Ichthyosaurs: The swimming reptiles of the ancient seas

Ichthyosaurs are not dinosaurs and not fish. They are a large group of reptiles that became fully adapted to life in the sea, looking rather like today's dolphins. The "ichthyosaur" in the museum stands for a whole group of animals, not a single named individual.

Its body

Ichthyosaurs had streamlined bodies built for moving quickly through water. They had large round eyes for seeing in dim, deep water, and rows of small pointed teeth for gripping fish and squid. The shape of the tail differs between species and through time — some were long and whip-like, others more like a crescent — and it helped push the animal forward.

It is worth saying that ichthyosaurs were not all the same. Different species varied enormously in size, from under a metre to well over twenty metres long.

Where it lived and what it ate

Ichthyosaurs fed on fish and other sea creatures, making them important predators of the ancient oceans. Most strikingly, some fossils show mothers giving birth to live young in the water rather than laying eggs. They almost certainly stayed in the sea their whole lives, like dolphins today.

Where their fossils were found

Ichthyosaur fossils have been found in ancient marine rocks around the world. The group first appeared in the Triassic and survived until the Late Cretaceous — a very long time. Recent finds in high-latitude regions have shown they appeared earlier and spread more widely than we used to think.

What the fossils tell us — and what they do not

Fossils preserve the skeleton, the size and the shape of the eye, but the colour of the skin, the exact outline of the flippers, and the precise style of swimming are rarely preserved. So the colours and some details in this exhibit are evidence-informed reconstructions — a way of making the science approachable, not a photograph of a living animal.

A note for grown-ups

You could ask a child to compare: "Ichthyosaurs look a lot like dolphins, but what kind of animal are they really?" Noticing the difference between "looking like" and "being" is one of the best parts of learning about evolution.