The rapid radiation of angiosperms during the Late Cretaceous has been thought to reflect their rise to vegetational dominance1-3. The number of species in a clade and its vegetational importance are not necessarily related, however. Quantitative studies of the recently discovered Big Cedar Ridge flora, found preserved in situ in a mid-Maastrichtian volcanic ash in central Wyoming, USA, reveal that dicotyledonous angiosperms accounted for 61% of the species but constituted just 12% of vegetational cover. Dicots, many of which appear to have been herbaceous, were abundant only in areas disturbed just before burial. By contrast, 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 in two other nearly contemporaneous floras buried by volcanic ash, whereas coeval floras from fluvial environments are dominated by dicots4. This shows that, even as late as the mid-Maastrichtian, in northern mid-latitudes there were areas away from streams that were not yet dominated by dicots. Despite vigorous taxonomic diversification during the previous 30 Myr3, dicots played a subordinate role in these areas of fern-dominated vegetation.
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