If you want to find sea monsters, there’s hardly a better place to look than western Kansas. Not that you’re going to see any live ones slithering around. They’ve been dead for more than 66 million years. In rocks deposited by a warm, shallow sea that once washed over the middle of North America, paleontologists have pulled dozens of marine reptiles that ruled the marine realm while non-avian dinosaurs stomped around on land and pterosaurs soared through the air. And among the most impressive of all were the great mosasaurs.

Enormous aquatic lizards related to today’s monitor lizards, mosasaurs like Tylosaurus and Platecarpus were totally at home in the sea. Descendants of terrestrial reptiles,