Mosasaurs, particularly the genus Mosasaurus, is not dinosaurs but large marine reptiles that lived during the Late Cretaceous period. They are known for their elongated bodies, similar to modern-day monitor lizards (to which they are closely related), and limbs that evolved into flippers for efficient swimming. These creatures are apex predators in their marine ecosystems, preying on fish, ammonites, and smaller marine reptiles. Their powerful jaws and sharp teeth are well-adapted for their carnivorous diet.