Editing Placoderm (section)

==Evolution and extinction==
[[File:Evolution of placoderms.png|thumb|260px|right|Evolution and extinction of [[placoderms]]. The diagram is based on [[Michael Benton]], 2005.<ref name="Benton, M. J. 2005 page 73">Benton, M. J. (2005) Vertebrate Palaeontology, Blackwell, 3rd edition, Figure 3.25 on page 73.</ref>]]
[[File:DunkleosteusSannoble.JPG|thumb|right|260px|''[[Dunkleosteus]]'', among the first of the [[vertebrate]] [[apex predator]]s, was a giant armoured placoderm [[Predation|predator]].]]
[[File:Amazichthys.jpg|thumb|261x261px|''[[Amazichthys]]'', a [[Pelagic fish|pelagic]] [[Arthrodira|arthrodire]] from the [[Famennian|Middle Famennian]] of the [[Late Devonian]].]]
[[File:Eczematolepis.jpg|thumb|right|260px|Fin spine of ''[[Eczematolepis]]'', from the [[Middle Devonian]] of [[Wisconsin]].]]
{{See also|Evolution of fish}}

It was thought for a time that placoderms became extinct due to competition from the first [[Osteichthyes|bony fish]] and early [[shark]]s, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from the environmental catastrophes of the [[Late Devonian extinction|Late Devonian]] and [[Hangenberg event|end-Devonian extinctions]].

===Fossil record===
The earliest identifiable placoderm fossils are of Chinese origin and date to the early [[Silurian]]. At that time, they were already differentiated into [[Antiarchi|antiarchs]] and [[Arthrodira|arthrodires]], as well as other, more primitive, groups. Earlier fossils of [[basal (phylogenetics)|basal]] placoderms have not yet been discovered.
[[File:Xiushanosteus.jpg|left|thumb|''[[Xiushanosteus]]'' is one of the oldest known placoderms, living in what is now China during the [[Telychian]] stage of the [[Early Silurian]]. ]]
The Silurian fossil record of the placoderms is both literally and figuratively fragmented. Until the discovery of ''[[Silurolepis]]'' (and then, the discoveries of ''[[Entelognathus]]'' and ''[[Qilinyu]]''), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of a Silurian placoderm, ''[[Wangolepis]]'' of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps:
* the basal antiarch ''[[Silurolepis]]'', from the [[Ludlow epoch]] of [[Yunnan]], China, known from an almost complete thoracic armor
* ''[[Entelognathus]]'', a placoderm ''[[incertae sedis]]'' that combines features of primitive arthrodires with jaw anatomy otherwise only seen in [[bony fish]] and [[tetrapod]]s.
* ''[[Qilinyu]]'', a close relative of ''Entelognathus'' that further links ''Entelognathus'' as a transitional form between placoderms and other stem-gnathostomes and crown-group gnathostomes.

