Sixty-seven coalified compression specimens of leaves, one attached to a stem, have been found in the St. Mary River Formation (uppermost Campanian–lowermost Maastrichtian, Upper Cretaceous) near Cardston, Alberta, Canada. Leaves have long multi-veined petioles with clasping bases, and an ellipsoid to obovate lamina with a convex to almost flat apex, and a crenate margin with one order of convex-convex chloranthoid teeth with angular sinuses. Tooth apices often end in a small concavity interpreted as a hydathode. Several primary veins enter the lamina in a flabellate manner. The central primary vein branches pinnately while lateral primaries, dichotomize toward the margin producing a reticulodromous pattern of secondaries. Secondaries arising from the lateral primary veins are the result of consistent dichotomies and anastomoses with a concurrent loss of gauge. Tertiary veins are alternate percurrent, and quaternaries form an irregular reticulate fabric. Quaternary veins form polygonal areoles with freely ending veinlets. Some specimens show a polygonal pattern of sub-epidermal aerenchyma like that of many other aquatics. Plants are compared to other fossil aquatic taxa, including those from the St. Mary River Formation and extant members of Ranunculaceae, Menyanthaceae and Nymphaeaceae. The specimens are described as a new amphibious aquatic: Zlatkovia crenulata Rothwell & Stockey gen. et sp. nov., eudicot family incertae sedis. This assemblage meaningfully enriches our understanding of aquatic and adjacent wetland habitats near the close of the Cretaceous, and emphasizes that the structure of modern wetland communities was well established before the end of the Mesozoic. [ABSTRACT FROM AUTHOR]