Cosmine is a spongy, bony material that makes up the

dentine 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 e ...

-like layers in the scales of the

lobe-finned fish Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...

es of the class

Sarcopterygii Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...

. Fish scales 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 scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

Description

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

coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...

s and extant lungfish proposing that coelacanths, for example, retained a juvenile scale morphology through pedomorphosis. 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 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 In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

of Osteichthyes 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