Scleral Rings

Some years ago I puzzled over the eyebones of Dunkelosteus. I didn't do quite enough research Mea culpa. The layout of the skull and bones mislead me into thinking that the bones were above the eye. But no, other examples clearly showed they weren't--I'd forgotten about the skin and muscles over the bone.

In fact, far from being unusual, they're quite common. All birds have them, for instance, and all reptiles except snakes and crocodiles. They're just not so big and obvious. Different species have different numbers of bones, and the number can even vary from left to right eye.

Notice where the lens is in relation to these bits of bone in bird's eyes. The one at the bottom, labeled "tubular", is from an owl. (Hummingbird scleral rings are almost tissue-paper thin.)

I got these images from Why are bones in vertebrate eyes?.

Why? Night-active creatures seem to have larger and more conical ones, day-active ones have smaller holes. Yet they have irises also to control the amount of light, so I don't quite see the benefit.

Some say this helps keep vision sharp, possibly by holding the eyeball more stiffly in place. If you live and die by your vision as birds do, I can imagine that reducing jitter might be important. Though in the human eye a little jitter (saccades) seems to help give individual rods/cones time to recover by spreading the signal around a bit. Or something--there's disagreement. Birds have saccades too.

There are other plates in some eyes too--near the lids (if present).

From this reference: "The function of the ocular skeleton is not well understood but is believed to be as follows. The scleral cartilage cup provides structural support to the delicate retina. The scleral ossicles either assist in visual accommodation by preventing the eyeball shape from distorting as the cornea changes shape to focus light onto the retina or it also provides structural support."

They wouldn't have them if they weren't useful. Mammals don't have them (except for the platypus, which has cartilege ones), so they aren't essential. But how do they help?