XVI International Botanical Congess

The early Tertiary biotic exchange between the Old and New Worlds likely occurred by both the Bering and the North Atlantic Land Bridges (NALB). Of these, the NALB lay at a lower paleolatitude, and was warmer and less affected by winter darkness, preferentially allowing exchange of thermophilic and evergreen taxa. The amount and significance of post-Eocene NALB migration is equivocal, land connections are largely inferred from North Atlantic oceanic circulation patterns and suggest limited exchange. The historical influence of the NALB was layered over Cretaceous exchange, and allowed newly evolving Paleogene taxa to spread between western Eurasia and North America, contributing to their phytogeographic similarity. The degree to which post-Eocene floristic similarities between these areas derive from this period or from later exchange is unclear.