Figure: Strata of the Chinle Formation below the prominent silcrete horizon exposed in the Petrified Forest National Park. Photo courtesy of Andrew V. Kearns, 2012.

The Norian Chinle Formation in the Southwestern United States provides a snapshot into an ancient terrestrial ecosystem with its famous petrified tree trunks and various plant and vertebrate remains. The fossil plant assemblages, including spores and pollen grains, provide useful information on past vegetation and the response of the vegetation to climate changes. New pollen and spore data from the Chinle Formation at the Petrified Forest National Park, Arizona, suggest that a extinction of plants occurred between 213 and 217 million years ago in tandem with an extinction of several reptile groups. The predominance of plants adapted to drier conditions after the extinction event is consistent with the gradual aridification of the North American continent due to the uplift of the Cordilleran volcanic mountain range and the probable northward shift of North America through plate tectonics. Plant community analysis reveals that the floral turnover was followed by the colonization of new plant groups such as the varieties of conifer trees and the decrease in the contribution of seed ferns in the vegetation along waterways.