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. 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.