Palynomorphs and selected mesofossils from the Cretaceous Dakota Formation, Minnesota, USA

The middle Cenomanian palynomorphs and selected mesofossils from the Dakota Formation of south central Minnesota were investigated. A total of 218 palynomorphs were recovered. Terrestrial palynomorphs include 41 types of angiosperm pollen in which five types are described as new species, 42 types of gymnosperm pollen, and 78 types of spores of ferns and fern allies. Spores of ferns and fern allies are most diverse among the terrestrial palynomorphs. Other palynomorphs include two types of megaspores, ten types of algal spores and colonies, seven types of fungal spores and fruiting body, 18 types of dinoflagellate cysts, and 20 types of acritarchs. Based upon the occurrence of Artiopollis indivisus, Balmeisporites glenelgensis, Cicatricosisporites crassiterminatus, Dictyophyllidites impensus, and Nyssapollenites sp., the age of the Cretaceous sediments exposed in south central Minnesota is probably middle Cenomanian. Based upon the analysis of angiosperm pollen morphological characters, the pollen types that appear to be insect-pollinated accounted for 77% on average, and the pollen types which appear to be wind-pollinated accounted for 23% on average during the middle Cenomanian. The characteristic vegetation elements of the coastal swamps during middle Cenomanian were diverse angiosperms, dominant ferns and fern allies, and a relative low abundance of gymnosperms. The Trochodendrales and Buxales of the eudicots, which were not recovered from leaf fossil records, probably were present during the middle Cenomanian based upon the angiosperm pollen records. Two new marattioid ferns, Goolangia minnesotensis Hu, Dilcher, H. Schneid. et Jarzen gen. et sp. nov. and Mesozoisynangia trilobus Hu, Dilcher, H. Schneid. et Jarzen gen. et sp. nov., are described based on charcoalified isolated sporangia and synangia. These fossils provide evidence for the existence of marattioid ferns during the mid-Cretaceous in North America and give the first unequivocal documentation of the Marattiaceae in post Jurassic times. Spores of Goolangia minnesotensis are comparable with the dispersed spore Dictyophyllidites impensus, which was distributed from Arizona to Alberta in west central North America during the middle Cenomanian.