An incomplete specimen of the plioplatecarpine mosasaur Plioplatecarpus from the lower Maastrichtian locality of Scabby Butte (Saint Mary River Formation) closely resembles the slightly older Plioplatecarpus primaevus from Saskatchewan (Bearpaw Formation). However, quantitative and qualitative differences are observed: the vertebrae of the Scabby Butte specimen are more gracile and, although of similar shape and proportions, are relatively smaller. Since these differences cannot be readily explained by allometry or sexual dimorphism, the hypothesis that this specimen represents a new species is provisionally accepted. However, in the absence of diagnostic cranial features, a new species is not named. The morphology of the well-preserved forelimb indicates that the animal cannot be reconstructed as a subaqueous flyer, but probably used the forelimbs for paddling. The associated matrix indicates preservation in overbank deposits in a deltaic system exhibiting no evidence of open marine environments. This suggests that this mosasaur was able to exploit estuarine and freshwater environments.