IMPLICATIONS OF AN EXCEPTIONAL FOSSIL FLORA FOR LATE CRETACEOUS VEGETATION

THE rapid radiation of angiosperms during the Late Cretaceous has been thought to reflect their rise to vegetational dominance1-3. The numbe r of species in a clade and its vegetational importance are not necess arily related, however. Quantitative studies of the recently discovere d Big Cedar Ridge flora, found preserved in situ in a mid-Maastrichtia n volcanic ash in central Wyoming, USA, reveal that dicotyledonous ang iosperms accounted for 61% of the species but constituted just 12% of vegetational cover. Dicots, many of which appear to have been herbaceo us, were abundant only in areas disturbed just before burial. By contr ast, free-sporing plants were 19% of the species but 49% of cover. The only abundant and ubiquitous angiosperm was a single species of palm (about 25% of cover). A comparably low abundance of dicots was found i n two other nearly contemporaneous floras buried by volcanic ash, wher eas coeval floras from fluvial environments are dominated by dicots4. This shows that, even as late as the mid-Maastrichtian, in northern mi d-latitudes there were areas away from streams that were not yet domin ated by dicots. Despite vigorous taxonomic diversification during the previous 30 Myr3, dicots played a subordinate role in these areas of f ern-dominated vegetation.