Sclerotic rings are rings of bone found in the eyes of many animals in several groups of vertebrates, except for mammals, amphibians, and crocodilians.[1] The ring is located within the soft tissue of the sclera, commonly known as the "white" of the eye. The structure has also been known as a scleral ring, to prevent confusion with sclerosis, an unrelated medical condition.[2]
They 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.[3] The exact arrangement, size, shape, and number of ossicles is diverse from group to 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 sclerotic rings are known for a variety of extinct animals, including ichthyosaurs, pterosaurs, and non-avian dinosaurs,[4][5] but are often not preserved.
The rings help to provide structure to 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 sclerotic 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, sclerotic 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]
A combination of scleral cartilage and ossicles are present, in which the cartilage acts as a cup around the posterior (rear) position of the eye and ossicles at the anterior (front) position of the eye form the ring.[3]
Within Lepidosaurs (snakes, lizards, tuatara, and relatives), sclerotic rings have been found in all major lineages except Serpentes, or snakes, and two families within Anguimorpha: Dibamidae and Rhineuridae, which are both legless lizard families.[3] All of these clades that lack a sclerotic ring share either a burrowing lifestyle or lack of limbs, indicating a possible correlation among these traits and loss of the sclerotic ring. Lizards typically have 14 ossicles in the ring, though this can vary.[2]
Within Archelosauria (turtles, birds, crocodilians, and relatives), only birds and turtles retain the sclerotic rings. Fossil evidence shows that extinct marine crocodiles living in the Mesozoic had sclerotic rings, so the trait was lost over time.[6] Sclerotic rings of varying lengths, curvatures, numbers of ossicles, and thickness are found in all birds.[7] Birds typically have 12-18 ossicles, with 14 being the most common number.[2]
While all fish have scleral cartilage, Teleost fish are the only family to retain sclerotic rings, with the rings being absent in the more basal clades Cladistia, Chondrostei, Lepisosteiformes, and Amiiformes.[2]
Teleost fish typically have only one or two ossicles per ring, and fish with no ossicles still retain cartilage.[8] Most teleosts do not have ossicles, but this can vary even within groups.[8] As a general trend, more basal groups (such as Elopomorpha and Osteoglossomorpha) tend to have no ossicles, while more derived groups (such as Percomorpha) are likely to have a variable number of ossicles (zero to two).[8]
More active fish are more likely to have sclerotic rings, indicating that the rings help keep the eye stable during rapid swimming.[8]
