Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent every weekday. Email address Sign up Thank you! Terms of Service and Privacy Policy. Nearly 70 million years ago, mosasaurs were the stuff of nightmares. Multiple species of the apex marine reptiles lived during the Late Cretaceous, often growing anywhere from 30 to 40 feet-long. But as dangerous as the ancient, great white shark-sized were for their prehistoric ocean prey, paleontologists have long assumed mosasaurs stuck to saltwater. A tooth recently found in the famous Hell Creek formation…