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The biogeographic affinities of the Florissant flora are in need of reevaluation. We give a critical review, based on megafossil and pollen records representing genera whose affinities we accept as well founded. The Florissant assemblage includes taxa of diverse modern geographic distribution. The flora is composed mainly of Laurasian elements, some of which are now confined to Asia (Ailanthus, Dipteronia, Eucommia, Platycarya, Pteroceltis) and a wide number co-occurring in the eastern United States and Asia. Others are now confined to western North America (Sequoia, Cercocarpus, Sarcobatus) and many occur in Mexico. The major geographic affinities of the Florissant genera discussed here are broad and include the present-day warm temperate and subtropical floras of Mexico, central and southern China, and the southeastern United States. Many taxa appear to have been shared between North America and Asia by Eocene time. The Rocky Mountain flora was distinct from that of the southeastern United States, probably because of the barrier represented by the Cannonball epeiric sea that traversed the Midcontinent in the Paleocene. Similarity of Florissant taxa to the South American flora is low.

The deterioration of climate after the time of Florissant deposition represents one of the most significant decreases in temperature of the entire Tertiary. Following the warm interval of the latest Eocene, a few Florissant genera were locally extirpated, a few became extinct, some were already at or dispersed to lower-elevation regions, and others persisted in the southern Rocky Mountains. Over longer geologic time spans, some taxa seem to have persisted on the West Coast of North America through the Miocene, and in a few cases even up to the present. Many deciduous taxa have persisted in the summer-wet climate area of the eastern United States.

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