Ichthyosaur Eyes

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Fossils / Expeditions

Ichthyosaur Eyes

a mirror of the content of Ryosuke Degus's excellent webpage on Ichthyosaurs
in memory of Australia's ichthyosaur hunter Mary Wade 1928-2005


2.2.1 Eye Bones
You must be wondering how we can talk about the eyes of something extinct. Well, the secret is what is called a sclerotic ring. This doughnut shaped bone is embedded within the eyeball of most vertebrates, except mammals (like us), crocodiles, and some others. Ichthyosaurs had exceptionally well-developed sclerotic rings, as in the below photograph, which is very good for us since we can take reliable measurements off them.

The most commonly asked question about these bones is about their function. In the 1950s, an observation was made that sclerotic rings were particularly well-developed in the animals whose eyes were not spherical. So it is believed that the ring helps keep the eyes in shape, at least in some animals. Ichthyosaurs had very flattened eyeballs, so it could well be that the sclerotic ring was useful in keeping the eyeballs in shape.
There is another reason why shape-keeping may be necessary for ichthyosaurian eyes. When a fish-shaped object moves forward in water, some part of it is pushed by the surrounding water while the other parts are pulled. It is usually the front part of the fish that is pushed most strongly, and then there is no pull or push near the eye area (figure below). As you go further posteriorly, the force changes to pulling by the opercular area. So, for a fish, eye is located in a safe spot. In ichthyosaurs, however, eyes are disproportionally large, as will be discussed below. It is expected that the anterior part of their eyeball be pushed while the posterior part is pulled just by swimming. So, it is probably beneficial to have a bony structure in the eyeball to keep its shape. This applies regardless of water depth.

2-2-2. Eye Sizes
You might have heard that ichthyosaurs had enormous eyes. Yes, it is true. The largest ichthyosaurian eye that I've measured was 264 mm across, and belonged to a Temnodontosaurus platyodon. This is the largest eye ever recorded for any animal (when I wrote a paper in Nature, the largest I had then was 253 mm). This measurement is conservative, since the specimen is not entirely prepared.
You can appreciate how big the eyes are in ichthyosaurs by comparing eye sizes among animals. See the figure below for example.

As can be seen in the figures above and below, there is another ichthyosaur with the largest eye. Ophthalmosaurus is the beast. When considering what scientists call relative growth, Ophthalmosaurus has the largest eyes of all. The blue ichthyosaur shape indicates Ophthalmosaurus in the figure below, where you can see it is far above dinosaurs and even other ichthyosaurs.

2-2-3. Eye F-number
Another interesting thing about ichthyosaurian eyes is their f-numbers.
As you might know from camera lenses, f-numbers indicate the relative brightness of optical systems. If the number is small, that means the systems takes in more light. In cameras, you can shorten your shutter speed by using more expensive lenses with lower f-numbers.

Standard Camera Lenses (50 mm or so)
Kind Minimum f-number
Low End f/3.5 or higher
Middle Class f/2.8
High End f/1.0 to f/1.2

Eyes of animals have f-numbers too. It is known that nocturnal animals (night dwellers) have low f-numbers of about f/1.0, whereas diurnal animals (day dwellers) have relatively high f-numbers exceeding f/2.0. This is hardly surprising since nocturnal animals can use ability to collect extra light to enhance their vision.
Animal Eyes
Animal Minimum f-number
Human about f/2.1
Owl about f/1.1
Cat about f/0.9
Ophthalmosaurus f/0.8 to f/1.1
Ichthyosaurus f/1.1 to f/1.3

Using equations established in comparative ophthalmology, it is possible to estimate the f-number of ichthyosaurian eyes with ranges. It turned out that some fish-shaped ichthyosaurs, such as Ophthalmosaurus and Ichthyosaurus, had low f-numbers comparable to those of nocturnal animals. Other fish-shaped ichthyosaurs, such as Stenopterygius, had slightly higher f-numbers, whereas some lizard-shaped ichthyosaurs had much higher f-numbers comparable to human eyes.