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Premise of research. Sixteen compression/impression specimens of a new floating aquatic angiosperm that enrich our knowledge of Late Cretaceous vegetation have been identified from the early Maastrichtian St. Mary River Formation near Cardston, Alberta, Canada. Methodology. Fossils were studied using dégagement and photographed under reflected light. Leaves were morphotyped using methodology in the Manual of Leaf Architecture and compared to other extant and extinct rosette-forming floating aquatic angiosperms. Pivotal results. Stems of this aquatic dicot show determinate growth of floating shoots with opposite/decussate attachment of heterophyllous leaves. Leaves are simple and sessile at the apicalmost node, simple and petiolate at the second node, and compound with three leaflets at nodes 3–6. Compound leaves are petiolate and ternate (trifoliate). Leaflet shape is unlobed, obovate, and symmetrical; base is acute-obtuse; apex angle is obtuse; base shape is convex; petiolar attachment is marginal; apex shape is rounded; and margin is crenate. Primary veins are flabellate to pinnate with three to five basal veins that dichotomize distally. Secondary veins are craspedodromous with uniform spacing and angles. Teeth are of a single order, three per centimeter near apex, and convex-convex; the sinus is angular, and the apex is foraminate. Conclusions. While the basic growth habit of this plant is similar to that of several extant taxa in the Onagraceae, Lythraceae, Plantaginaceae, and Hydrocharitaceae, leaf venation and the presence of compound leaves differ greatly from the living species. In growth habit and the presence of both simple and compound leaves, the new plant is similar to Quereuxia angulata , also found at this fossil site. These fossils are described as a new genus and species, Tolmania aquatica Edmonds, Stockey & Rothwell, which differs from Quereuxia in pattern of heterophylly, venation, larger plant size, stem morphology, and the presence of hydathodes in the leaves. [ABSTRACT FROM AUTHOR]

Several new broad-leaved monocots were identified in gray siltstones and fine-grained sandstones from the Upper Cretaceous (Campanian-Maastrichtian) St. Mary River Formation near Cardston, Alberta, Canada. Specimens are compression/impressions of long-petiolate aquatic plants that were probably buried in situ. Leaf blades are entire, ovate to elliptic, with deeply cordate bases; leaf blades are 5-12 cm long and 3.5-8.5 cm wide. Petioles are at least 4 mm wide with five to seven primary veins that enter the leaf blade. Venation is campylodromous, resulting in 23-27 primary veins with three medial veins that remain unbranched to the apex. Major and minor secondary veins (ABAB pattern) diverge at angles of 45°-65° near the midveins and 900 near the leaf margin. Tertiary veins are usually unbranched, but occasional dichotomies and anastomoses occur. No freely ending veinlets are visible. The Cardston specimens are compared with extant leaves of Alismatales and show closest similarities to those of Lirnnocharis L., Hydrocleys Rich., and Butomopsis Kunth. These leaves are also similar to fossil leaves of Haemanthophyllum Budantsev, in particular H. cordatum Golovneva from the Maastrichtian-Danian deposits of the Koryak Highlands, Russia. A reexamination of the genus Haemanthophyllum, the generitype (H. kamtschaticum Budantsev), the holotype for H. cordatum, and the Cardston specimens results in the description of a new genus, Cardstonia tolmanii gen. et sp. nov. (Limnocharitaceae). This study points to the need for reexamination of the remaining species in the genus Haemanthophyllum, which appear to represent a diverse assemblage of leaves of varying morphology that are probably not a natural group. [ABSTRACT FROM AUTHOR]