Open main menu
Home
Random
Recent changes
Special pages
Community portal
Preferences
About Wikipedia
Disclaimers
Incubator escapee wiki
Search
User menu
Talk
Dark mode
Contributions
Create account
Log in
Editing
Ostracoderm
(section)
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
{{Short description|Armored jawless fish of the Paleozoic}} [[File:Osteostraci Janvier.gif|thumb|Various ostracoderms of the class [[Osteostraci]] ('bony-shields')]] [[File:Cardipeltis bryanti fossil fish, Bighorn Mountains WY.jpg|thumb|''Cardipeltis bryanti'', a lower [[Devonian]] ostracoderm from the [[Bighorn Mountains]] of Wyoming. Ventral (underside) exposed.|upright]] '''Ostracodermi''' ({{lit|shell-skins}}) or '''ostracoderms''' is an informal group of [[vertebrate]] [[animal]]s that include all armored [[Agnatha|jawless fish]] of the [[Paleozoic Era]]. The term does not often appear in classifications today because it is [[paraphyletic]] (excluding [[jawed fish]]es and possibly the [[Cyclostomi|cyclostomes]] if [[anaspid]]s are closer to them) and thus does not correspond to one evolutionary lineage.<ref>[[Michael Benton|Benton, Michael]] (2009) [https://books.google.com/books?id=VThUUUtM8A4C&dq=Ostracoderms+Pituriaspida+Osteostraci&pg=PA44 ''Vertebrate Palaeontology''] Edition 3, page 44, John Wiley & Sons. {{ISBN|9781405144490}}.</ref> 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 [[gill]]s not for feeding, but exclusively for [[Respiration (physiology)|respiration]]. Earlier [[chordates]] with [[pharyngeal slits|gill precursors]] used them for both respiration and feeding.<ref>{{cite book| title=Functional Anatomy of the Vertebrates An Evolutionary Perspective| last1 = Walker | last2 = Liem | edition=2 | isbn=0-03-096846-1|publisher=Sanders College Publishing | date=1994|quote="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}}</ref> Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective [[Operculum (fish)|operculum]]. Unlike [[invertebrate]]s that use [[Cilium|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 {{wikt-lang|grc|ὄστρακον}} {{Transliteration|grc|óstrakon}} + {{wikt-lang|grc|δέρμα}} {{Transliteration|grc|dérma}}).<ref>{{cite book |title=Discovering Fossil Fishes |url=https://archive.org/details/discoveringfossi0000mais |url-access=registration |last=Maisey |first=John G. |year=1996 |publisher=Henry Holt & Company|location=New York |page=[https://archive.org/details/discoveringfossi0000mais/page/37 37] |isbn=9780805043662 |edition=illustrated}}</ref> Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth-like structures."<ref name="Shubin 2009 85–86">{{cite book|last=Shubin|first=Neil|title=Your Inner Fish: A Journay into the 3.5 Billion Year History of the Human Body|year=2009|edition=reprint|publisher=Pantheon Books|location=New York|isbn=9780307277459|pages=85–86}}</ref> [[Neil Shubin]] writes: "Cut the bone of the [ostracoderm] skull open…pop it under a microscope and…you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull--one of the earliest in the fossil record--is made entirely of little teeth. Teeth originally arose to bite creatures (see [[Conodonts]]); later a version of teeth was used in a new way to protect them."<ref name="Shubin 2009 85–86"/> Ostracoderms existed in two major groups, the more primitive [[Heterostraci|heterostracans]] and the [[cephalaspidomorphi|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.<ref>{{Cite web |url=http://planetearth.nerc.ac.uk/news/story.aspx?id=1043 |title=Fossil fish reveals how jaws evolved - Planet Earth Online |access-date=2014-04-11 |archive-url=https://web.archive.org/web/20120703111253/http://planetearth.nerc.ac.uk/news/story.aspx?id=1043 |archive-date=2012-07-03 |url-status=dead }}</ref><ref>{{Cite web |url=http://www.geol.umd.edu/~tholtz/G331/lectures/331vertsI.htm |title=GEOL 331 Invertebrate Paleontology |access-date=2014-04-11 |archive-date=2016-10-20 |archive-url=https://web.archive.org/web/20161020150428/http://www.geol.umd.edu/~tholtz/G331/lectures/331vertsI.htm |url-status=dead }}</ref> 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.<ref>[https://www.sciencedaily.com/releases/2011/07/110706134130.htm Vertebrate jaw design locked down early]</ref> The Subclass Ostracodermi has been placed in the division [[Agnatha]] along with the extant Subclass [[Cyclostomata]], which includes [[lamprey]]s and [[hagfish]]es.
Edit summary
(Briefly describe your changes)
By publishing changes, you agree to the
Terms of Use
, and you irrevocably agree to release your contribution under the
CC BY-SA 4.0 License
and the
GFDL
. You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license.
Cancel
Editing help
(opens in new window)