoliths form when silica fills epidermal cells that change in shape and size with changing light conditions. We measured epidermal phytoliths from nine late Eocene–early Miocene sites. Reconstructions suggest that habitats remained closed through much of this period, with canopy cover comparable to modern closed forests at all sampled sites. Although preliminary, these results are consistent with the persistence of a closed canopy in the region even after open habitat grasses became ecologically dominant. Currently, we are working to increase the number of phytoliths sampled at each site, and to expand the geographic and temporal coverage our dataset by incorporating additional fossil localities. Ultimately, understanding how habitat structure changed during the lead up to, and initial expansion of grasslands in the Great Plains Region will contribute to a better understanding of the drivers and consequences of this major transition.