Angiosperm diversification and Cretaceous floristic trends: a comparison of palynofloras and leaf macrofloras

Abstract

Fossil leaves and palynomorphs represent different phases of the plant life cycle, are studied as systematically independent entities, and are subject to different taphonomic, sampling, and recognition biases in the paleobotanical record. They thus provide parallel, and largely independent, documentation of long-term trends in land plant diversity, and the palynological record may be used as a comparative test of floristic trends inferred from macrofossil evidence. Analyses of relative “species” richness in 91 macrofossil and 860 palynomorph Cretaceous “floras” (assemblages) from between 25° and 65° N paleolatitude show a major mid-Cretaceous increase in the within-flora diversity of angiosperms, from near 0% prior to the Aptian (120 Ma) to 50–80% by the end of the Maastrichtian (65 Ma). This level of diversity is attained rapidly in macrofloras, but more slowly in palynofloras. In the latest Cretaceous, macrofloras and palynofloras both indicate that “pteridophytes,” conifers, and other “gymnosperms” are generally less diverse than angiosperms. In both data sets, “pteridophyte” diversity shows a clear decline through the Cretaceous, whereas conifer diversity shows no marked temporal trend. Broad congruence of these patterns, in spite of different biases in the macrofossil and palynomorph records, indicates that they provide a robust reflection of floristic trends through the Cretaceous. Nevertheless, discrepancies between the patterns do occur and underline the importance of complementary macrofossil and palynological analyses for accurate resolution of long-term vegetational change.