The scleral ring is a hardened ring composed of multiple often bone-derived plates found in the eyes of many animals across several groups of vertebrates. Exceptions are found among mammals, amphibians, and crocodilians. The ring is located within the fibrous outer layer of the eyes called the sclera. The structure is commonly referred to as the sclerotic ring. However, as this term implies a pathology of the sclera (see: sclerosis, an unrelated medical condition.), recent authors have urged avoiding the use of this term so as to avoid confusion and increase the utility of character comparisons.
Scleral rings can be made up of cartilaginous material (scleral cartilage) or bony material (scleral ossicles), or often a combination of both, that come together to form a ring. The exact arrangement, size, shape, and number of ossicles is diverse from group to group. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater. Fossil scleral rings are known for a variety of extinct animals, including ichthyosaurs, pterosaurs, and non-avian dinosaurs, but are often not preserved.
Scleral rings may help support inner structures of the eye, especially in animals that do not have round eyes. Animals that move rapidly, including both fast flying birds and fast swimming fish have the most robust scleral rings, indicating that these thick rings are used to protect the eye during intense changes in pressure in the air and in the water. Additionally, scleral rings may help the eye adjust to different viewing differences, also known as visual accommodation. Muscles are used to adjust the shape of the eye for accommodation, and the rings provide attachment sites for these muscles. In aquatic animals, the lens is squeezed in a different way to compensate for differences in light refraction underwater, and so the shape of the ring can be different than those in terrestrial animals.
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