Cosmineis a spongy, bony material that makes up thedentine-like layers in the scales of thelobe-finned fishesof theclassSarcopterygii.Fishscalesthat include layers of cosmine are known ascosmoid scales.[1]
Description
editAs traditionally described,[2]cosmine consists of a layer of dentine covered by a continuous sheet of enamel. Pulp cavities, which secretedentinetubules, are surrounded by a complex polygonal network of 'pore cavities' which pierce the overlyingenamellayer, 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
editCosmine was first described in theOsteolepiformMegalichthys hibbertiby 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 chondryicthianplacoidscale) 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 incoelacanthsand extantlungfish[6]proposing that coelacanths, for example, retained a juvenile scale morphology throughpedomorphosis.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
editNew 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 asMeemanniaeos,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 thatMeemanniais 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 oldersynapomorphyofOsteichthyesas opposed to a definitive sarcopterygian trait. The exact phylogenetic significance of cosmine (as classically described) remains unclear.
See also
editReferences
edit- ^Schultze, Hans-Peter (2016-01-01)."Scales, Enamel, Cosmine, Ganoine, and Early Osteichthyans".Comptes Rendus Palevol.15(1–2): 83–102.Bibcode:2016CRPal..15...83S.doi:10.1016/j.crpv.2015.04.001.ISSN1631-0683.
- ^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.JSTOR108487.
- ^abc(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.
- ^Donoghue, Philip C.J."Evolution of Development of the Vertebrate Dermal and Oral Skeletons: Unraveling Concepts, Regulatory Theories, and Homologies"(PDF).[permanent dead link]
- ^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.ISSN1469-7998.
- ^abØrvig, Tor (1969-09-01). "Cosmine and Cosmine Growth".Lethaia.2(3): 241–260.Bibcode:1969Letha...2..241O.doi:10.1111/j.1502-3931.1969.tb01850.x.ISSN1502-3931.
- ^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.
- ^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.ISSN1476-4687.PMID16672968.S2CID1840338.
- ^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.Bibcode:2016CBio...26.1602L.doi:10.1016/j.cub.2016.04.045.ISSN0960-9822.PMID27212403.
- ^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.ISSN1469-185X.PMID27194072.S2CID3497774.
Further reading
edit- Ørvig, Tor (1969). "Cosmine and cosmine growth".Lethaia.2(3): 241–260.Bibcode:1969Letha...2..241O.doi:10.1111/j.1502-3931.1969.tb01850.x.
- Meinke, Deborah K. (1984). "A review of cosmine: Its structure, development, and relationship to other forms of the dermal skeleton in osteichthyans".Journal of Vertebrate Paleontology.4(3): 457–470.Bibcode:1984JVPal...4..457M.doi:10.1080/02724634.1984.10012022.
- Thomson, Keith S. (1975)."On the biology of cosmine".Bulletin of the Peabody Museum of Natural History.40:1–59.