Editing Placoderm

{{Short description|Extinct paraphyletic class of fishes}}
{{Automatic taxobox
| name = Placoderms
| fossil_range = {{geological range|439|358.9|latest=recent}}Late [[Llandovery epoch|Llandovery]] – Late [[Devonian]]{{efn|If [[paraphyletic]] in relation to the rest of [[Gnathostomata]], then modern jawed vertebrates represent extant forms.}}
| image = Bothriolepis panderi.jpg
| image_caption = Fossil of ''[[Bothriolepis|Bothriolepis panderi]]'', an [[antiarch]] placoderm showing its [[caliper|caliper-like]] [[pectoral fin]]s
| image2 = Coccosteus reconstruction.png
| image2_caption = Life restoration of ''[[Coccosteus]]'', an [[arthrodire]] placoderm
| display_parents = 2
| taxon = Placodermi
| authority = [[Frederick McCoy|McCoy]], 1848
| subdivision_ranks = Orders
| subdivision = * [[extinction|†]]'''[[Antiarchi]]'''
* †'''[[Arthrodira]]''' (includes [[Phyllolepida]])
* †[[Petalichthyida]]
* †[[Ptyctodontida]]
* †[[Rhenanida]]
* †[[Acanthothoraci]] ([[paraphyletic]])
* "Maxillate placoderms"
** †''[[Qilinyu]]''
** †''[[Entelognathus]]''
** †''[[Silurolepis]]''
** †''[[Bianchengichthys]]''
** †''[[Minjinia]]''
** ?[[Eugnathostomata]]
* †''[[Xiushanosteus]]''
* †''[[Brindabellaspis]]''
* ?†[[Pseudopetalichthyida]]
* ?†''[[Stensioella]]''
| synonyms = * Placodermata
}}

'''Placoderms''' (from [[Ancient Greek]] πλάξ [''plax'', ''plakos''] '[[Plate (animal anatomy)|plate]]' and δέρμα [''derma''] 'skin')<ref>{{cite book |last1=Colbert |first1=Edwin H. (Edwin Harris) |last2=Knight |first2=Charles Robert |title=The dinosaur book: the ruling reptiles and their relatives |date=1951 |publisher=McGraw-Hill |location=New York |page=153 |url=https://archive.org/details/bookruli00colb/page/152/mode/2up}}</ref> are [[vertebrate]] [[animal]]s of the [[class (biology)|class]] '''Placodermi''', an [[extinct]] group of [[prehistoric fish]] known from [[Paleozoic]] [[fossil]]s during the [[Silurian]] and the [[Devonian]] [[geological period|period]]s. While their [[endoskeleton]]s are mainly [[cartilaginous]], their [[head]] and [[thorax]] were covered by articulated [[armour (zoology)|armoured plate]]s (hence the name), and the rest of the body was [[scale (zoology)|scaled]] or naked depending on the [[species]].

Placoderms were among the first [[jawed fish]] (their [[fish jaw|jaws]] likely [[Evolution|evolve]]d from the first pair of [[gill arch]]es), as well as the first vertebrates to have true [[tooth|teeth]]. They were also the first fish [[clade]] to develop [[pelvic fin]]s, the second set of [[paired fins]] and the [[homology (biology)|homologous]] precursor to [[hindlimb]]s in [[tetrapod]]s.<ref name="Rucklin2012">{{Cite journal | last1 = Rücklin | first1 = M. | last2 = Donoghue | first2 = P. C. J. | last3 = Johanson | first3 = Z. | last4 = Trinajstic | first4 = K. | last5 = Marone | first5 = F. | last6 = Stampanoni | first6 = M. | title = Development of teeth and jaws in the earliest jawed vertebrates | doi = 10.1038/nature11555 | journal = Nature | volume = 491 | issue = 7426 | pages = 748–751 | year = 2012 | pmid = 23075852| bibcode = 2012Natur.491..748R | s2cid = 4302415 }}</ref> 380-million-year-old fossils of three other genera, ''[[Incisoscutum]]'', ''[[Materpiscis]]'' and ''[[Austroptyctodus]]'', represent the oldest known examples of [[viviparity|live birth]].<ref name="BBC">{{cite news | publisher = BBC | url = http://news.bbc.co.uk/2/hi/science/nature/7424281.stm | title = Fossil reveals oldest live birth | date = May 28, 2008 |access-date = May 30, 2008 }}</ref>

