Editing Placoderm (section)

===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.