The scleral ring or sclerotic 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 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. [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), 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. [3] 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. [2]
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. [6] Scleral 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 scleral 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 scleral rings, indicating that the rings help keep the eye stable during rapid swimming. [8]
Anatomy is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. Anatomy is inherently tied to developmental biology, embryology, comparative anatomy, evolutionary biology, and phylogeny, as these are the processes by which anatomy is generated, both over immediate and long-term timescales. Anatomy and physiology, which study the structure and function of organisms and their parts respectively, make a natural pair of related disciplines, and are often studied together. Human anatomy is one of the essential basic sciences that are applied in medicine, and is often studied alongside physiology.
A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal frame to which the organs and soft tissues attach; and the hydroskeleton, a flexible internal structure supported by the hydrostatic pressure of body fluids.
The skull is a bone protective cavity for the brain. The skull is composed of three types of bone: cranial bones, facial bones, and ear ossicles. Two parts are more prominent: the cranium and the mandible. In humans, these two parts are the neurocranium (braincase) and the viscerocranium that includes the mandible as its largest bone. The skull forms the anterior-most portion of the skeleton and is a product of cephalisation—housing the brain, and several sensory structures such as the eyes, ears, nose, and mouth. In humans, these sensory structures are part of the facial skeleton.
Crocodylia ) is an order of semiaquatic, predatory reptiles known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchian, a subset of archosaurs that appeared about 235 million years ago and were the only survivors of the Triassic–Jurassic extinction event. The order includes the true crocodiles, the alligators and caimans, and the gharial and false gharial. Although the term "crocodiles" is sometimes used to refer to all of these, it is less ambiguous to use "crocodilians".
The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber.
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Fish anatomy is the study of the form or morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish.
The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.
Cladoselache is an extinct genus of shark-like chondrichthyan from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks, but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.
Ophthalmosaurus is a genus of ichthyosaur known from the Middle-Late Jurassic. Possible remains from the earliest Cretaceous, around 145 million years ago, are also known. It was a relatively medium-sized ichthyosaur, measuring 4 m (13 ft) long and weighing 940 kg (2,070 lb). Named for its extremely large eyes, it had a jaw containing many small but robust teeth. Major fossil finds of this genus have been recorded in Europe with a second species possibly being found in North America.
Euparkeria is an extinct genus of archosauriform reptile from the Triassic of South Africa. Euparkeria is close to the ancestry of Archosauria, the reptile group that includes crocodilians, pterosaurs, and dinosaurs.
A nose is a sensory organ and respiratory structure in vertebrates. It consists of a nasal cavity inside the head, and an external nose on the face. The external nose houses the nostrils, or nares, a pair of tubes providing airflow through the nose for respiration. Where the nostrils pass through the nasal cavity they widen, are known as nasal fossae, and contain turbinates and olfactory mucosa. The nasal cavity also connects to the paranasal sinuses. From the nasal cavity, the nostrils continue into the pharynx, a switch track valve connecting the respiratory and digestive systems.
The evolution of mammalian auditory ossicles was an evolutionary process that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.
Chacaicosaurus is a genus of neoichthyosaurian ichthyosaur known from the Middle Jurassic of Argentina. The single known specimen of this genus was excavated from the Los Molles Formation in Neuquén Province, and is housed at the Museo Olsacher under the specimen number MOZ 5803. This specimen consists of a skull, forelimb, some vertebrae, and some additional postcranial elements. The genus was named by Marta Fernández in 1994, and contains a single species, Chacaicosaurus cayi, making it the first named distinctive ichthyosaur from the Bajocian stage. It is a medium-sized ichthyosaur with a very long snout, which bears a ridge running along each side. The forelimbs of Chacaicosaurus are small and contain four main digits.
The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.
Athabascasaurus is an extinct genus of platypterygiine ophthalmosaurid ichthyosaur known from Alberta, Canada.
Reptiles arose about 320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds that are descended from early traditionally-defined reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.
Acamptonectes is a genus of ophthalmosaurid ichthyosaurs, a type of dolphin-like marine reptiles, that lived during the Early Cretaceous around 130 million years ago. The first specimen, a partial adult skeleton, was discovered in Speeton, England, in 1958, but was not formally described until 2012 by Valentin Fischer and colleagues. They also recognised a partial subadult skeleton belonging to the genus from Cremlingen, Germany, and specimens from other localities in England. The genus contains the single species Acamptonectes densus; the generic name means "rigid swimmer" and the specific name means "compact" or "tightly packed".
Leninia is an extinct genus of basal ophthalmosaurine ichthyosaur known from the late Early Cretaceous of western Russia. Leninia was first named by Valentin Fischer, Maxim S. Arkhangelsky, Gleb N. Uspensky, Ilya M. Stenshin and Pascal Godefroit in 2013 and the type species is Leninia stellans. It was named for Vladimir Lenin, one of the leaders of the Communist Revolution in Russia, but not directlу: the museum where fossils is housed is located within the Lenin Memorial and Lenin school complex in Ulyanovsk; accordingly, the generic name reflects the geohistorical location of the find.
Kyhytysuka is an extinct genus of ophthalmosaurian ichthyosaur from Early Cretaceous Colombia. The animal was previously assigned to the genus Platypterygius, but given its own genus in 2021. Kyhytysuka was a mid-sized ophthalmosaurian with heterodont dentition and several adaptations suggesting that it was a macropredatory vertebrate hunter living in shallow waters. It contains a single species, Kyhytysuka sachicarum.