Placoderms are thought to be [[paraphyly|paraphyletic]], consisting of several distinct [[outgroup (cladistics)|outgroup]]s or [[sister taxon|sister taxa]] to all living jawed [[vertebrate]]s, which originated among their ranks.<ref>{{cite journal |last1=Li |first1=Qiang |last2=Zhu |first2=You-an |last3=Lu |first3=Jing |last4=Chen |first4=Yang |last5=Wang |first5=Jianhua |last6=Peng |first6=Lijian |last7=Wei |first7=Guangbiao |last8=Zhu |first8=Min |title=A new Silurian fish close to the common ancestor of modern gnathostomes |journal=Current Biology |date=August 2021 |volume=31 |issue=16 |pages=3613–3620.e2 |doi=10.1016/j.cub.2021.05.053 |pmid=34146483 |s2cid=235477130 |doi-access=free |bibcode=2021CBio...31E3613L }}</ref> In contrast, one 2016 analysis concluded that Placodermi is likely [[monophyletic]].<ref>{{cite journal |last1=King |first1=Benedict |last2=Qiao |first2=Tuo |last3=Lee |first3=Michael S. Y. |last4=Zhu |first4=Min |last5=Long |first5=John A. |title=Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates |journal=Systematic Biology |date=5 December 2016 |volume=66 |issue=4 |pages=499–516 |doi=10.1093/sysbio/syw107 |pmid=27920231 |doi-access=free }}</ref>

The first identifiable placoderms appear in the [[fossil record]] during the late [[Llandovery epoch]] of the early Silurian.<ref name="BurrowTurner">{{cite journal | title=A review of placoderm scales, and their significance in placoderm phylogeny |author1=Burrow, Carol |author2=Turner, Susan |name-list-style=amp | journal=Journal of Vertebrate Paleontology |date=September 1998 | volume=19 | issue=2 | pages=204–219 | doi=10.1080/02724634.1999.10011135}}</ref> They eventually outcompeted the previously [[dominance (ecology)|dominant]] marine [[arthropod]]s (e.g. [[eurypterid]]s) and [[cephalopod]] [[mollusc]]s (e.g. [[orthocone]]s), producing some of the first and most infamous vertebrate [[apex predator]]s such as ''[[Eastmanosteus]]'', ''[[Dinichthys]]'' and the massive ''[[Dunkleosteus]]''. Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-Devonian [[Hangenberg event]] 358.9 million years ago,<ref name="SallanCoates">{{cite journal | title=End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates |author1=Sallan, Lauren |author2=Coates, Michael |name-list-style=amp | journal=Proceedings of the National Academy of Sciences |date=June 2010 | volume=107 | pages=10131–10135| doi= 10.1073/pnas.0914000107 | pmid=20479258 | issue=22 | pmc=2890420| bibcode=2010PNAS..10710131S |doi-access=free }}</ref> leaving the [[ecological niche|niche]]s open for the [[osteichthyan]] and [[chondrichthyan]] survivors who subsequently [[evolutionary radiation|radiate]]d during the [[Carboniferous]].

