Ostracoderm

Ostracodermi (lit. 'shell-skins'^[1]) or ostracoderms is an informal group of vertebrate animals that include all armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes and possibly the cyclostomes if anaspids are closer to them) and thus does not correspond to one evolutionary lineage.^[2] However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.

An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gill precursors used them for both respiration and feeding.^[3] Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths.

Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeon, but later realized that they lacked movable jaws. Hence, he classified them in 1844 as a new group, named "ostracoderms" to mean 'shell-skinned' (from Greek ὄστρακον óstrakon + δέρμα dérma).^[4]

Their heads "look like they are composed of little tooth-like structures."^[5] Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.

It was long assumed that pteraspidomorphs and thelodonts were the only ostracoderms with paired nostrils, while the other groups have just a single median nostril. It has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs.^[6]^[7]

After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years.^[8]

The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.

^ Woodward, Arthur Smith (1911). "Ostracoderms" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. p. 361.
^ Benton, Michael (2009) Vertebrate Palaeontology Edition 3, page 44, John Wiley & Sons. ISBN 9781405144490.
^ Walker; Liem (1994). Functional Anatomy of the Vertebrates An Evolutionary Perspective (2 ed.). Sanders College Publishing. ISBN 0-03-096846-1. "Gills are not present in the pharyngeal pouches of protochordates as they are in fishes; rather the [pharangeal] slits of protochordates are part of their feeding mechanism." - p 32 "Water is drawn into the pharynx ... The pharynx wall is perforated by many vertically elongated, pharyngeal slits ... Cells in the endostyle of the pharyngeal floor secrete mucus that entraps minute food particles." - p 35 "Amphioxus also gains oxygen and discharges carbon dioxide from the water flowing through the pharynx even though gills are not present." - p 35
^ Maisey, John G. (1996). Discovering Fossil Fishes (illustrated ed.). New York: Henry Holt & Company. p. 37. ISBN 9780805043662.
^ Shubin, Neil (2009). Your Inner Fish: A Journay into the 3.5 Billion Year History of the Human Body (reprint ed.). New York: Pantheon Books. pp. 85–86. ISBN 9780307277459.
^ "Fossil fish reveals how jaws evolved - Planet Earth Online". Archived from the original on 2012-07-03. Retrieved 2014-04-11.
^ "GEOL 331 Invertebrate Paleontology". Archived from the original on 2016-10-20. Retrieved 2014-04-11.
^ Vertebrate jaw design locked down early

[1] Woodward, Arthur Smith (1911). "Ostracoderms" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. p. 361.

[2] Benton, Michael (2009) Vertebrate Palaeontology Edition 3, page 44, John Wiley & Sons. ISBN 9781405144490.

[3] Walker; Liem (1994). Functional Anatomy of the Vertebrates An Evolutionary Perspective (2 ed.). Sanders College Publishing. ISBN 0-03-096846-1. "Gills are not present in the pharyngeal pouches of protochordates as they are in fishes; rather the [pharangeal] slits of protochordates are part of their feeding mechanism." - p 32 "Water is drawn into the pharynx ... The pharynx wall is perforated by many vertically elongated, pharyngeal slits ... Cells in the endostyle of the pharyngeal floor secrete mucus that entraps minute food particles." - p 35 "Amphioxus also gains oxygen and discharges carbon dioxide from the water flowing through the pharynx even though gills are not present." - p 35

[4] Maisey, John G. (1996). Discovering Fossil Fishes (illustrated ed.). New York: Henry Holt & Company. p. 37. ISBN 9780805043662.

[Shubin_2009_85–86-5] Shubin, Neil (2009). Your Inner Fish: A Journay into the 3.5 Billion Year History of the Human Body (reprint ed.). New York: Pantheon Books. pp. 85–86. ISBN 9780307277459.

[6] "Fossil fish reveals how jaws evolved - Planet Earth Online". Archived from the original on 2012-07-03. Retrieved 2014-04-11.

[7] "GEOL 331 Invertebrate Paleontology". Archived from the original on 2016-10-20. Retrieved 2014-04-11.

[8] Vertebrate jaw design locked down early

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Ostracoderm

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