Three hundred specimens of a new aquatic angiosperm have been excavated from the Upper Cretaceous deposits of the St. Mary River Formaton (Maastrichtian) of southern Alberta, Canada. Plants are compression/impressions and are represented primarily by isolated leaflets found in grey siltstones and fine-grained sandstones. These sediments probably represent a sudden overbank flood. The remains of 13 complete or partial rosettes of this dicot have been identified in the grey siltstones. Growth habit is similar to that described for Quereuxia (=Trapago) angulata from the same locality. The leaves of the new plant, however, are more than twice the size of those of Q. angulata and the largest rosettes measure at least 16 cm in diameter. Leaves are opposite, simple at the first two nodes from the apex, and compound with three leaflets for at least four nodes below. Blades of leaves or leaflets are symmetrical, obovate, microphyllous with obtuse to rounded apices and bases. The margin is unlobed and crenate. Five to seven primary veins enter from the petiole/petiolule exhibiting reticulate actinodromous to pinnate, craspedodromous branching. Primary veins are thicker near the base and weaker near the apex and show deflections near the leaflet tip. Secondary veins branch dichotomously and thin toward the margin. Tertiary veins are random reticulate with quaternaries forming irregular areolae. Leaves were probably aerenchymatous and fleshy with the abaxial epidermis being preserved on several specimens. Teeth appear to have been chloranthoid or rosoid. Rosettes are attached to a vertical axis, the largest being at least 11 cm in length. The rosettes were probably floating on the surface on a shallow pond or oxbow lake. These plants are found in the same deposits as Quereuxia angulata and the two species shared the same habitat.

Key words: Alberta, aquatic, dicot, Quereuxia, rosette, Upper Cretaceous