They were very fast swimmers and possibly were capable of reaching speeds of 15 mph. Information gained from analysing fossil ichthyosaur stomachs and coprolites (droppings) shows that their main diet was fish, ammonites, nautiloids and squid. The hard hooks found on the tentacles of squids were indigestible and collected in the stomach. One fossil ichthyosaur showed that it had gulped down at least 1,500 squids in its lifetime! The largest ichthyosaurs were about 7.6m (25 feet) long but some species were no longer than a human.

Ichthyosaurs swam by moving their powerful tails from side to side. When the first fossil ichthyosaur skeletons were discovered it was thought they had broken tails. Then it was realised that the backbone actually had a downward kink, and this shape supported a vertical tail fin (unlike the horizontal one found in dolphins and whales). The dorsal fin and paddles were used for steering.

The skull of the ichthyosaur had a long narrow snout and the nostrils were placed far back on the sides of the head to allow the animal to feed and breathe at the same time. Ichthyosaurs had huge eyeballs that were strengthened by a ring of bones. In life the ring prevented distortion of the eyeballs when the animal was diving.

Beside their fish-shaped body, many other features of the ichthyosaur indicated that they alone amongst the reptiles were totally adapted to a marine life. This included giving birth to live young in the water.

In Britain, fossil reptile remains, including those of ichthyosaurs, attracted the attention of collectors from a very early period in the study of natural history. The earliest illustrations now known to be ichthyosaurs appeared in Lithophylacii Britainnici Ichnographicia (1699), the first book devoted solely to British fossils. Mary Anning (1799-1847) was famous for the fossils she collected close to her home in Lyme Regis. Between 1810 and 1812 she and her brother excavated a complete ichthyosaur which was 17 ft long with a 4 ft skull. They sold it for about £23 and it was deposited in William Bullock's London Museum of Natural History in Piccadilly. Quarrying operation during the 19th century revealed many specimens of ichthyosaurs and reports of these striking finds often caused excitement among the general public. Interest was reflected in the publication of cartoons and poems.

There was once an Ichthyosaurus
Who lived when the world was all porous
He fainted with shame
When he first heard his name
And departed a long time before us

(Anon.)