Paleoecological reconstruction of Mid Cretaceous plant communities from the Dakota Formation of Iowa, USA

The Dakota Formation (DF) of Iowa (outcrops also in KS, NE, and MN) was deposited along the eastern shore of the Western Interior Seaway during the early Late Cretaceous (112-93.5 Ma). The DF comprises mostly non-marine strata which were deposited in fluvial to estuarine environments. The time interval preserved in this formation marks the early stage of angiosperm evolution, and the macroflora previously described from this formation are predominantly angiospermous. However, biostratigraphic analyses indicate that the palynoflora is dominated by free-sporing palynomorphs. Three localities (one from the lower Nishnabotna Member and two from the upper Woodbury Member) were sampled and eight of the samples were productive. A 300-grain count was conducted for each sample to identify the palynomorphs to a specific morphotype group (free-sporing, gymnosperm-conifer, gymnosperm-other, angiosperm, other, and unidentified). A total of 46 different taxa (genus/species) were also identified. The preservational quality of the samples were also assessed and the preservation does not vary significantly between samples. The morphotype groups, taxonomic identifications, and presence/absence data were used to perform Detrended Correspondence Analyses and k-means and hierarchical cluster analyses in order to test for spatial, temporal, and environmental patterns in vegetation composition. The composition of the palynoflora appears to be primarily driven by differences in the depositional environment, with the highest diversity in taxa occurring in fluvial/estuarine deposits and the lowest diversity in coastal swamp deposits. Additional factors affecting the composition of palynoflora include the preservation of samples and the DF Member, both of which are also strongly correlated with depositional environment.