Largest reptile eyes

Based upon estimates derived from palaeontological studies of fossil remains, the largest reptile eyes are believed to be those of the prehistoric ichthyosaurs or fish-lizards. Examining the sclerotic ring aperture's diameter in fossil ichthyosaur skulls to estimate the eyeball diameter, palaeontologists consider that the eyeball of one of the largest known ichthyosaurs, Temnodontosaurus, may have exceeded 300 mm. It lived in the deep oceans of what is today Europe (specifically those around England, France, Germany and Belgium), during the Early Jurassic Period (200–175 million years ago).

Although Temnodontosaurus is thought to have had the largest of all reptile eyes in the absolute sense, the somewhat smaller ichthyosaur Ophthalmosaurus is believed to have had the largest eyes (over 220 mm in diameter) of any ichthyosaur relative to its body length, and it also had the largest sclerotic ring aperture, with a diameter of about 100 mm.

There is much speculation regarding why these aquatic prehistoric reptiles possessed such large eyes, and one popular theory is that it enabled them to detect light, including bioluminescence, at great depths.