Mosasaur Fossil Tooth

Mosasaurs (from Latin Mosa meaning the 'Meuse River' in The Netherlands where they were first found in 1764, and Greek sauros meaning 'lizard') are an extinct group of large marine reptiles. During the last 20 million years of the Cretaceous period, with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators, and their diet included ammonites. They went extinct at the end of the Cretaceous period, about 66 million years ago. Mosasaurs breathed air, were powerful swimmers, and ranged from 3 to 55 feet in length, depending on the species.