Editing Placoderm (section)

===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]].