Plesiosaurus was a marine reptile that lived during the late Jurassic and early Cretaceous periods, around 200 to 66 million years ago. It is known for its distinctive long neck, small head, and large, paddle-like limbs, which helped it navigate through the water. Plesiosaurus was not a dinosaur but belonged to a group called Plesiosauria, which included various similar species. This creature could grow up to 15 feet long and primarily fed on fish and other marine animals. Fossils of Plesiosaurus have been found in Europe and North America, providing valuable insights into the diversity of life in ancient oceans.