==Characteristics==
Many placoderms, particularly the [[Rhenanida]], [[Petalichthyida]], [[Phyllolepida]], and [[Antiarchi]], were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian [[Genus (biology)|genus]], ''[[Bothriolepis]]'', known from over 100 valid species.{{sfn|Long|1983}} The vast majority of placoderms were [[predator]]s, many of which lived at or near the [[Substrate (marine biology)|substrate]]. Many, primarily the [[Arthrodira|arthrodires]], were active, [[nekton]]ic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire ''[[Compagopiscis]]'' published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined [[pulp (tooth)|pulp cavities]] and were made of both bone and [[dentine]]. However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.<ref name="Rucklin2012" />

[[File:Placoderm anatomy.png|thumb|left|400px|{{center|External anatomy of the placoderm ''[[Coccosteus|Coccosteus decipiens]]''}}]]

{{clear left}}

One of the largest known arthrodires, ''[[Dunkleosteus]] terrelli'', was {{Convert|3.5 - 4.1|m|ft}} long,<ref>{{Cite journal |last=Engelman |first=Russell K. |date=2023-02-21 |title=A Devonian Fish Tale: A New Method of Body Length Estimation Suggests Much Smaller Sizes for Dunkleosteus terrelli (Placodermi: Arthrodira) |journal=Diversity |language=en |volume=15 |issue=3 |pages=318 |doi=10.3390/d15030318 |doi-access=free |issn=1424-2818}}</ref> and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as the world's first [[vertebrate]] "superpredator", preying upon other predators. Other, smaller arthrodires, such as ''[[Fallacosteus]]'' and ''[[Rolfosteus]]'', both of the [[Gogo Formation]] of Western Australia, had streamlined, bullet-shaped head armor, and ''[[Amazichthys]]'', with [[Morphology (biology)|morphology]] like that of other fast-swimming [[Pelagic fish|pelagic]] [[organism]]s,<ref>{{Cite journal |last1=Jobbins |first1=Melina |last2=Rücklin |first2=Martin |last3=Ferrón |first3=Humberto G. |last4=Klug |first4=Christian |date=2022 |title=A new selenosteid placoderm from the Late Devonian of the eastern Anti-Atlas (Morocco) with preserved body outline and its ecomorphology |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.969158 |issn=2296-701X |doi-access=free }}</ref> strongly supporting the idea that many, if not most, [[Arthrodira|arthrodires]] were active swimmers, rather than passive [[Ambush predator|ambush-hunters]] whose armor practically anchored them to the sea floor. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire ''[[Holonema]]'', and some were [[planktivore]]s, such as the gigantic arthrodire ''[[Titanichthys]]'', various members of [[Homostiidae]], and ''[[Heterosteus]]''.

Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near [[Fitzroy Crossing]], [[Kimberley (Western Australia)|Kimberley]], Western Australia,{{sfn|Long|Trinajstic|2010}} of a small female placoderm, about {{convert|25|cm|0|abbr=on}} in length, which died in the process of giving birth to a 6&nbsp;cm ({{frac|2|1|2}}&nbsp;in) offspring and was fossilized with the umbilical cord intact.{{sfn|Long|Trinajstic|Young|Senden|2008}} The fossil, named ''[[Materpiscis attenboroughi]]'' (after scientist [[David Attenborough]]), had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("[[Viviparity|viviparous]]"),<ref name="BBC"/> pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire ''[[Incisoscutum|Incisoscutum ritchei]]'', also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability.{{sfn|Long|Trinajstic|Johanson|2009}} The males reproduced by inserting a long [[clasper]] into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such as ''[[Austrophyllolepis]]''{{sfn|Long|1984}} and ''[[Cowralepis]]'', both from the Middle Devonian of Australia, suggesting that the basipterygia were used in copulation.

