A remarkable encounter between two Cretaceous mosasaurs of the same species and a sick or recently dead nautiloid (Argonautilus catarinae Sundberg) is recorded. It is possible that the mosasaur was training its young to attack shelled cephalopods. This is the first recorded attack by two mosasaurs on a nautiloid in the eastern Pacific Ocean, although ammonites preyed upon by mosasaurs are known from the west coast of North America.