MANCHESTER, STEVEN R.* AND BRUCE H. TIFFNEY. Paleobotanical Laboratory, Florida Museum of Natural History, Gainesville, FL 32611-7800; Department of Geological Sciences, University of California, Santa Barbara, CA 93106. - Paleobotanical data and the phytogeographic history of holarctic angiosperm clades.
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