The placoderm claspers are not [[homology (biology)|homologous]] with the claspers in [[Chondrichthyes|cartilaginous fishes]]. The similarities between the structures has been revealed to be an example of [[convergent evolution]]. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in the [[hox gene]]s hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward.<ref>{{Cite web |url=https://theconversation.com/the-first-vertebrate-sexual-organs-evolved-as-an-extra-pair-of-legs-27578 |title=The first vertebrate sexual organs evolved as an extra pair of legs |date=8 June 2014 |access-date=2014-06-27 |archive-url=https://web.archive.org/web/20161220230411/http://theconversation.com/the-first-vertebrate-sexual-organs-evolved-as-an-extra-pair-of-legs-27578/ |archive-date=2016-12-20 }}</ref>

A study on ''[[Kolymaspis]]'' showcases that the vertebrate [[shoulder girdle]] evolved from gill arches.<ref>Brazeau et al, Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle, Nature volume 623, pages550–554 (2023)</ref>

==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.

==Taxonomy and phylogeny==
Currently, Placodermi are divided into eight recognized [[Order (biology)|orders]]. There are two further controversial orders: One is the [[monotypic]] Stensioellida, containing the enigmatic ''[[Stensioella]]''; the other is the equally enigmatic [[Pseudopetalichthyida]]. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the [[Hunsruck]] [[lagerstatten]].

===Placoderm orders===
====Arthrodira====
[[File:Dunkleosteus terrelli 2024 reconstruction.jpg|left|thumb|''[[Dunkleosteus]]'']]
[[Arthrodira]] ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giant [[apex predator]]s to [[detritus]]-nibbling [[Benthic zone|bottom dweller]]s. They had a movable joint between armour surrounding the head and body. As the lower jaw moved down, the head shield moved, allowing for a larger opening. All arthrodires, save for ''Compagopiscis'', lacked teeth, and used instead the sharpened edges of a bony plate, termed a "tooth plate", as a biting surface (''Compagopiscis'' had true teeth in addition to tooth plates). The eye sockets are protected by a bony ring, a feature shared by birds and some [[ichthyosaurs]]. Early arthrodires, such as the genus ''[[Arctolepis]]'', were well-armoured fishes with flattened bodies. The largest member of this group, ''[[Dunkleosteus]]'', was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length. In contrast, the long-nosed ''[[Rolfosteus]]'' measured just 15&nbsp;cm. Fossils of ''[[Incisoscutum]]'' have been found containing unborn fetuses, indicating that arthrodires gave birth to live young.<ref>{{cite news |url=https://www.sciencedaily.com/releases/2008/06/080606104814.htm |publisher=[[Science Daily]] |title=Fish 380 Million Years Old Found With Unborn Embryo |date=June 6, 2008}}</ref>

====Antiarchi====
[[File:Bothriolepis_canadensis_(2024).png|thumb|''[[Bothriolepis canadensis]]'']]
[[Antiarchi]] ("opposite anus") were the second most successful order of placoderms known, after the [[Arthrodira]]. The order's name was coined by [[Edward Drinker Cope]], who, after incorrectly identifying the first fossils as being those of an armored [[tunicate]], mistakenly thought the [[Orbit (anatomy)|eye-hole]] was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to the point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming [[wiktionary:sinuous|sinuous]], particularly with later forms. The pair of [[pectoral fins]] were modified into a pair of [[caliper]]-like, or [[arthropod]]-like limbs. In primitive forms, such as ''[[Yunnanolepis]]'', the limbs were thick and short, while in advanced forms, such as ''[[Bothriolepis]]'', the limbs were long and had elbow-like joints. The function of the limbs is still not perfectly understood, but most hypothesize that they helped their owners pull themselves across the substrate, as well as allowing their owners to bury themselves into the substrate.{{ctn|date=August 2013}}

====Brindabellaspida====
[[File:Brindabellaspis.png|thumb|upright|left|''[[Brindabellaspis stensioi]]'']]
''[[Brindabellaspis]]'' ("[[Brindabella Ranges|Brindabella's]] shield") was a long-snouted placoderm from the [[Early Devonian]]. When it was first discovered in 1980, it was originally regarded as a [[weejasperaspid]] [[acanthothoracid]] due to anatomical similarities with the other species found at the same locality. According to [[Philippe Janvier]], anatomical similarities in the brain of ''[[Brindabellaspis stensioi]]'' and the brain of a [[jawless fish]] suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placoderm ''[[incertae sedis]]'' have also been inserted in the order.

