Large eyeballs in diving ichthyosaurs. Nature 1999;402:747.

Abstract

Eye size generally reflects the importance of vision in animals, although eye size scales with body size. For example, the blue whale has the largest eye diameter of any living vertebrate, measuring 150 mm, considered small for such a large body. Eye size and sclerotic ring aperture diameter may be used as indicators of how deep aquatic animals dive. For instance, the giant squid has the largest eyeball diameter of any living animal, measuring 250 mm in diameter. Ichthyosaurs are marine reptiles with fish-like bodies that existed approximately 250 million to 90 million years ago. The authors report that Ophthalmosaurus had the largest eyes (greater than 220 mm in diameter) and largest sclerotic ring aperture (100 mm diameter) of any ichthyosaurs. Estimating the f-number (the same measure of relative aperture as used for camera lenses), the authors found low numbers for Ophthalmosaurus, indicating that the genus could see in low-light conditions. This information, in addition to information regarding Caisson disease (the bends) in the humeri and femora of various ichthyosaurs, provide evidence that Ophthalmosaurus dove to depths of approximately 500 meters or more.— Hans E. Grossniklaus