An ichthyosaur looks like a fish or maybe a dolphin, but it was actually a reptile. It lived in the Mesozoic seas while dinosaurs were walking on the land. They are not dinosaurs but they are so unlike any other reptile that palaeontologists aren’t sure what they are closely related to.
Ichthyosaurs went extinct about 90 million years ago so, unlike pterosaurs or non-bird dinosaurs, they didn’t become extinct because of a large meteor. Instead, palaeontologists think they wentextinct because newly evolved species of fish were faster and harder to catch. Other marine reptiles were more adaptable and replaced them in the late Mesozoic seas before the meteor hit.
Ichthyosaurs had fins instead of legs that they used for swimming. On their back they had a dorsal fin like those on many fish, and their tails also had a large crescent (moon-shaped) fin. They lived in the sea and were incapable of returning to land.
Most reptiles lay eggs but eggs are permeable to water. This means that if an egg falls in water then the infant inside would drown. But ichthyosaurs lived in water and couldn’t get onto land so ichthyosaurs gave birth to live young, as mammals do. Some fossil ichthyosaurs have even been found with babies half way out of the body. Palaeontologists think these ichtyosaurs died while giving birth.
Ichthyosaurs could be really large. The largest so far found is about 23m long. They also had massive eyes. One species called Opthalmosaurus has the largest eyes for its body size of any animal that has ever existed.
Here is a video of an ichthyosaur from the BBC’s Walking with Dinosaurs series: