New anatomical observations of the holotype skull of Plotosaurus bennisoni from the Maastrichtian Moreno Formation of California, U.S.A., are used as a framework to examine cranial kinesis in derived members of the Mosasaurinae. Enlarged posteromedial flanges of the frontal and extensive lateral contacts of the prefrontal and postorbitofrontal contributed to increased rigidity along the frontoparietal suture (the mesokinetic joint). Suturai contacts of the parietal with the supraoccipital posteriorly and the prootic ventrally would have restricted metakinetic movements. Furthermore, the unusual shape of the epipterygoid, and its dorsal contact with the prootic and parietal, shows that the epipterygoid and pterygoid were probably not capable of anteroposterior movements. Most strikingly, Plotosaurus exhibits a tight association of the quadrate with the temporal arcade, suggesting that streptostyly was limited or lost in this derived mosasaurine, the loss of such a feature having never been described in a mosasaur. These charcteristics are placed in a functional context to examine aquatic adaptations in mosasaurs. As one of the most specialized mosasaurs known, the loss of cranial kinesis may have evolved as a result of its piscivorous diet.