Ichthyosaurs, the reptiles that swam in the seas during the time of the dinosaurs, had monster-sized eyeballs.

The latest research suggests that some of the marine creatures could have had eyeballs that measured more than 30 centimetres (12 inches) across. This is far larger than any known vertebrate, and bigger even than the eyes of giant squids which are about 25 cm across.

A team of scientists, led by Ryosuke Montani, from the University of California Museum of Palaeontology, Berkeley, US, examined the fossil remains of ichthyosaur specimens to estimate their eyeball diameters. The largest eye aperture was 25.3 cm across and belonged to a species called Temnodontosaurus, which had a body length of nine metres.

Absolute size is a very important property of eyes, the team say in their report to the science journal Nature. Larger eyes can house more light-sensitive cells and let in more light. Eye size, therefore, usually reflects the importance of vision to an animal.

Deep diver

The clear suggestion here, of course, is that ichthyosaurs were able to dive to great depths where sunlight hardly penetrates. Montani's group looked for evidence that ichthyosaurs were deep divers and found one type, Opthalmasaurus, whose bones bore evidence of "the bends" (Caisson disease). This indicates that some of these creatures which lived between 250 and 90 million years ago could dive to depths of 500 metres (1,640 ft) or more.

Of all the ichthyosaurs, Opthalmasaurus probably had the biggest eyeballs for its size (four metres) at 22 cm in diameter.

But Opthalmasaurus was by no means the biggest ichthyosaur to have swum in the oceans. And the researchers make the observation that eye size generally tends to scale with body size - they quote the example of the blue whale which has the biggest eyes of any living vertebrate at 15 cm in diameter.

"There is a poorly known parvipelvian ichthyosaur that may have been 15 m long," the researchers point out, "so the largest ichthyosaurian eye was probably more than 30 cm in diameter."

Computer impression from the BBC Science series Walking With Dinosaurs