====Phyllolepida====
[[File:Phyllolepis12DB.jpg|thumb|''[[Phyllolepis|Phyllolepis orvini]]'']]
[[Phyllolepida]] ("leaf scales") were flattened placoderms found throughout the world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such as [[Stargazer (fish)|stargazer]]s or [[flatfish]], which have eyes on the top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the phyllolepids may have been blind.

====Ptyctodontida====
[[File:Kimbryanodus williamburyensis.jpg|thumb|left|''Kimbryanodus williamburyensis'']]
[[Ptyctodontida]] ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have a strong but superficial resemblance to modern day [[chimaera]]s. Their armour was reduced to a pattern of small plates around the head and neck. Like the extinct and related [[acanthothoracids]], and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on [[shellfish]]. On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, but [[holocephalians]] or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that the similarities between these two groups are superficial. The major differences were that holocephalians have [[shagreen]] on their skin, while ptyctodontids do not; the armoured plates and scales of holocephalians are made of [[dentine]], while those of ptyctodontids are made of bone; the craniums of holocephalians are similar to sharks, while those of ptyctodontids are similar to those of other placoderms; and, most importantly, that holocephalians have true teeth, while ptyctodonts have beak-like tooth plates. Ptyctodontids were [[sexually dimorphic]], with the males having pelvic [[clasper]]s and possibly claspers on the head as well.

====Rhenanida====
[[File:Asterosteus stenocephalus.jpg|thumb|upright|''[[Asterosteus]]'']]
[[Rhenanida]] ("[[Rhine]] fish") were flattened, [[Batoidea|ray-like]], bottom-dwelling [[predator]]s with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to the solidified plates of "advanced" placoderms, such as [[antiarch]]s and [[arthrodire]]s. However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their "mosaics" come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species.

====Acanthothoraci====
[[File:Palaeacanthaspis vasta.jpg|thumb|upright|left|''[[Palaeacanthaspis]]'']]
[[Acanthothoraci]] ("spine chests") were a group of [[Chimaera|chimaera-like]] placoderms closely related to the rhenanid placoderms. Superficially, acanthoracids resembled scaly [[chimaera]]s or small, scaly arthrodires with blunt [[Rostrum (anatomy)|rostrums]]. They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in the anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to the mid-Devonian extinction event.

====Petalichthyida====
[[File:Lunaspis.png|thumb|upright|''[[Lunaspis]]'']]
[[Petalichthyida]] ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the [[Early Devonian]] and were found throughout the world. The petalichthids ''[[Lunaspis]]'' and ''Wijdeaspis'' are among the best known. There was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida", named after the first discovered species there, ''[[Quasipetalichthys haikouensis]]''. Soon after the petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that pursued or ambushed smaller fish. Their diet is not clear, as none of the fossil specimens found have preserved mouth parts.

====Pseudopetalichthyida ====
[[File:Pseudopetalichthys problematica.jpg|thumb|upright|left|''[[Pseudopetalicthys problematica]]'']]
[[Pseudopetalichthyida]] ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata in [[Hunsrück]], Germany. Like ''[[Stensioella heintzi]]'', and the [[Rhenanida]], the pseudopetalichthids had armour made up of a mosaic of tubercles. Like ''[[Stensioella heintzi]]'', the pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms, they may be a group more advanced than the [[ptyctodont]]s. As such, placoderm experts consider [[Pseudopetalichthyida]] to be the sister group of the [[Arthrodires]] + [[Phyllolepida]] + [[Antiarchi]] [[Speciation#Genetics|trichotomy]] and the [[Acanthothoraci]] + [[Rhenanida]] [[dichotomy]].

