ABSTRACT
Evidence accumulated during the past two decades has shown that the appearance of the angiosperms in the fossil record was, in contradiction to previously held views, neither more sudden nor more mysterious than other groups of organisms. The principal radiation appears to have begun during the Barremian stage of the Early Cretaceous, or slightly earlier. Evolutionary patterns are best shown in sequences of pollen morphotypes from the eastern United States, where there are clear increases in diversity and morphological complexity upwards through the Barremian to Cenomanian stages. Application of advancement indices to the sequence shows the pollen flora, even at the top of the sequence in the Cenomanian, to be more limited in its morphological range than pollen of extant taxa. Leaf floras show a similar progression from early, poorly diversified floras with ‘first rank’ leaf architecture, to more complex forms. These data accord with the idea of an initial radiation of the angiosperms having occurred entirely within the Early Cretaceous, rather than with the postulate that the group had a long pre-Cretaceous history.
In Australia, which formed part of the Southern Gondwana Province in the mid-Cretaceous, monosulcate morphotypes appear in the early Albian, some 10 million years after their appearance elsewhere, testifying to an immigrant origin for the flowering plants on this continent. Statistical data from Queensland basins suggest the early producers of angiospermous pollen were concentrated in the coastal zones.
The acquisition of the complex of characters that collectively define angiospermy seems likely to have been initially independent of large-scale environmental change, and to have occurred through single gene mutations, evolving under selective pressures of variable moisture availability in streamside or coastal habitats. Rates of evolution were high in the early 102Aptian-Albian phase, consistent with the occupation of a new adaptive zone by a group that had physical, evolutionary and ecological access to it. A slowing of rates after the first phase is compatible with adjustment of the group to new adaptive subzones.
Plots of the overall diversity of fossil plants through the geological record show the appearance of the angiosperms to be linked with a major increase in total plant species diversity beginning in the mid-Cretaceous. Their rise to dominance coincides with a decline in gymnosperm species numbers. Comparisons of all plant groups shows the angiosperms to have the highest origination rates and the shortest species durations.
Diversity plots for the Australian record are possible only in a limited number of areas because of paucity of data and uncertainties about the age of sedimentary rocks. Late Cretaceous and Tertiary pollen data from the Gippsland Basin, including graphs of total diversity, originations and extinctions, reveal little perturbation over the Cretaceous/Tertiary boundary, but show a clear response to climatic events in the Early Tertiary. The record is biased toward moister environments where pollen and other plant remains are most likely to be preserved; semi-arid habitats, where speciation rates are highest in the extant flora, are poorly represented in the fossil record.
