Floristic composition and comparison of middle Eocene to late Eocene and Oligocene floras in North America

 

Authors: Devore ML, Pigg KB

Published in: Bulletin of Geosciences, volume 85, issue 1; pages: 111 - 134; Received 7 April 2009; Accepted in revised form 5 October 2009; Online 8 January 2010

Keywords: Claiborne, Eocene/Oligocene transition, Florissant, Okanogan Highlands, Tertiary floras,

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Abstract

In comparison to the early and middle Eocene, the late Eocene and particularly the Oligocene floral record is sparse in North America. Changing tectonic, environmental and climatic conditions during these times resulted in the development of fewer depositional systems favorable for fossil preservation. Floras are known from the Southeast, the Pacific Northwest and the Rocky Mountains. Each area has a distinct geological history that shaped both the vegetation adjacent to sites of deposition as well as the depositional environments themselves. The floristic change from middle to late Eocene, and then to Oligocene reflects a changing paleoclimate from the thermal maximum to cooler and drier conditions in the late Paleogene. In the present paper, major middle, and then late Eocene and finally Oligocene floras of North America are summarized, with an emphasis on their regional geology, depositional setting, paleoclimate and significant floral elements. The North American occurrences of coryphoid palms (Sabal) and cycads are reviewed in relationship to their biogeographic history. Finally, we suggest several directions for future research that will further illuminate the floristic changes from middle, to late Eocene and Oligocene that occurred in North America.

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