A new, exquisitely preserved specimen of a small mosasaur, referable to Mosasaurus missouriensis, is reported from the Bearpaw Formation (ca. 75 Ma, upper Campanian) of southern Alberta, Canada. Many calcified cartilaginous elements, including tracheal rings and the sternum, are preserved. The sternum most closely resembles that of Clidastes propython, bearing five shallow sternal rib facets on each side. Our comparative study of the new material with the holotype, referred material, and the genotype M. hoffmannii is congruent with the preexisting hypothesis that M. missouriensis and M. hoffmannii are phylogenetically more closely related to each other than to the other congeners, in spite of a temporal gap of nearly 10 million years between them. Also preserved with the mosasaur, inside its ribcage and around the specimen, are well-preserved aulopiform fish bones, including a skull. The fish skull is punctured and its centra truncated, suggesting macrophagy was employed by M. missouriensis despite the apparent lack of tooth wear. A sympatric specimen of Prognathodon overtoni is known to have consumed a sea turtle as well as fishes, and consistently exhibits apical wear across marginal teeth. We hypothesize that coexistence of these apex predators in the Bearpaw Sea was possible because of niche partitioning. Finally, the mosasaur carcass was likely scavenged by at least three lamniform sharks, based on their shed teeth and a series of truncated transverse processes on the mosasaur tail.