Giant sea monsters lived in rivers at the end of the dinosaur age

Giant mosasaurs, once thought to be strictly ocean-dwelling predators, may have spent their final chapter prowling freshwater rivers alongside dinosaurs and crocodiles. A massive tooth found in North Dakota, analyzed using chemical isotope techniques, reveals that some mosasaurs adapted to river systems as seas gradually freshened near the end of the age of dinosaurs. These enormous reptiles, possibly as long as a bus, appear to have hunted near the surface, perhaps even feeding on drowned dinosaurs.