The first officially described Silurian placoderm is an antiarch, ''[[Shimenolepis]]'', which is known from distinctively ornamented plates from [[Hunan]], China. It was originally considered to be from the late [[Llandovery epoch|Llandovery]], although later study reconsidered its age at [[Ludfordian]].<ref>{{Cite journal |last1=Pan |first1=Zhaohui |last2=Niu |first2=Zhibin |last3=Xian |first3=Zumin |last4=Zhu |first4=Min |date=2023-01-03 |title=A novel specimen-based mid-Paleozoic dataset of antiarch placoderms (the most basal jawed vertebrates) |url=https://essd.copernicus.org/articles/15/41/2023/ |journal=Earth System Science Data |language=English |volume=15 |issue=1 |pages=41–51 |doi=10.5194/essd-15-41-2023 |issn=1866-3508 |doi-access=free |bibcode=2023ESSD...15...41P }}</ref> ''Shimenolepis'' plates are very similar to the early Devonian [[Yunnanolepiformes|yunnanolepid]] ''[[Zhanjilepis]]'', also known from distinctively ornamented plates.<ref name="BurrowTurner" /><ref name="WangJ">{{cite journal | url=http://article.geobiology.cn/lunwen/%E6%9C%9F%E5%88%8A%E8%AE%BA%E6%96%87/%E5%BF%97%E7%95%99%E7%B3%BB/014/%E6%B9%98%E8%A5%BF%E5%8C%97%E5%BF%97%E7%95%99%E7%BA%AA%E8%83%B4%E7%94%B2%E9%B1%BC%E5%8C%96%E7%9F%B3.pdf | title=The Antiarchi from Early Silurian Hunan | author=Wang Junqing | journal=Vertebrata PalAsiatica | year=1991 | volume=21 | issue=3 | pages=240–244 |id={{INIST|19733953}} | archive-url=https://web.archive.org/web/20131212161921/http://article.geobiology.cn/lunwen/%E6%9C%9F%E5%88%8A%E8%AE%BA%E6%96%87/%E5%BF%97%E7%95%99%E7%B3%BB/014/%E6%B9%98%E8%A5%BF%E5%8C%97%E5%BF%97%E7%95%99%E7%BA%AA%E8%83%B4%E7%94%B2%E9%B1%BC%E5%8C%96%E7%9F%B3.pdf | archive-date=2013-12-12 }}</ref> In 2022, ''[[Xiushanosteus]]'' is described from complete fossils from [[Telychian]], late Llandovery of [[Chongqing]], China.<ref>{{Cite journal |last1=Zhu |first1=You-an |last2=Li |first2=Qiang |last3=Lu |first3=Jing |last4=Chen |first4=Yang |last5=Wang |first5=Jianhua |last6=Gai |first6=Zhikun |last7=Zhao |first7=Wenjin |last8=Wei |first8=Guangbiao |last9=Yu |first9=Yilun |last10=Ahlberg |first10=Per E. |last11=Zhu |first11=Min |date=2022 |title=The oldest complete jawed vertebrates from the early Silurian of China |url=https://www.nature.com/articles/s41586-022-05136-8 |journal=Nature |language=en |volume=609 |issue=7929 |pages=954–958 |doi=10.1038/s41586-022-05136-8 |pmid=36171378 |bibcode=2022Natur.609..954Z |s2cid=252569910 |issn=1476-4687}}</ref>

Paleontologists and placoderm specialists suspect that the scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the [[Devonian]].

During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both [[freshwater]] and [[seawater|saltwater]].<ref name=PlacodermUCMP>{{cite web|last=Waggoner|first=Ben|title=Introduction to the Placodermi|url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html|publisher=UCMP|access-date=18 July 2011}}</ref> But this diversity ultimately suffered many casualties during the extinction event at the [[Frasnian]]–[[Famennian]] boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into the [[Carboniferous]].

===History of study===
The earliest studies of placoderms were published by [[Louis Agassiz]], in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to [[ostracoderms]]. Some naturalists even suggested that they were shelled invertebrates or even [[turtle]]-like vertebrates.

In the late 1920s, Dr. [[Erik Stensiö]], at the [[Swedish Museum of Natural History]] in [[Stockholm]], established the details of placoderm anatomy and identified them as true jawed fishes related to [[shark]]s. He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in [[wax]]. Once the specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with [[shark]]s; however, as more fossils were found, placoderms were accepted as a sister group of [[Chondrichthyes|chondrichthyans]].

Much later, the exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other [[gnathostome]] groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase.<ref>{{cite journal | last1 = Young | first1 = G.C. | last2 = Goujet | first2 = D. | last3 = Lelievre | first3 = H. | year = 2001 | title = Extraocular muscles and cranial segmentation in primitive gnathostomes – fossil evidence | journal = Journal of Morphology | volume = 248 | page = 304 }}</ref><ref>{{cite book |last1=Goujet |first1=Daniel |last2=Young |first2=Gavin |year=2004 |chapter=Placoderm anatomy and phylogeny: new insights |pages= |chapter-url=http://www.pfeil-verlag.de/wp-content/uploads/2015/05/3_52d06.pdf |editor1-first=G. |editor1-last=Arratia |editor2-first=M. V. H. |editor2-last=Wilson |editor3-first=R. |editor3-last=Cloutier |title=Recent Advances in the Origin and Early Radiation of Vertebrates |publisher=Verlag Dr. Friedrich Pfeil |location=Munchen, Germany |isbn=3-89937-052-X }}</ref>

Placoderms also share certain anatomical features only with the jawless [[osteostracan]]s; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes.