New material of Plesiobaena antiqua from the mid-Campanian Judith River Group of Alberta, Canada, fully documents the skeleton in this species and provides evidence regarding variation within the taxon. Size-independent variation is present in the size of the nasals. Variation in the degree of entry of the frontal into the orbital margin is size-related. The shell varies in the presence or absence of the seventh neural, presence of a ninth neural, and fusion of the eleventh and twelfth peripherals. Plesiobaena antiqua is restricted to the Campanian. The late Maastrichtian specimens previously included in the species differ in the absence of the following features: pointed crista supraoccipitalis, parietal-postorbital sutures strongly divergent from midline, maxilla meeting postorbital excluding jugal from orbit margin, short contact between pterygoids at midline, fused first and second peripheral, and first marginal located mostly, or entirely, on nuchal. The late Maastrichtian specimens are distinct from the Paleocene species P. putorius in the degree of development of the skull roof and presence of sulci on the skull roof, but the two taxa share two derived features suggesting that they form a monophyletic clade relative to P. antiqua. The distribution of P. antiqua within the Campanian of North America suggests that it was a member of a northern faunal assemblage.