====Stensioellida ====
[[File:Stensioella heintzi.png|thumb|upright|''[[Stensioella heintzi]]'']]
[[Stensioellida]] ("[Heintz's] little [[Erik Stensio|Stensio]]") contains another problematic placoderm of uncertain affinity, known only from the [[Lower Devonian]] [[Hunsrück]] slates of Germany. ''Stensioella'' was a thin fish that, when alive, looked vaguely like an elongated [[chimaeridae|ratfish]], or a skinny ''[[Gemuendina]]'' with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the [[rhenanids]]. It is tentatively placed within Placodermi as a primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist [[Philippe Janvier]], as well as other paleontologists, has suggested that ''Stensioella'' is not a placoderm, but instead is a [[holocephali]]an.<ref>{{Cite web |last=Carr |first=Robert K. |display-authors=etal |date=2010 |title=The ancestral morphotype for the gnathostome pectoral fin revisited and the placoderm condition |url=https://www.academia.edu/48849838 |website=Academia}}</ref><ref>{{Cite web | url=http://tolweb.org/onlinecontributors/app?service=external/ContributorDetailPage&sp=488&sp=X | title=Philippe Janvier Tree of Life Contributor Profile}}</ref> If this is true, then the holocephalians diverged from sharks before the [[Chondrichthyes|Chondrichthyan]] [[Devonian]] radiation. Critics of Janvier's position say that aside from a [[bodyplan]] superficially similar to primitive [[holocephali]]ans, the two groups have little else in common anatomically.

==Cladogram==
{{See also|Agnatha#Groups}}
The following [[cladogram]] shows the interrelationships of placoderms according to Carr ''et al.'' (2009):<ref name=CJR09>{{cite journal |last1=Carr |first1=Robert K. |last2=Johanson |first2=Zerina |last3=Ritchie |first3=Alex |title=The phyllolepid placoderm ''Cowralepis mclachlani'': Insights into the evolution of feeding mechanisms in jawed vertebrates |journal=Journal of Morphology |date=July 2009 |volume=270 |issue=7 |pages=775–804 |doi=10.1002/jmor.10719 |pmid=19215000 |s2cid=45258255 }}</ref>

[[File:Homostius2DB.jpg|thumb|right|''[[Homostius]]'' and ''[[Pterichthys]]'']]
[[File:Diandongpetalichthys baojiaoshanensis.jpg|thumbnail|''[[Diandongpetalichthys]]'']]
{{clade| style=font-size:100%;line-height:85%
|1={{clade
   |1=''[[Stensioella]]''
   |label2='''Placodermi'''
   |2={{clade
      |1=''[[Pseudopetalichthys]]''
      |2={{clade
         |1=''[[Brindabellaspis]]''
         |2=[[Acanthothoraci]]}}
      |3={{clade
         |1=[[Rhenanida]]
         |2={{clade
            |1={{clade
               |1=''[[Yunnanolepis]]''
               |2=[[Euantiarcha]]}}
            |2={{clade
               |1={{clade
                  |1=[[Petalichthyida]]
                  |2=[[Ptyctodontida]]}}
               |label2=[[Arthrodira]]
               |2={{clade
                  |1=''[[Wuttagoonaspis]]''
                  |2=[[Actinolepidae]]
                  |3={{clade
                     |1=[[Phyllolepida]]
                     |2={{clade
                        |1=[[Phlyctaeniida]]
                        |label2=[[Brachythoraci]]
                        |2={{clade
                           |1={{clade
                              |1=''[[Holonema]]''
                              |2=''[[Antineosteus]]''
                              |3=[[Buchanosteidae]]}}
                           |label2=[[Eubrachythoraci]]
                           |2={{clade
                              |1={{clade
                                 |1={{clade
                                    |1=''[[Pholidosteus]]''
                                    |2=''[[Tapinosteus]]''}}
                                 |2={{clade
                                    |1=''[[Coccosteus]]''
                                    |2=''[[Torosteus]]''}} }}
                              |2=''[[Plourdosteus]]''
                              |3={{clade
                                 |1=''[[Dunkleosteus]]''
                                 |2={{clade
                                    |1=''[[Brachyosteus]]''
                                    |2=''[[Erromenosteus]]''
                                    |3=''[[Gorgonichthys]]''
                                    |4=''[[Titanichthys]]''
}} }} }} }} }} }} }} }} }} }} }} }} }}

