Plesiosaurs: The marine reptiles that swam with four flippers

Plesiosaurs are not dinosaurs and not fish. They are a large group of extinct marine reptiles that moved through the water with four flippers, looking a little like a "swimming turtle with a long neck". The plesiosaur in the museum stands for a whole group of animals, not one named individual.

Its body

The most striking thing about a plesiosaur is its four paddle-like flippers, rather than the two front flippers of a sea turtle or the tail-driven motion of a fish. Scientists have studied how four flippers work together to push the animal forward. Some research suggests they moved them like four underwater wings — a kind of "underwater flight" that keeps the body stable as it glides. Some plesiosaurs had very long, thin necks; others had short, thick necks, and their body and tail shapes varied accordingly.

Where it lived and what it ate

Plesiosaurs ate fish, cephalopods and other marine animals. Long-necked forms may have used their flexible necks to creep quietly toward a school of fish, while short-necked forms could chase prey more directly. The group lived from the Late Triassic to the end of the Cretaceous — a very long span of time.

Where their fossils were found

Plesiosaur fossils have been found in ancient marine rocks around the world. Because they were so widespread and varied, their fossils are often displayed in museums as "sea monsters", recording a real chapter of life in Earth's oceans.

What the fossils tell us — and what they do not

The skeleton tells us the size of the animal and where the four flippers sat, but the colour of the skin, the soft edge of the flippers and the exact rhythm of swimming are rarely preserved. So the colours and movements you see are evidence-informed reconstructions. How the four flippers actually coordinated is still a lively topic in palaeontology biomechanics.

A note for grown-ups

You could ask a child to look at the four flippers: "Having four flippers seems complicated — why didn't it use just two?" Hidden in an apparently awkward number is often a clever structure. That kind of structure-to-function thinking is how palaeontologists come to understand a long-vanished animal.