Plesiosaur

Plesiosaurs were marine reptiles that lived at the same time as the dinosaurs and also became extinct with the dinosaurs around 66 million years ago. They propelled themselves through the water by means of their strong, paddle-like flippers, which were moved up and down like wings. During this so-called “underwater flight,” the front and hind flippers probably moved at counterpoint to each other. The body was too inflexible and the tail too short to generate propulsion through lateral motion.