Funky ichthyosaur eyes

[bettashark]

Help, I think I'm slowly losing my mind here! 
Ok, everyone knows that ichthyosaurs have big, and usually well-preserved, sclerotic rings. That's a good starting point for reconstructing the size of the eye. The problem is that many ichthyosaurs are depicted with the sclerotic ring as the iris rather than the sclera, leading to huge fishy eyeballs. Now I'm all for fishy eyeballs...on fish, where they belong! Near as I can tell, the sclerotic ring would have been in a part of the eye covered by soft tissue, leaving only the central hole of the fossilized eye exposed. But finding models or images of icthyosaurs with reptilian, rather than piscine, eyes is very difficult. I've looked at so many I'm having trouble telling what looks right and what doesn't, especially after a lifetime of fish-eyed ichthyosaurs.
I'm especially having trouble deciding what to do with the model I'm repainting, as it as the funky eyeballs. I think.
The problem is that I've looked at so many incorrect reconstructions that I can't tell what's right anymore, and I can't tell if there's a problem or not with this model, or what to do about it.

[Libraraptor]

Well, this is a very special concern. I think we do have experts here, @dinotoyforum for example; although he´s rather concerned with Plesiosaurs, I think he knows more.
So you say the sclerotic ring was INSIDE of the eyeball, and not its frame, right?

[bettashark]

Right, the sclerotic ring bones are actually embedded within the eyeball, not just surrounding it from the outside. Pretty wild, but that's how living animals are, both birds and reptiles. They're especially thick in more fish-shaped ichthyosaurs, probably to counteract water pressure, as these guys apparently did a lot of deep diving. (Modern cetaceans handle this by just having really thick and sturdy eye tissues.)

[stargatedalek]

Picture the scleral ring as if it were an iris, it most decidedly is not, but in terms of the eyes overall proportions that's a decent enough way to get a good idea of how large it should be. The edges of the ring are where the eye ends, and the pupil is somewhere within the center opening.

[EmperorDinobot]

I too was wondering about this. So they're inside the eye? What other animals have massive sclerotic ring bones?

[stargatedalek]

I too was wondering about this. So they're inside the eye? What other animals have massive sclerotic ring bones?

Sort of. The outer edge of the eye is the outer edge of the ring, as opposed to either the ring being a structure within the eye or the eye sitting inside of the opening in the ring.

Most birds and many lizards have quite large eyes too. Though they generally also have proportionally smaller heads relative to Ichthyosaurs.



The hard part about reconstructing eye size based on them is they are rarely as well preserved as they look. Scleral rings aren't flat discs in life, but angled more like the bottom part of a cone. So the eye is going to be somewhat larger than you'd get estimating just on the rings opening from a fossil or a diagram.

[Halichoeres]

I too was wondering about this. So they're inside the eye? What other animals have massive sclerotic ring bones?

They are internal structures of the eyeball, yes. They are just under the sclera but superficial to the musculature that controls the shape of the lens. As mentioned, birds, lizards, ichthyosaurs, many dinosaurs, pterosaurs, and fishes like arthrodires and acanthodians. Whales are sort of oddballs in that they experience wide ranges of pressure, but have no sclerotic rings. They do have extra-thick sclera, however, which might accomplish the same thing.