Plesiosaurs inhabited the seas from around 200 million to 65 million years ago. They were not dinosaurs, despite living at the same time as dinos. It is thought that plesiosaurs fed mainly on fish, breathed air and laid their eggs on beaches. The largest of these sea monsters reached 15 meters (49 feet) in length.

Is a plesiosaur considered a dinosaur?

Marine reptiles, such as ichthyosaurs, plesiosaurs and mosasaurs are not dinosaurs. Nor is Dimetrodon or other reptiles in the same group (previously called 'mammal-like reptiles' and now called synapsids).

Is a plesiosaur a mammal?

Like whales, humans, and most other mammals, plesiosaurs—giant, long-necked marine reptiles of dinosaur times—gave birth to live young, a new fossil study suggests.