Editing Placoderm (section)

==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]]''}}]]

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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>