Cosmine

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Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the class Sarcopterygii. Fish scales that include layers of cosmine are known as cosmoid scales . [1]

Contents

Description

As traditionally described, [2] 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. [3] 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. [3]

History

Cosmine was first described in the Osteolepiform Megalichthys hibberti by Williamson in 1849, in a purely descriptive, pre-Darwinian, non-evolutionary framework. [4] Goodrich [5] 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 [3] 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, [6] 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 [6] 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. [7]

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. [8] However, later studies on cranial characters [9] have indicated that Meemannia is likely a basal actinopterygian. Newer imaging studies [10] including synchrotron tomography show that pore canal systems in association with dentine occur outside the crown sarcopterygian clade, implying an older synapomorphy of Osteichthyes as opposed to a definitive sarcopterygian trait. The exact phylogenetic significance of cosmine (as classically described) remains unclear.

See also

Related Research Articles

<span class="mw-page-title-main">Actinopterygii</span> Class of ray-finned bony fishes

Actinopterygii, members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webs of skin supported by radially extended bony spines, as opposed to the bulkier, fleshy lobed fins of the sister class Sarcopterygii. Resembling folding fans, the actinopterygian fins can change shape and wetted area easily, providing superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton.

<span class="mw-page-title-main">Chondrichthyes</span> Class of jawed cartilaginous fishes

Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyians, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opecula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

<span class="mw-page-title-main">Osteichthyes</span> Diverse group of fish with skeletons of bone rather than cartilage

Osteichthyes, commonly referred to as the bony fish, is a diverse superclass of vertebrate animals that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage. The vast majority of extant fish are members of Osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, over 435 families and 28,000 species. It is the largest class of vertebrates in existence today.

<span class="mw-page-title-main">Scale (anatomy)</span> Small rigid plate that grows out of an animals skin

In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times through convergent evolution, with varying structure and function.

<span class="mw-page-title-main">Lungfish</span> A type of bony fish

Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Lungfish represent the closest living relatives of the tetrapods. The mouths of lungfish typically bear tooth plates, which are used to crush hard shelled organisms.

<span class="mw-page-title-main">Sarcopterygii</span> Class of fishes

Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a taxon of the bony fish known as the lobe-finned fish or sarcopterygians, characterised by prominent muscular buds (lobes) within the fins. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only bony spines supporting the fins.

<span class="mw-page-title-main">Dentin</span> Calcified tissue of the body; one of the four major components of teeth

Dentin or dentine 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 enamel on the crown and cementum on the root and surrounds the entire pulp. By volume, 45% of dentin consists of the mineral hydroxyapatite, 33% is organic material, and 22% is water. Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel. Dentin rates approximately 3 on the Mohs scale of mineral hardness. There are two main characteristics which distinguish dentin from enamel: firstly, dentin forms throughout life; secondly, dentin is sensitive and can become hypersensitive to changes in temperature due to the sensory function of odontoblasts, especially when enamel recedes and dentin channels become exposed.

<i>Psarolepis</i> Extinct genus of fishes

Psarolepis is a genus of extinct bony fish which lived around 397 to 418 million years ago. Fossils of Psarolepis have been found mainly in South China and described by paleontologist Xiaobo Yu in 1998. It is not known certainly in which group Psarolepis belongs, but paleontologists agree that it probably is a basal genus and seems to be close to the common ancestor of lobe-finned and ray-finned fishes. In 2001, paleontologist John A. Long compared Psarolepis with onychodontiform fishes and refer to their relationships.

<span class="mw-page-title-main">Onychodontiformes</span> Extinct order of fishes

Onychodontiformes is an order of prehistoric sarcopterygian fish that lived during the Devonian period. The onychodontiforms are generally regarded as early-diverging members of the coelacanth lineage.

<i>Styloichthys</i> Extinct genus of fishes

Styloichthys is a prehistoric sarcopterygian, lobe-finned fish which lived during the Early Devonian (Lochkovian) period of East Yunnan, South China.

Andreolepis is an extinct genus of prehistoric fish, which lived around 420 million years ago. It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden. It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.

The Xitun Formation is a palaeontological formation which is named after Xitun village in Qujing, a location in South China. This formation includes many remains of fossilized fish and plants of the Early Devonian period. It was originally referred to as the Xitun Member of the Cuifengshan Formation.

<span class="mw-page-title-main">Megalichthyidae</span> Extinct family of tetrapodomorphs

Megalichthyidae is an extinct family of tetrapodomorphs which lived from the Middle–Late Devonian to the Early Permian. They are known primarily from freshwater deposits, mostly in the Northern Hemisphere, but one genus (Cladarosymblema) is known from Australia, and the possible megalichthyid Mahalalepis is from Antarctica.

Guiyu oneiros is one of the earliest articulated bony fish discovered. Fossils of Guiyu have been found in what is now Qujing, Yunnan, China, in late Silurian marine strata, about 425 million years old.

