Editing Cephalaspidomorphi

{{Short description|Extinct class of jawless fishes}}
{{Automatic taxobox
| name = Cephalaspidomorphs
| fossil_range = {{Fossil range|438|359|latest=0|ref=<ref>Gai Z, Lu L, Zhao W, Zhu M (2018) New polybranchiaspiform fishes (Agnatha: Galeaspida) from the Middle Palaeozoic of China and their ecomorphological implications. PLoS ONE 13(9): e0202217. {{doi|10.1371/journal.pone.0202217}}</ref><ref name="Sansom2015">{{cite journal | last1 = Sansom | first1 = Robert S. | last2 = Randle | first2 = Emma | last3 = Donoghue | first3 = Philip C. J. | date = February 7, 2015 | title = Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history | journal = [[Proceedings of the Royal Society B]] | volume = 282 | issue = 1800 | page = 20142245 | doi = 10.1098/rspb.2014.2245 | pmid=25520359 | pmc=4298210}}</ref>}}<small>Cephalaspidomorphs may have survived to the [[Holocene|present day]] if lampreys and/or gnathostomes are their descendants</small>
| image = Cephalaspis Wiki2.png
| image_caption = Life restoration of ''[[Cephalaspis|Cephalaspis lyelli]]''.
| classification_status = 
| taxon = Cephalaspidomorphi
| authority = 
| type_species = {{extinct}}''[[Cephalaspis lyelli]]''
| type_species_authority = Agassiz, 1835
| subdivision_ranks = Subgroups<ref>{{Cite book |last=Nelson |first=Joseph Schieser |title=Fishes of the world |date=2016 |publisher=John Wiley & Sons |isbn=978-1-119-17484-4 |edition=Fifth |location=Hoboken, New Jersey}}</ref>
| subdivision = {{extinct}}[[Osteostraci|Cephalaspidiformes]]<br/>
{{extinct}}[[Galeaspida|Galeaspidiformes]]<br/>
{{extinct}}[[Pituriaspida|Pituriaspidiformes]]<br/>
[[Gnathostomata]]?
}}'''Cephalaspidomorphi''' (alternatively called '''Monirhina''', or simply '''cephalaspids''')<ref name=":0">{{Cite book |last=Nelson |first=Joseph S. |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9781119174844 |title=Fishes of the World |last2=Grande |first2=Terry C. |last3=Wilson |first3=Mark V. H. |date=2016-02-22 |publisher=Wiley |isbn=978-1-118-34233-6 |edition=1 |language=en |doi=10.1002/9781119174844}}</ref> is a [[Class (biology)|class]] of [[agnatha|jawless fishes]] that is presently regarded as uniting the [[Osteostraci|osteostracans]], [[Galeaspida|galeaspids]] and [[Pituriaspida|pituriaspids]]. Most [[Biologist|biologists]] regard this [[taxon]] as extinct, but the name is still sometimes used in the classification of [[lamprey]]s because they were once thought to be descended. If lampreys are included they would extend the known range of the group from the [[Silurian]] and [[Devonian]] periods, when they are traditionally assumed to have lived, to the [[Holocene|present day]]. Modern works typically assume the cephalaspidimorphs to be the closest relatives of [[jawed fishes]], who may have emerged from within them; if this is true, they would survive if the [[Gnathostomata|jawed fish]] are included. The cephalaspidomorphs possessed armored head-shields, a [[heterocercal]] tail fin, and in some groups paired pectoral fins.

==Biology and morphology==
[[File:Cephalaspis Lyellii.jpg|thumb|254px|left|Reconstruction of ''[[Cephalaspis]] lyelli'']]
Cephalaspidomorphs possessed armored plates on the exterior of their bodies. The head shield was particularly well developed, protecting the head, [[gill]]s and the anterior section of the [[viscera]]. The body was in most forms well armored as well. The head shield had a series of grooves over the whole surface, forming an extensive [[lateral line]] organ. The eyes were rather small and placed on the top of the head. There was no [[jaw]], and instead the mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.<ref name=Colbert&Morales>{{cite book|last=Morales|first=Edwin H. Colbert, Michael|title=Evolution of the vertebrates : a history of the backboned animals through time|year=1991|publisher=Wiley-Liss|location=New York|isbn=978-0-471-85074-8|edition=4th}}</ref>

