Scleral ring

The scleral ring (also sclerotic ring) is a hardened ring of plates, often derived from bone, found in the eye of animals in several groups of vertebrates. Mammals, amphibians, snakes, and crocodilians lack scleral rings.^[1] The ring is in the fibrous outer layer of the eye, called the sclera.

Thumb — Skull of the red tegu (*Salvator rufescens*) including a scleral ring

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.^[2] The arrangement, size, shape, and number of ossicles vary by group.^[1] They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater.^[3]

Terminology

The structure is referred to as scleral ring or the sclerotic ring. The individual bones in the ring are the scleral ossicles if bony, or scleral cartilage if cartilaginous. The Latin name is Anulus [Annulus] ossicularis sclerae.

In some species, there may be a cartilaginous cup in the posterior half of the eye. Any ossification in the cup is called Ossicula posteriora sclerae. In birds, there is sometimes an ossification around the optic nerve, called the os opticus.^[4]

Because the word sclerotic often implies pathology of the sclera (see "sclerosis", an unrelated medical condition^[1]), some authors have urged using only "scleral ring" and avoiding "sclerotic ring", to avoid confusion and to increase the utility of character comparisons.^[2]

Rudolf Leuckart claims that the scleral ring was already noted in De arte venandi cum avibus.^[5] It was first discovered by Volcher Coiter in the 1570s,^[6] then rediscovered by Malpighi in the eye of an eagle.^[7]^[8] A 1931 thesis made an exhaustive study.^[5]

Function

Summarize

Perspective

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.^[1]

In birds, the eye is generally of a hemispherical shape on the posterior half. The anterior half may be of 3 shapes: flat (as in most diurnal birds), globose (as in Falconiformes and many Passeriformes), and tubular (as in owls). In all cases, the eye shape diverges significantly from an ellipsoidal shape in the anterior end, and the corneal limbus is usually curved inwards. This is hypothesized to require mechanical reinforcement by the scleral ring. A cartilaginous cup reinforces the posterior end.^[9]

Additionally, scleral rings may help the eye adjust to different viewing distances, 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.^[1]

The ratio between the inner radius of the scleral ring and the radius of the orbit is correlated with ecology. It is larger in modern nocturnal birds than modern diurnal birds. This has been used as evidence for arguing that some dinosaurs were nocturnal.^[10]

Distribution

Summarize

Perspective

Reptiles

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.^[2]

Within lepidosaurs (snakes, lizards, tuatara, and relatives), scleral 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.^[2] All of these clades that lack a scleral ring share either a burrowing lifestyle or lack of limbs, indicating a possible correlation among these traits and loss of the scleral ring. Lizards typically have 14 ossicles in the ring, though this can vary.^[1]

Mosasaurs, a clade of extinct marine squamates, also have scleral rings, as do members of the non-squamate Ichthyopterygia.^[1]

Within Archelosauria (turtles, birds, crocodilians, and relatives), only birds and turtles retain the scleral rings. Fossil evidence shows that extinct marine crocodiles living in the Mesozoic had scleral rings, so the trait was lost over time.^[11] Scleral rings of varying lengths, curvatures, numbers of ossicles, and thickness are found in all birds.^[12] Birds typically have 12-18 ossicles, with 14 being the most common number.^[1]

Fish

While all fish have scleral cartilage, teleost fish are the only family to retain scleral rings, with the rings being absent in the more basal clades Cladistia, Chondrostei, Lepisosteiformes, and Amiiformes.^[1]

Teleost fish typically have only one or two ossicles per ring, and fish with no ossicles still retain cartilage.^[13] Most teleosts do not have ossicles, but this can vary even within groups. 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).^[13] In the Arthrodira, the typical number is 4.^[14]

More active fish are more likely to have scleral rings, indicating that the rings help keep the eye stable during rapid swimming.^[13]

Extinct animals

In the Osteolepis, the parietal eye also has a scleral ring.^[14]^{: 201, Fig. 16}^[15]^{: 906, Fig. 42}

Homology

The scleral rings of fishes and those of reptiles and birds are not homologous.^{[citation needed]}

The scleral rings of fishes are on the eyeball's equator, and in modern teleosts number in 1 or 2, though in extinct species can go up to 4. It develops via ossification of the equatorial region of cartilaginous cup. The scleral rings in birds and reptiles are on the corneal limbus, and typically 10 to 18 in number. It develops directly from the mesenchyme, anteriorly to the cartilaginous cup.^[16]^[17]

Gallery

Tawny frogmouth (Podargus strigoides), showing large scleral rings
Great horned owl (Bubo virginianus), showing tubular scleral rings typical of owls
Satanic leaf-tailed gecko (Uroplatus phantasticus), showing large scleral rings.
Reconstructed skull and scleral ring of †Mosasaurus hoffmanni
†Stenopterygius quadriscissus, an ichthyosaur with a large scleral ring
†Viatkogorgon, a gorgonopsian therapsid ('mammal predecessor')
Prosaurolophus, a hadrosaurid dinosaur with a small scleral ring
Extinct Dunkleosteus terrelli, showing four ossicles in the ring characteristic of the Arthrodira
Extinct Osteolepis, showing prominent scleral ring on its parietal eye