PHYTOLITH ASSEMBLAGE ANALYSIS: NEW INTERPRETATIONS OF GRASSLAND EVOLUTION IN THE GREAT PLAINS DURING THE LATE EOCENE TO EARLY MIOCENE

Phytolith assemblage analysis is commonly used for reconstructing vegetation types in Quaternary paleoecology but is less commonly applied to pre-Pleistocene floras. Studies of sub-modern assemblages with well-known modern analogs commonly rely on system-specific methods of inference. For ancient phytolith assemblages whose modern analogs are less well understood, a more general analytical approach is necessary. This analytic approach includes using all size fractions (2-250 microns), employing a classification scheme based on literature and a comprehensive modern phytolith reference collection, and conducting multivariate analysis of all morphotypes.

Phytolith assemblages from late Eocene to early Miocene sediments of the White River, Arikaree and Ogallala Groups in northwestern Nebraska were reanalyzed using this analytic approach, and the results compared to a previous study that used a more specific technique. In both studies phytoliths were extracted using heavy liquid separation. The two techniques (general analytic vs. specific) produced results that differ markedly. These differences are principally due to inclusion of phytoliths in the >50 micron size fraction in the more general approach, as well as refinement of the identification of phytolith morphotypes from the expanded reference collection.

Previous work suggested open grasslands were established in northwestern Nebraska by the late Eocene. In contrast, results from the general analytic approach indicate the presence of relatively closed habitats in late Eocene to early Oligocene, typified by the presence of bambusoid phytolith morphotypes and abundant forest indicator phytoliths (those from woody and herbaceous dicotyledons and palms). Moreover, the introduction and spread of modern, open-habitat grasses (pooids, panicoids) seems to have occurred between the late Oligocene and early Miocene, resulting in a savanna type habitat by the early Miocene. These results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record.