Species with very large eyeball sizes typically are mobile predators that hunt in the deep ocean, where light is scarce and large eyes are very helpful. The largest known eyeball among living animals is that of the Colossal Squid (Mesonychoteuthis hamiltoni), with a maximum diameter of as much as 30 centimeters¹. The blue whale, whose maximum eyeball diameter is around 15 cm, is another example.
The biggest eyeball in the fossil record is probably that of Temnodontosaurus, an ichthyosaur. Ichthyosaurs were air-breathing, ocean-dwelling animals that lived during the Jurassic, about 150 million years ago. Paleontologists estimate the maximum diameter of Temnodontosaurus’s eyeball at around 26 centimeters. The estimate is based on the size of a bone called the sclerotic ring, which occurs in the eyes of ichthyosaurs and can be preserved as a fossil.
Eric J. Warrant and N. Adam Locket.
Vision in the deep sea.
Biological Reviews, volume 79, no. 3, pages 671-712 (2004). doi: 10.1017/S1464793103006420
Rulers of the Jurassic Seas.
Scientific American, volume 283, number 6. pages 52-59.(December 2000)
Ryosuke Motani, Bruce M. Rothschild and William Wahl, Jr.
Large eyeballs in diving ichthyosaurs.
Nature, volume 402, page 7474. (December 16, 1999)
Among birds that greet the day with song, the larger a species’ eyes, the earlier in the morning it begins singing. A bird doesn't benefit from informing predators of its location before it can see them coming.
Robert J. Thomas, Tamás Székely, Innes C. Cuthill, David G. C.
Harper, Stuart E. Newson, Tim D. Frayling, Paul D. Wallis.
Eye size in birds and the timing of song at dawn.
Proceedings of the Royal Society B, volume 269, number 1493, pages 831-837 (April 22, 2002).
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Last revised: 10 May 2008.