|Cryptoclidid plesiosaurs are best known from the Oxford Clay of England, but have also been found in the western US and Cuba. Two new specimens from the Upper Jurassic Sundance Formation of Wyoming, tentatively referred to the taxon Pantosaurus striatus, provide significant new information on the osteology of these lesser known plesiosaurs. In addition, examination of two exceptionally well-preserved caudal regions from Cryptoclidus eurymerus and Muraenosaurus leedsi has revealed previously undescribed features indicating the presence of a tail fin in these taxa. Comparisons of these features to fossil and extant animals with tail fins allow the shape of the tail fin in plesiosaurs to be constrained. These comparisons also reveal shared similarities in patterns of flexibility of the tail in plesiosaurs, scombroid fishes, and lamnid sharks. These data strongly suggest that the tail was involved, along with the limbs, in active thrust production during plesiosaur locomotion.