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

Abstract

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.

Decoupled taxonomic radiation and ecological expansion of open-habitat grasses in the Cenozoic of North America

“…The hypothesis should be tested by detailed systematic work on phytoliths in tandem with sampling in other geographic regions and in older strata. If true, this pattern reiterates, at a lower taxonomic rank, the well known lag (Յ30 Ma) between angiosperm taxonomic radiation and rise to ecological dominance during the Cretaceous (44,45) and parallels patterns in other highly successful clades [e.g., ants, termites, and wasps (46)]. On a smaller scale, the presence of possible C 4 grasses long before the worldwide expansion of C 4 -dominated ecosystems in the Late Miocene, documented herein (at Ϸ19 Ma) and elsewhere (17, 37), points to a similar ecological delay.…”

Adaptive Radiation, Ecological Opportunity, and Evolutionary Determinism

“…This idea has fallen into general disfavor. Examination of the fossil record finds that evidence of competitive replacement of one clade by another is relatively weak; in most cases, the stratigraphic occurrence of the decline of one clade and the rise of the other is not consistent with this hypothesis (Benton 1996; see discussion in Jablonski 2008), although there are some exceptions, such as the rise of cheilostome bryozoans at the expense of cyclostomes (Sepkoski et al 2000) and perhaps that of angiosperms versus gymnosperms (Lupia et al 1999).…”

Testing the Biases in the Rich Cenozoic Angiosperm Macrofossil Record

“…How angiosperms achieved such high diversity has long challenged biologists, but clues may be found in their Cenozoic diversification history. Fossil-based analyses of the patterns of angiosperm evolution for the Cretaceous have led to important insights into the initial disparification and species radiation of the group Crane 1988, 1990;Crane and Lidgard 1989;Lupia et al 1999). However, the diversification of angiosperms during the Cenozoic, and the causes of such changes in diversity, remains unclear.…”