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Some limitations in using leaf physiognomic data as a precise method for determining paleoclimates with an example from the Late Cretaceous Pautût Flora of West Greenland

1994; Elsevier BV; Volume: 112; Issue: 3-4 Linguagem: Inglês

10.1016/0031-0182(94)90076-0

1872-616X

Austin Boyd,

Paleontology and Stratigraphy of Fossils

Plant parts of the Late Cretaceous West Greenland Pautût Flora are derived from vegetation growing in lower delta plain, delta upland, and levee habitats. A large river system drained the uplands and deposited leaves from species there and levee areas into a shallow bay setting. The large drainage area of the rivers caused a great diversity of species to be preserved in the bay. When these upland and levee species are combined with those that grew and were deposited in the lower delta plain areas, 12 fern, six cycad/cycadeoid, 1 ginkgoalean, 35 conifer, and over 85 angiosperm species represent the Pautût Flora. When the paleotemperature derived from the angiosperm leaf margin ratio of the complete Pautût Flora (8°C) is compared to other paleobotanical data and oxygen isotope values (warm temperate to subtropical there is a serious discrepancy that puts the leaf physiognomic approach for precisely determining paleoclimate in the Cretaceous in serious doubt. Also, paleoclimatic interpretations based on the angiosperm leaf margin data of leaves from the lower delta plain species would indicate slightly cooler conditions than that suggested by the upper delta plain species. Leaf size would indicate wetter conditions in the lower delta plain even though other leaf features (e.g. drip tips) suggest moist conditions did occur in the upper delta plain. These preservation enough that regional paleoclimate scenarios drawn from only one depositional setting or loclity can be erroneous. A sampling size of less than 30 angiosperm species for a fossil flora also is suspect for reliably indicating climatic conditions, unless the flora was originally impoverished of angiosperm species. Taphonomic factors play a significant role in fossil leaf preservation, so collected fossil assemblages must closely represent the original vegetation before paleoclimatic interpretations based on leaf physiognomy can have a degree of credibility. A limit to plausibility is especially significant in Cretaceous floras because they have a limited diversity, due to evolutionary constraints, and appear to have consisted of more successional species than younger floras, thus displaying intermediate climatic conditions. Moisture and Arctic light levels further restrict determination of paleoclimatic conditions derived from leaf margin ratios and leaf size.