There was once an ancient inland seaway that split North America in two. In this seaway there was some of the most awesome predators ever to prowl Earth's oceans. One of these predators was the a marine reptile called Tylosaurus . It was not a dinosaur, but instead a mosasaur : an extinct group of reptiles that are closely related to lizards and snakes.
Tylosaurus was quite large when full grown. It grew to about 40 feet in length and weighed about 3 tons. It had four flippers, a large head with razor sharp teeth , a stout body, and long flat tail. It was a fast swimmer. If we would seen one today we would call it a sea serpent.
Fossils of Tylosaurus have been found in areas where there once was inland seas. They been found in North America (Kansas) and Australia. Tylosaurus scientific name means knob lizard.
The Carnegie Museum of Natural History in Pittsburgh has one on display in their exhibit, Dinosaurs in Their Time. Its shown in the Cretaceous Seaway part of the exhibit.
National Geographic has a Tylosaurus in their documentary video called SEA MONSTERS : A Prehistoric Adventure.