The scleral ring is a hardened ring of plates, often derived from bone, that is found in the eyes of many animals in several groups of vertebrates. Some species of mammals, amphibians, and crocodilians lack scleral rings.[1] The ring is in the fibrous outer layer of the eye, called the sclera. The structure is commonly referred to as the sclerotic ring; but, because the word sclerotic often implies pathology of the sclera (see "sclerosis", an unrelated medical condition[2]), recent authors have urged avoiding the use of this term, to avoid confusion and to increase the utility of character comparisons.[3]
Scleral rings can be made of cartilaginous material (scleral cartilage) or bony material (scleral ossicles), or often a combination of both, that comes together to form a ring.[3] The arrangement, size, shape, and number of ossicles vary by group.[2] They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater.[1] Fossil scleral rings are known for a variety of extinct animals, including ichthyosaurs, pterosaurs, and non-avian dinosaurs,[4][5] 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.[2] 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.[2]