However, the cladogram had changed significantly over the years, and the placoderms are now thought to be [[paraphyletic]],<ref name="Giles et al 2015">{{closed access}} {{cite journal |last1=Giles |first1=Sam |last2=Friedman |first2=Matt |last3=Brazeau |first3=Martin D. |title=Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome |journal=[[Nature (journal)|Nature]] |volume=520 |issue=7545 |pages=82–85 |date=2015-01-12 |issn=1476-4687  |doi=10.1038/nature14065 |pmid=25581798 |pmc=5536226 |bibcode=2015Natur.520...82G }}</ref> with some being more closer to the [[Eugnathostomata]] than others. The updated cladogram (Zhu et al., 2016):<ref name=zhu2016>{{cite journal |last1=Zhu |first1=Min |last2=Ahlberg |first2=Per E. |last3=Pan |first3=Zhaohui |last4=Zhu |first4=Youan |last5=Qiao |first5=Tuo |last6=Zhao |first6=Wenjin |last7=Jia |first7=Liantao |last8=Lu |first8=Jing |title=A Silurian maxillate placoderm illuminates jaw evolution |journal=Science |date=21 October 2016 |volume=354 |issue=6310 |pages=334–336 |doi=10.1126/science.aah3764 |pmid=27846567 |bibcode=2016Sci...354..334Z |s2cid=45922669 }}</ref>
{{Clade|style=font-size:85%;line-height:85%
|1={{Clade
  |state1=double
   |1=[[Cephalaspidomorphi]]
   |label2=[[Gnathostomata]]
   |2={{Clade
      |1=[[Antiarchi]]
      |2={{Clade
         |1=[[Petalichthyida]]
         |2={{Clade
            |1={{Clade
               |1=[[Arthrodira]]
               |2=[[Ptyctodontida]]}}
            |2={{Clade
               |1=''[[Qilinyu]]''
               |2={{Clade
                  |1=''[[Entelognathus]]''
                  |2={{Clade
                     |1=''[[Janusiscus]]''
                     |2={{Clade
                        |label1=[[Eugnathostomata]]
                        |1={{Clade
                           |1=[[Chondrichthyes]] (total group, including [[Acanthodii]] as stem lineages)
                           |2=[[Osteichthyes]]
}} }} }} }} }} }} }} }} }} }}

==See also==
{{Portal|Paleontology|Fish}}
* [[Acanthodii]]
* [[List of placoderms]]
* [[Ostracoderm]]
* [[Chondrichthyes]]
* [[Entelognathus]]

