Reptile that saw better than a cat

IT HAD the biggest eyeballs in history for any animal with a backbone. Now scientists think they can explain why an extinct reptile needed to see better in the dark than a cat.

The eyes of ophthalmosaurus, a member of the tuna-like ichthyosaurs that lived between 250 and 90 million years ago, were nearly nine inches across, a record for an animal that was 15 feet long.

The scientists who made the discovery, led by Ryosuke Motani, of the University of California at Berkeley, state in the journal Nature that eye size was a vital factor in the reptile's habit of deep diving for prey.

Absolute size is an important property of eyes, because larger eyes can house more photosensitive cells and capture more light from the surrounding environment, they state. "Eye size also usually reflects the importance of vision in animals: for example, the horse has among the largest eyeballs of any land animal alive today, about 50mm [2in] across, which may be important, given its fast speed," they say. Bigger animals, however, also tend to have larger eyes - the blue whale's eye is nearly 6in across and is the largest of any living vertebrate - although eye size can still be small in comparison to overall body size.

The scientists found that some larger ichthyosaurs had eyes nearly 10in in diameter and one species probably had eyes about a foot across.

They found that species with bigger eyes, which were probably better at seeing in the dark, were also more likely to shows signs of the bends - when nitrogen dissolves out of the blood on rapid ascent from a deep dive.

The large eyes of the ichthyosaurs were probably an adaptation to deep diving for food under low-light conditions, the scientists say. Ophthalmosaurus probably dived to about 2,000ft and could see in the murky depths better than a cat at night.