Long-necked plesiosaurs were marine reptiles that lived during the Mesozoic Era, particularly in the Jurassic and Cretaceous periods. They are known for their distinctive long necks, which could have contained up to 30 vertebrae, allowing them to reach and catch prey in the water. Plesiosaurs had a broad body, four flippers, and a small head, which helped them navigate through the ocean. These creatures were not dinosaurs but belonged to a separate group of reptiles. Long-necked plesiosaurs primarily fed on fish and other marine animals, using their long necks to ambush prey. Their unique body structure made them well-adapted to life in the sea.