PALEOECOLOGICAL IMPLICATIONS OF A MID-CRETACEOUS (ALBIAN) FOSSIL PLANT COMMUNITY FROM THE DAKOTA FORMATION OF IOWA, USA

Although flowering plants dominate nearly all modern biomes, there remain significant uncertainties in our understanding of the early angiosperm radiation. Well-preserved, Early Cretaceous fossil floras are critical for expanding our knowledge of the morphology, phylogeny and ecology of these early lineages. The Dakota Formation (Fm) is a sequence of Albian-Cenomanian fluvial/estuarine mudstones and sandstones deposited on a broad coastal plain along the Western Interior Seaway in North America. The lower Nishnabotna Member (Albian) of the Dakota Fm crops out in at least three locations on the Whiterock Land Conservancy of Guthrie County, Iowa. The Long Creek site (N 41.86^o, W 94.68^o) preserves a compression macroflora (including both vegetative and reproductive parts) and charcoalified mesofossils, in addition to a diverse palynoflora. The compression flora contains ±6 angiosperm leaf morphotypes (including probable Laurales and Proteales), in addition to an ovulate cone and probable fruiting axis. Palynological analysis of the leaf horizon indicates that the palynoflora (ca. 31 taxa) was dominated by pteridophytes (±61%), with some gymnosperms (±23%) and angiosperms (±16%) also represented.

The Long Creek palynoflora records a very different vegetation than is indicated by the macroflora (dominated by broad-leaved angiosperms), probably due to taphonomic differences in pre-depositional transport and fluvial concentration. Sedimentological evidence indicates some marine influence at the Long Creek site, suggesting that sections of the eastern shoreline along the Western Interior Seaway may have deviated much farther east during the Albian than is usually interpreted. When the Long Creek palynoflora is placed in the context of other new collections from central and western Iowa, they will provide a critical window into the changing ecosystem structure during one of the warmest global climate intervals in Earth’s history.