Editing Ostracoderm (section)

==Major groups==
{| class="wikitable"
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! colspan=4| Major groups of ostracoderms
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! Group
! Class
! Image
! Description
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! rowspan=4 | [[Cephalaspidomorphi|Cephalaspido-<br />morphi]]
| style="background:#eefcfc;" colspan="3" | Cephalaspidomorphi or cephalaspids ('head-shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, [[gill]]s and the anterior section of innards. The body were in most forms well armoured too. 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 atop the head. There was no [[jaw]] proper. 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> Most biologists regard this [[taxon]] as extinct, but the name is sometimes used in the classification of [[lamprey]]s because lampreys were once thought to be related to cephalaspids.  If lampreys are included, they would extend the known range of the group from the [[Silurian]] and [[Devonian]] periods to the present day. 
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! [[extinction|<sup>†</sup>]][[Galeaspida]]<br /><small>(extinct)</small>
| align=center | [[File:Galeaspida 1.JPG|80px]]
| [[Galeaspida]] ('helmet-shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the [[Silurian]] and [[Devonian]] times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their [[morphology (biology)|morphology]] appears more similar to that of [[Heterostraci]] than [[Osteostraci]], and one species, ''Tujiaaspis vividus'', had paired fins.<ref>[https://phys.org/news/2022-09-dead-fish-life-evolutionary-fins.html Dead fish breathes new life into the evolutionary origin of fins and limbs]</ref> Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the [[braincase]].
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! <sup>†</sup>[[Pituriaspida]]<br /><small>(extinct)</small>
| align=center | [[File:Pituriaspida.jpg|80px]]
| [[Pituriaspida]] ('[[Duboisia|pituri]]-shields') are a small group of extinct armoured jawless fishes with tremendous nose-like [[rostrum (anatomy)|rostrum]]s, which lived in the marine, deltaic environments of [[Middle Devonian]] Australia (about 390 Ma). They are known only by two species, ''[[Pituriaspis doylei]]'' and ''[[Neeyambaspis enigmatica]]'' found in a single sandstone location of the [[Georgina Basin]], in Western [[Queensland|Queensland, Australia]].
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! <sup>†</sup>[[Osteostraci]]<br /><small>(extinct)</small>
| align=center | [[File:Osteostraci Janvier.gif|140px]]
| [[Osteostraci]] ('bony-shells') lived in what is now North America, Europe and Russia from the [[Wenlock epoch|Middle Silurian]] to [[Late Devonian]]. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to [[lamprey]]s than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group of pituriaspids. Together, these two taxa of jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone.<ref name=SRS09>{{Cite journal | last1 = Sansom | first1 = R. S. | doi = 10.1017/S1477201908002551 | title = Phylogeny, classification and character polarity of the Osteostraci (Vertebrata) | journal = Journal of Systematic Palaeontology | volume = 7 | pages = 95–115| year = 2009 | s2cid = 85924210 }}</ref>
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! rowspan=4 | Other<br />groups
| style="background:#eefcfc;" colspan="3" align=center | Other groups
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! [[extinction|<sup>†</sup>]][[Pteraspidomorphi|Pteraspido-<br />morphi]]<br /><small>(extinct)</small>
| [[File:Larnovaspis stensioei.jpg|140px]]
| colspan=2| <sup>†</sup>[[Pteraspidomorphi]] ('wing-shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water.
The taxon contains the subgroups [[Heterostraci]], [[Astraspida]], [[Arandaspida]].

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! <sup>†</sup>[[Thelodonti]]<br /><small>(extinct)</small>
| [[File:Thelodonti.gif|140px]]
| colspan=2| [[Thelodonti]] ('feeble-teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids<ref name=Turner1982>{{cite journal|author=Turner, S.|author2=Tarling, D. H. |year=1982 |title=Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=39 |pages=295–311 |doi=10.1016/0031-0182(82)90027-X|issue=3–4|bibcode=1982PPP....39..295T }}</ref>) represent a [[Monophyly|monophyletic grouping]], or disparate stem groups to the major lines of jawless and [[Gnathostome|jawed fish]]. Thelodonts are united in possession of '[[thelodont scales]]'. This defining character is not necessarily a result of shared ancestry, as it may have been [[Convergent evolution|evolved independently by different groups]]. Thus the thelodonts are generally thought to represent a polyphyletic group,<ref>{{cite book | url = https://books.google.com/books?id=B3leE3TTeuIC&pg=PA146 | title = Vertebrate fossils and the evolution of scientific concepts: writings in tribute to Beverly Halstead | isbn = 978-2-88124-996-9 | author1 = Sarjeant, William Antony S. | author2 = L. B. Halstead | year = 1995}}</ref> although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.<ref>{{cite journal |author=Donoghue, P. C., P. L. Forey & R. J. Aldridge |year=2000 |title=Conodont affinity and chordate phylogeny |journal=[[Biological Reviews of the Cambridge Philosophical Society]] |volume=75 |issue=2 |pages=191–251 |doi=10.1111/j.1469-185X.1999.tb00045.x |pmid=10881388|s2cid=22803015 }}</ref>{{rp|206}} Thelodonts were morphologically very similar, and probably closely related, to fish of the classes [[Heterostraci]] and [[Anaspida]], differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.<ref name=Turner1999>{{cite book |author=Turner, S. |year=1999 |chapter=Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance |editor1=A. J. Boucot |editor2=J. Lawson |title=Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report |publisher=[[Cambridge University Press]] |pages=42–78}}</ref><ref name=palaeos>The early and mid Silurian. See {{cite web|author=Kazlev, M.A., White, T.|title=Thelodonti|work=Palaeos.com|date=March 6, 2001|access-date=October 30, 2007|url=http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.100.html#Thelodonti|url-status=dead|archive-url=https://web.archive.org/web/20071028153634/http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.100.html#Thelodonti|archive-date=October 28, 2007}}</ref> The fish lived in both freshwater and marine environments, first appearing during the [[Ordovician]], and perishing during the [[Late Devonian extinction|Frasnian–Famennian extinction event]] of the Late [[Devonian]]. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms.
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! <sup>†</sup>[[Anaspida]]<br /><small>(extinct)</small>
| [[File:Anaspida.png|140px]]
| colspan=2 | [[Anaspida]] ('no-shields') is an extinct group of primitive jawless vertebrates that lived during the [[Silurian]] and [[Devonian]] periods.<ref name="isbn0-415-23370-4">{{cite book |author=Ahlberg, Per Erik |title=Major events in early vertebrate evolution: palaeontology, phylogeny, genetics, and development |publisher=Taylor & Francis |location=Washington, DC |year=2001 |isbn=0-415-23370-4 |url=https://books.google.com/books?id=zeyRZNZl-74C&pg=PA188 |page= 188}}</ref> Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly [[hypocercal]] tail.  They first appeared in the [[Llandovery epoch|Early Silurian]], and flourished until the [[Late Devonian extinction]],<ref name="isbn0-226-31568-1">{{cite book |author1=Hall, Brian Keith |author2=Hanken, James |author2-link=James Hanken |title=The Skull |publisher=University of Chicago Press |location=Chicago |year=1993 |isbn=0-226-31568-1 |url= https://books.google.com/books?id=fB-hO7i50esC&pg=PA131 |page= 131}}</ref> where most species, save for [[lamprey]]s, became extinct due to the environmental upheaval during that time.
|}