The extinct marine reptile from the Mesozoic Era with a long neck and four flippers was a plesiosaur. Plesiosaurs had broad, flattened bodies, short tails, and two pairs of powerful paddle-like limbs adapted for swimming.
They lived in marine environments from the Late Triassic until the end-Cretaceous extinction, roughly 203 to 66 million years ago. The group included long-necked forms with small heads, often called plesiosauroids, as well as short-necked, large-headed predators traditionally known as pliosaurs. Thus, not every plesiosaur had the iconic extremely long neck.
Their four-flipper arrangement was unusual among marine reptiles. The limbs likely worked as underwater wings, producing lift and thrust through coordinated strokes, while the short tail was used mainly for steering. Fossil evidence also indicates that at least some plesiosaurs gave birth to live young rather than laying eggs on land.
Plesiosaurs were reptiles, not dinosaurs. Pterosaurs were flying reptiles, ichthyosaurs had fish-like bodies, and mosasaurs were later marine lizards that generally relied on a powerful tail for propulsion.