<span class="mw-page-title-main">Skull roof</span> Roofing bones of the skull

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.

<span class="mw-page-title-main">Fish scale</span> Rigid covering growing atop a fishs skin

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 well as possible hydrodynamic advantages. The term scale derives from the Old French escale, meaning a shell pod or husk.

<span class="mw-page-title-main">Fish fin</span> Bony skin-covered spines or rays protruding from the body of a fish

Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of Actinopterygii, Dipnomorpha, Actinistia and Chondrichthyes fishes. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as seen in sharks. Apart from the tail or caudal fin, fish fins have no direct connection with the spine and are supported only by muscles. Their principal function is to help the fish swim.

<span class="mw-page-title-main">Evolution of fish</span> Origin and diversification of fish through geologic time

The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.

<span class="mw-page-title-main">Ganoine</span> Fish scale covering

Ganoine or ganoin is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some non-teleost ray-finned fishes, such as gars and bichirs, as well as lobe-finned coelacanths. It is composed of rod-like, pseudoprismatic apatite crystallites, with less than 5% of organic matter. Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa. It is a characteristic component of ganoid scales.

Cladistic classification of Sarcopterygii is the classication of Sarcopterygii as a clade containing not only the lobe-finned fishes but also the tetrapods, which are closely related to lungfish. The taxon Sarcopterygii was traditionally classified as a paraphyletic group considered either a class or a subclass of Osteichthyes. Identification of the group is based on several characteristics, such as the presence of fleshy, lobed, paired fins, which are joined to the body by a single bone.

References

  1. ↑ Schultze, Hans-Peter (2016-01-01). "Scales, Enamel, Cosmine, Ganoine, and Early Osteichthyans". Comptes Rendus Palevol. 15 (1–2): 83–102. doi: 10.1016/j.crpv.2015.04.001 . ISSN   1631-0683.
  2. ↑ Williamson, W. C. (1849). "On the Microscopic Structure of the Scales and Dermal Teeth of Some Ganoid and Placoid Fish". Philosophical Transactions of the Royal Society of London. 139: 435–475. Bibcode:1849RSPT..139..435W. doi: 10.1098/rstl.1849.0023 . JSTOR   108487.
  3. 1 2 3 (geologist.), Walter Gross (1956). Über Crossopterygier und Dipnoer aus dem baltischen Oberdevon im Zusammenhang einer vergleichenden Untersuchung des Porenkanalsystems paläozoischer Agnathen und Fische (in German). Almqvist & Wiksell.
  4. ↑ Donoghue, Philip C.J. "Evolution of Development of the Vertebrate Dermal and Oral Skeletons: Unraveling Concepts, Regulatory Theories, and Homologies" (PDF).
  5. ↑ Goodrich, Edwin S. (1907-05-01). "On the Scales of Fish, Living and Extinct, and their importance in Classification". Proceedings of the Zoological Society of London. 77 (4): 751–773. doi:10.1111/j.1469-7998.1907.tb06953.x. ISSN   1469-7998.
  6. 1 2 Ørvig, Tor (1969-09-01). "Cosmine and Cosmine Growth". Lethaia. 2 (3): 241–260. doi:10.1111/j.1502-3931.1969.tb01850.x. ISSN   1502-3931.
  7. ↑ Bemis, William; Glenn Northcutt, R (2010-04-22). "Skin and Blood Vessels of the Snout of the Australian Lungfish, Neoceratodus forsteri, and their Significance for Interpreting the Cosmine of Devonian Lungfishes". Acta Zoologica. 73 (2): 115–139. doi: 10.1111/j.1463-6395.1992.tb00956.x .
  8. ↑ Zhu, Min; Yu, Xiaobo; Wang, Wei; Zhao, Wenjin; Jia, Liantao (2006). "A primitive fish provides key characters bearing on deep osteichthyan phylogeny". Nature. 441 (7089): 77–80. Bibcode:2006Natur.441...77Z. doi:10.1038/nature04563. ISSN   1476-4687. PMID   16672968. S2CID   1840338.
  9. ↑ Lu, Jing; Giles, Sam; Friedman, Matt; den Blaauwen, Jan L.; Zhu, Min (2016-06-20). "The Oldest Actinopterygian Highlights the Cryptic Early History of the Hyperdiverse Ray-Finned Fishes". Current Biology. 26 (12): 1602–1608. doi: 10.1016/j.cub.2016.04.045 . ISSN   0960-9822. PMID   27212403.
  10. ↑ Qu, Qingming; Sanchez, Sophie; Zhu, Min; Blom, Henning; Ahlberg, Per Erik (2017-05-01). "The origin of novel features by changes in developmental mechanisms: ontogeny and three-dimensional microanatomy of polyodontode scales of two early osteichthyans". Biological Reviews. 92 (2): 1189–1212. doi:10.1111/brv.12277. ISSN   1469-185X. PMID   27194072. S2CID   3497774.

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