The rise of angiosperms during the Cretaceous period is often portrayed as coincident with a dramatic drop in the diversity and abundance of many seed-free vascular plant lineages, including ferns. This has led to the widespread belief that ferns, once a principal component of terrestrial ecosystems, succumbed to the ecological predominance of angiosperms and are mostly evolutionary holdovers from the late Palaeozoic/early Mesozoic era. The first appearance of many modern fern genera in the early Tertiary fossil record implies another evolutionary scenario; that is, that the majority of living ferns resulted from a more recent diversification. But a full understanding of trends in fern diversification and evolution using only palaeobotanical evidence is hindered by the poor taxonomic resolution of the fern fossil record in the Cretaceous. Here we report divergence time estimates for ferns and angiosperms based on molecular data, with constraints from a reassessment of the fossil record. We show that polypod ferns (> 80% of living fern species) diversified in the Cretaceous, after angiosperms, suggesting perhaps an ecological opportunistic response to the diversification of angiosperms, as angiosperms came to dominate terrestrial ecosystems.

The Cretaceous radiation of angiosperms initiated a major reorganization of terrestrial plant communities as dominance by pteridophytic and gymnospermic groups eventually gave way to dominance by angiosperms. Previously, patterns of biotic replacement have been assessed using measures based on taxonomic diversity data. However, using measures of both abundance and diversity to investigate replacement patterns provides more information about macroecological change in the fossil record than either can provide alone. Analyses of an updated and expanded database of North American palynological samples from Cretaceous sediments document a rapid increase in angiosperm diversity and abundance within individual fossil palynofloras (represent ing local/subregional vegetation). New analyses of floristic diversity patterns support 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 more pronounced than the decline of free-sporing plants. Detailed examination of both data sets segregated by paleolatitude shows that this apparent contradiction reflects biogeographical differences in the patterns of vegetational change (e.g., free-sporing plants declined in abundance at lower latitudes) as well as sampling bias (e.g., greater sampling in the northern region in the Late Cretaceous). Analyses accounting for these biases support the conclusion that as an giosperms radiated, free-sporing plants rather than gymnosperms (in this case, mainly conifers) experienced the most pronounced decline. A thorough understanding of the Cretaceous radiation of angiosperms will require both abundance and diversity data. It also will require expanding the analyses presented here into other geographic regions as well as sampling more completely at all spatial scales.

No abstract

No abstract

No abstract