SIGNIFICANCE OF THE FLORISSANT FLORA TO PLANT COMMUNITY DYNAMICS OF THE EOCENE-OLIGOCENE TRANSITION

Fossil floras spanning the Eocene-Oligocene transition have long provided the basis for reconstructing terrestrial paleoclimate during this interval, indicating a mid-latitude cooling of mean annual temperature of about 8 to 10 ºC. Additionally, fossil floras that bracket this interval of time provide the basis for inferring biogeographic patterns of plant dispersal and for understanding the dynamics of changing biotic community composition and diversity as a response to this major climatic change. In this respect, the 34.07 Ma Florissant flora represents a critical interval in time and provides significant evidence for understanding upland biotic communities of the continental interior just preceding the end of the Eocene. This provides insight into the role of these taxa in the development of Oligocene communities following this cooling.

Plant communities responded to this change not en masse, but rather by the responses of individual species. Comparison of the composition of the late Eocene upland flora at Florissant with lowland late Eocene and early Oligocene floras of the Pacific Northwest and Oligocene upland floras of the Rocky Mountains indicate that genera either 1) became extinct or were regionally extirpated, 2) evolved in place or were pre-adapted, or 3) dispersed from areas of higher elevation or latitude. Generic examples from Florissant include the extinction of Deviacer, the apparent dispersal of Asterocarpinus/Paracarpinus and several Rosaceous genera to lower elevations, and the persistence of Abies, Picea, Populus, and Cercocarpus in the uplands of the Rocky Mountain region.