Comparing palynological abundance and diversity: implications for biotic replacement during the Cretaceous angiosperm radiation

The Cretaceous radiation of angiosperms initiated a major reorganization of terrestrial plant communities as dominance by pteridophytic and gymnosperm ic groups eventually gave way to dominance by angiosperms. Previously, patt erns of biotic replacement have been assessed using measures based on taxon omic diversify data. However, using measures of both abundance and diversit y to investigate replacement patterns provides more information about macro ecological change in the fossil record than either can provide alone. Analy ses of an updated and expanded database of North American palynological sam ples from Cretaceous sediments document a rapid increase in angiosperm dive rsity and abundance within individual fossil palynofloras (representing loc al/subregional vegetation). New analyses of floristic diversity patterns su pport previous results and indicate that the decline of free-sporing plants is more pronounced than the decline of gymnosperms. In contrast, analyses of abundance data appear to show that the decline of gymnosperms is far mor e pronounced than the decline of free-sporing plants. Detailed examination of both data sets segregated by paleolatitude shows that this apparent cont radiction reflects biogeographical differences in the patterns of vegetatio nal change (e.g., free-sporing plants declined in abundance at lower latitu des) as well as sampling bias (e.g., greater sampling in the northern regio n in the Late Cretaceous). Analyses accounting for these biases support the conclusion that as angiosperms radiated, free-sporing plants rather than g ymnosperms tin this case, mainly conifers) experienced the most pronounced decline. A thorough understanding of the Cretaceous radiation of angiosperm s will require both abundance and diversify data. It also will require expa nding the analyses presented here into other geographic regions as well as sampling more completely at all spatial scales.