No internal skeleton is known, outside of the head shield. If they had a [[vertebral column]] at all, it would have been [[cartilaginous|cartilage]] rather than bone. Likely, the [[axial skeleton]] consisted of an unsegmented [[notochord]]. A fleshy appendage emerged laterally on each side, behind the head shield, functioning as [[pectoral fin]]s. The tail had a single, wrap-around tail-fin. Modern fishes with such a tail are rarely quick swimmers, and the Cephalaspidomorphs were not likely very active animals. They probably spent much of their time semi-submerged in the mud. They also lacked a [[swim bladder]], and would not have been able to keep afloat without actively swimming. The head shield provided some lift though and would have made the Cephalaspidomorphs better swimmers than most of their contemporaries.<ref name=Colbert&Morales/> The whole group were likely algae- or filter-feeders, combing the bottom for small animals, much like the modern armored bottom feeders, such as [[Loricariidae]] or ''[[Hoplosternum]]'' catfish.<ref>{{cite book|last=Lucas|first=F.A.|title=Animals of the past : an account of some of the creatures of the ancient world|year=1922|publisher=American Museum of Natural History|location=New York}}</ref>

== Classification ==
In the 1920s, the biologists [[Johan Kiær]] and [[Erik Stensiö]] first recognized the Cephalaspidomorphi as including the osteostracans, [[anaspid]]s, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.<ref>Stensiö, E.A. (1927): The Devonian and Downtonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. ''Skrifter om Svalbard og Ishavet'', no. 12, pp. 1–391.</ref>

Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard [[Agnatha]] as [[paraphyletic]], having given rise to the [[gnathostomata|jawed fishes]]. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to Cephalaspidomorphi. Some biologists regard the name Cephalaspidomorphi as obsolete because relations among Osteostraci and Anaspida are unclear, and the relation of lampreys to these groups is no longer supported. Others, such as the authors behind [[Palaeos.com]] have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as [[Galeaspida]] and potentially the [[Pituriaspida]].<ref name = "Palaeos">{{Cite web
  | last = White
  | first = Toby
  | title = Thelodonti: Cephalaspidomorphi
  | work = [[Palaeos]]
  | url = http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.400.html#Cephalaspidomorphi
  | access-date = 2009-10-27
  | archive-url = https://web.archive.org/web/20090306221641/http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.400.html#Cephalaspidomorphi
  | archive-date = 2009-03-06
  | url-status = dead
  }}</ref> Also following the latter definition, the class (and its [[Monotypic taxon|monotypic]] superclass Osteostracomorphi) is assumed to be the [[Monophyly|monophyletic]] sister taxon to Gnathostomata by the 2016 edition of [[Fishes of the World]].<ref name=":0" />

== Lampreys ==
Some reference works and databases have regarded Cephalaspidomorphi as a [[Linnean taxonomy|Linnean]] class whose sole living representatives are the [[lamprey]]s.<ref name = "Nelson 1994">
{{cite book
 | author = Nelson, Joseph S.
 | year = 1994
 | title = Fishes of the World
 | edition = Third
 | publisher = John Wiley and Sons
 | isbn = 0-471-54713-1}}</ref> Evidence now suggests that lampreys acquired the characters they share with cephalaspids by [[convergent evolution]].<ref name = "Forey & Janvier">
{{Cite book
  | author = Forey, Peter
  | author2 = Janvier, Philippe
  | name-list-style = amp
  | year = 2012
  | contribution = Agnathans and the origin of jawed vertebrates
  | title = Shaking the tree: readings from Nature in the history of life
  | editor-last = Gee
  | editor-first = Henry
  | editor-link = Henry Gee
  | pages = 251–266
  | place = USA
  | publisher = University of Chicago Press; Nature/Macmillan Magazines
  | isbn = 978-0-226-28497-2
  }}</ref>
<ref name = "Janvier 2008">
{{cite journal
 | author = Janvier, Philippe
 | year = 2008 
 | title = Early Jawless Vertebrates and Cyclostome Origins
 | journal = Zoological Science
 | pmid = 19267641
 | volume = 25
 | issue = 10 | pages = 1045–1056
 | doi = 10.2108/zsj.25.1045
| doi-access = free
 }}</ref>
As such, many newer works about fishes classify lampreys in a separate group called [[Hyperoartia|Petromyzontida or Hyperoartia]].<ref name = "Nelson 2006">
{{cite book
 | author = Nelson, J. S.
 | year = 2006
 | title = Fishes of the World
 | publisher = John Wiley and Sons, Inc.
 | location = New York
 | edition = 4th
 | pages = 601 pp
 | isbn = 0-471-25031-7
}}
</ref>

==References==
{{Reflist}}

== Further reading ==
*Janvier, Philippe. ''Early Vertebrates''. Oxford, New York: Oxford University Press, 1996. {{ISBN|0-19-854047-7}}

==External links==
* [http://tolweb.org/Vertebrata/14829 The Tree of Life discusses relations among jawless fish]

{{Evolution of fish|state=collapsed}}
{{Taxonbar|from=Q857497}}
{{Authority control}}

[[Category:Paleozoic jawless fish]]
[[Category:Silurian first appearances]]
[[Category:Devonian extinctions]]