Sclerotic ring

Diagram showing the position of the sclerotic ring within a bird eye

A skull of a Uroplatus (leaf-tailed gecko), showing large sclerotic rings

The sclerotic ring of Ophthalmosaurus ("eye lizard") was one of the largest in the animal kingdom

Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater. Fossil sclerotic rings are known for a variety of extinct animals, including ichthyosaurs, pterosaurs, and dinosaurs, but are often not preserved.