Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

Sarcopterygii. Fish

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...

that include layers of cosmine are known as

cosmoid scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

Description

As traditionally described, cosmine consists of a layer of dentine covered by a continuous sheet of enamel. Pulp cavities, which secrete

dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...

e tubules, are surrounded by a complex polygonal network of 'pore cavities' which pierce the overlying enamel layer, giving cosmine its characteristic dotted appearance. The pulp cavities and pore chambers are connected by a complex, reticulated pore canal network which continues into a layer of vascular bone beneath the dentine. The exact configuration of the pore canal network and shape of the pore chambers differs between various taxa, although the general organization into a single layer of enamel over dentine with pore canals with vascular bone underneath remains consistent, at least within the Sarcopterygii.

History

Cosmine was first described in the

Osteolepiform Osteolepiformes, also known as Osteolepidida, is a group of prehistoric lobe-finned fishes which first appeared during the Devonian period. The order contains the families Canowindridae, Megalichthyidae, Osteolepididae and Tristichopteridae. Th ...

Megalichthys hibberti ''Megalichthys'' is a genus of prehistoric lobe-finned fish which lived during the Devonian and Carboniferous periods. It is the type genus of the family Megalichthyidae. The type species is ''M. hibberti''. The species ''M. mullisoni,'' named ...

'' by Williamson in 1849, in a purely descriptive, pre-Darwinian, non-evolutionary framework. Goodrich expanded on Williamson's descriptions, hypothesizing a transition from a monoodontode scale (like a chondryicthian

placoid A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

scale) to a complex polyodontode scale through fusion of discrete units. Gross' 1956 monograph provided the most elaborate description of cosmoid tissues detailing differences between the shape and configuration of pore canals within different clades of lobe finned fishes. Further descriptions of cosmine growth and development were advanced by Tor Ørvig, dealing specifically with the pattern of squamation, or scale formation across the body of a fish. Ørvig rationalized the observed patterns of cosmine in the fossil record with putative losses of the tissue in coelacanths and extant

lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...

proposing that coelacanths, for example, retained a juvenile scale morphology through

pedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...

. Keith Thomson later analyzed specific growth structures on the cosmine sheet- 'blisters' or 'islands' where cosmine had broken down, and deduced an electroceptive function for the pore chambers. Comparisons with electroceptive organs in extant sarcopterygians, however, have contradicted Thomson's functional hypothesis.

Phylogenetics

New fossils from China have altered current understanding of early fish evolution. Many of these fossils have been identified on the basis of histological characteristics, such as ''

Meemannia ''Meemannia'' is a genus of extinct bony fish from the early Devonian period. It was initially classified as a lobe-finned fish; however, a restudy conducted by Lu ''et al.'' (2016) indicates that it was actually an early-diverging ray-finned fis ...

eos,'' classified as an early diverging sarcopterygian on the basis of a pore canal system similar to cosmine. However, later studies on cranial characters have indicated that ''Meemannia'' is likely a basal actinopterygian. Newer imaging studies including synchrotron tomography show that pore canal systems in association with dentine occur outside the crown sarcopterygian clade, implying an older synapomorphy of

Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...

as opposed to a definitive sarcopterygian trait. The exact phylogenetic significance of cosmine (as classically described) remains unclear.

Description

History

Phylogenetics

See also

References

Further reading