==Notes==
{{Notelist}}

==References==
===Citations===
{{Reflist|33em}}

===Other references===
{{refbegin|2}}
* {{cite journal | last1 = Ahlberg | first1 = P.E. | last2 = Trinajstic | first2 = K. | last3 = Johanson | first3 = Z. | last4 = Long | first4 = J.A. | year = 2009 | title = Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires | journal = Nature | volume = 460 | issue = 7257| pages = 888–889 | bibcode = 2009Natur.460..888A | doi = 10.1038/nature08176 | pmid = 19597477 | s2cid = 205217467 }}
* Janvier, P. ''Early Vertebrates'' Oxford, New York: Oxford University Press, 1998. {{ISBN|0-19-854047-7}}
* {{cite journal |last1=Long |first1=J. A. |title=New bothriolepid fish from the Late Devonian of Victoria, Australia {{!}} The Palaeontological Association |journal=Palaeontology |date=May 1983 |volume=26 |issue=2 |pages=295–320 |url=https://www.palass.org/publications/palaeontology-journal/archive/26/2/article_pp295-320 }}
* {{cite journal | last1 = Long | first1 = J.A. | year = 1984 | title = New phyllolepids from Victoria and the relationships of the group | journal = Proceedings of the Linnean Society of New South Wales | volume = 107 | pages = 263–308 |url=https://www.biodiversitylibrary.org/page/34925577 }}
* Long, J.A. ''The Rise of Fishes: 500 Million Years of Evolution'' Baltimore: The Johns Hopkins University Press, 1996. {{ISBN|0-8018-5438-5}}
* {{cite journal | last1 = Long | first1 = J.A. | last2 = Trinajstic | first2 = K. | year = 2010 | title = The Late Devonian Gogo Formation Lagerstatte – Exceptional preservation and Diversity in early Vertebrates | journal = Annual Review of Earth and Planetary Sciences | volume = 38 | pages = 255–279 | doi = 10.1146/annurev-earth-040809-152416 | bibcode = 2010AREPS..38..255L }}
* {{cite journal | last1 = Long | first1 = J.A. | last2 = Trinajstic | first2 = K. | last3 = Young | first3 = G.C. | last4 = Senden | first4 = T. | year = 2008 | title = Live birth in the Devonian | journal = Nature | volume = 453 | issue = 7195| pages = 650–652 | doi=10.1038/nature06966| pmid = 18509443 | bibcode = 2008Natur.453..650L | s2cid = 205213348 }}
* {{cite journal | last1 = Long | first1 = J.A. | last2 = Trinajstic | first2 = K. | last3 = Johanson | first3 = Z. | year = 2009 | title = Devonian arthrodire embryos and the origin of internal fertilization in vertebrates | journal = Nature | volume = 457 | issue = 7233| pages = 1124–1127 | doi=10.1038/nature07732| pmid = 19242474 | bibcode = 2009Natur.457.1124L | s2cid = 205215898 }}
* {{cite journal |last1=Zhu |first1=Min |last2=Yu |first2=Xiaobo |last3=Choo |first3=Brian |last4=Wang |first4=Junqing |last5=Jia |first5=Liantao |title=An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates |journal=Biology Letters |date=23 June 2012 |volume=8 |issue=3 |pages=453–456 |doi=10.1098/rsbl.2011.1033 |pmid=22219394 |pmc=3367742 }}
{{refend}}

==External links==
{{Commons category|Placodermi}}
* [http://hoopermuseum.earthsci.carleton.ca/placoderms/first.html Annetta Markussen-Brown, "Devonian Armoured Fish" 2000]
* [http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html Introduction to the Placodermi Extinct armored fishes with jaws]
* {{cite web|url=http://www.btinternet.com/~vendian/FOSSILWEB/paleozoic_fish.htm|title=PALAEOZOIC FOSSILS UK|archive-url=https://archive.today/20121017142123/http://www.btinternet.com/~vendian/FOSSILWEB/paleozoic_fish.htm|archive-date=17 October 2012}}
* [https://web.archive.org/web/20060501194014/http://www.toyen.uio.no/palmus/galleri/montre/english/m_panserhai_e.htm Placoderms]
* [https://web.archive.org/web/20101220205158/http://palaeos.com/Vertebrates/Units/Unit060/060.000.html Placodermi: Overview]
* [http://news.bbc.co.uk/1/hi/sci/tech/6195188.stm BBC—report on ''Dunkleosteus terrelli'']

{{Placodermi}}
{{Evolution of fish}}
{{Taxonbar|from=Q130932}}
{{Authority control}}

[[Category:Placoderms| ]]
[[Category:Prehistoric fish classes]]
[[Category:Devonian fish]]
[[Category:Silurian fish]]
[[Category:Llandovery first appearances]]
[[Category:Devonian extinctions]]
[[Category:Taxa named by Frederick McCoy]]
[[Category:Paraphyletic groups]]