Eye eye, what’s going on here?

So here is the third installment of my blog. As I am still new to this kind of thing, I am sticking with familiar territory. For me, this is ichthyosaurs (not surprising). For this blog we are going to look at their eyes, and one paper, Motani and co’s interpretation as to the reasons behind why they are so big.

Ichthyosaurs are marine reptiles from the Mesozoic, around 259 million – 90 million years ago. They are exceptional among Mesozoic marine reptiles as they evolved a fish-shaped body (they look similar to dolphins, except with an upright tail fin like sharks). They also had exceptionally large eyes for their size (Fig. 1) and are completely awesome. But what is the purpose of such large eyes?

ImageFig. 1 – Pic by Sam Bennett. Red line showing the eye socket in a specimen of Stenopterygius from the Statliches Museum fur Naturkunde. Stuttgart, Germany

Motani et. al. (1999) say that the absolute size of an eye is important. This is because the bigger the eye, the more photoreceptor cells they can hold, and therefore the more light the eyes can receive. Also, in an evolutionary context, the size of an eye in an organism can reflect on how important the use of eyesight is. Some animals that live in caves in perpetual darkness have lost the use of their eyes. In such cases the eyes are either tiny, or non-existent. We can infer then that the eyes were big in ichthyosaurs as they are important.

The authors compared eye size to body length for ichthyosaurs (using the sclerotic ring, a circle of plates in the eye) to infer eye size in ichthyosaurs (Fig. 2).

ImageFig. 2. Graph showing eye size to body length for ichthyosaurs and other organisms (Motani et. al., 1999).

The graph illustrates how much larger the eyes are  compared to other organisms. The authors go on to use f-numbers compared to eye opening size. This is the same measure that is used for cameras. The results of this show that the ichthyosaur with the lowest f-value would have been capable of seeing in low light conditions, up to 500m depth. A conservative estimate would give it a similar visual range as a cat. It is inferred that light point sources could be spotted at depth. This means that an ichthyosaur could spot bio-luminescence from prey.

From the general ichthyosaur body plan, we can safely infer that they were mainly pursuit predators, similar to dolphins and some fish today. The results of this paper show that they were capable of hunting prey at depth and would dive to get a meal. This conclusion is further supported by evidence of the bends seen in the humerus and femur (upper arm and thigh bones) of the organism. This provides a valuable insight into the feeding strategies of these large reptiles.

As ever, if you’ve got here, thanks for reading. I hope you enjoyed it. Tune in next time for whatever takes my fancy.

References

Motani, R., Rothschild B. M. & Wahl, W. J. 1999. Large eyeballs in diving ichthyosaurs. Nature. 402:747-750

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