A clear understanding of the phytogeographic history of angiosperms in the Northern Hemisphere requires integration of data from both living and fossil plants. Attempts to reconstruct phytogeographic history based exclusively on modern distribution patterns are prone to error, because there are numerous instances of genera with holarctic paleobotanical distribution that are today confined to one or two continents. Centers of diversity have changed over time, undermining attempts to place centers of origin in areas of current high diversity. There has sometimes been a temptation to view the earliest known fossil representatives of a clade as an indication of the place of origin; however, this method is biased by the differential availability of pertinent stratigraphic sequences on different continents. Perhaps the most informative method is to conduct phylogenetic analyses of all the available fossil and extant representatives of a given clade and examine the sequence of geographic dispersal events indicated by the most parsimonious trees. The choice of taxa for such investigations should be based in part on the richness and quality of paleontological data. Carefully documented whole plant reconstructions of fossil species, providing characters from foliage, stem anatomy, flower, fruit, seed, and/or pollen morphology, provide the best opportunities for the successful integration of fossil and modern taxa in phylogenetic analyses. Preliminary investigations illustrate varied patterns of intercontinental dispersal, but two of the more common are: Asian-North American diversification with subsequent dispersal to Europe (e.g., Acer, Aesculus, Eucommia, Paliurus), North American-European diversification with subsequent dispersal to Asia (e.g., Nyssa, Platycarya, Tilia, Ulmus). Greater resolution of the directions and timing of dispersal events throughout the Northern Hemisphere will require intensive investigations of taxa with excellent fossil records, with attention to both vegetative and reproductive morphological characters, complemented by molecular-based phylogenetic analyses of the living representatives.

Key words: Northern Hemisphere, Paleobotany, Phylogeny, Phylogeography, Phytogeography