Premise of research. A large number of floating aquatic aroid fossils have been recovered from pond sediments in the Hell Creek Formation (Upper Cretaceous) of South Dakota, providing valuable new data about aquatic vegetation of the uppermost Cretaceous, that are used to describe a new species of the genus Cobbania, and to evaluate associated reproductive structures and phylogenetic relationships among floating aquatic monocots. Methodology. Fossils were uncovered as needed with fine needles to reveal surface features of the specimens. Images were captured with a digital scanning camera, and phylogenetic analyses were conducted with TNT implemented through WinClada. Pivotal results. The new species, Cobbania hickeyi Stockey, Rothwell & Johnson, extends the range of the genus to the uppermost Cretaceous, supports the taxonomic integrity of the genus Cobbania, and increases our understanding of structural variation and species richness within the genus. Associated reproductive structures include an aroid spadix, strengthening the assignment of Cobbania to the Araceae. Phylogenetic analyses using "total-evidence" data help resolve conflicting results from either morphological or nucleotide sequence analyses of relationships among floating aquatic aroids, and the fossil taxon Aquaephyllum does not nest among the other floating aquatic species in any of our results. Conclusions. Species of the genus Cobbania were an important component of aquatic vegetation across the Northern Hemisphere during the Late Cretaceous. In tests of competing hypotheses for relationships among Pistia stratiotes, Cobbania spp., and species of Araceae subfamily Lemnoideae, the results from a "total-evidence" analysis suggest that specializations for the floating aquatic life form may be overwhelming other characters in the results of morphological analyses alone.
