Mosasaurus was a large, aquatic reptile that lived during the Late Cretaceous period, approximately 70 to 66 million years ago. Related to modern lizards and snakes, it could grow up to 50 feet long and was a formidable predator in the ancient seas. With a streamlined body, powerful tail, and elongated limbs adapted to flippers, Mosasaurus was well-suited for swimming. Fossils have been found in various parts of the world, indicating it was a widespread species.