Mosasaurs were large marine reptiles that lived during the late Cretaceous period, around 70 to 66 million years ago. They were not dinosaurs but belonged to a group called Lepidosauria, which also includes modern lizards and snakes. Mosasaurs had long, streamlined bodies, powerful tails, and paddle-like limbs, making them efficient swimmers. These creatures could grow up to 50 feet long and were apex predators in their ocean habitats. They primarily fed on fish, cephalopods, and other marine animals. Fossils of mosasaurs have been found in various parts of the world, indicating their widespread distribution in ancient seas.