Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an [[Extinction (biology)|extinct]] [[Class (biology)|]] of armoured [[prehistoric fish]], known from [[fossil]]s, which lived from the [[Silurian]] to the end of the [[Devonian]] Period. Their [[head]] and [[thorax]] were covered by articulated armoured plates and the rest of the body was [[scale (zoology)|scaled]] or naked, depending on the species. Placoderms were among the first [[jawed fish]]; their [[jaw]]s likely evolved from the first of their [[gill arch]]es. Placoderms are [[paraphyly|paraphyletic]], and comprise several distinct or [[sister taxon|sister taxa]] to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, [[Entelognathus]], from China, which is the only known placoderm with a type of [[dermal bone|bony]] jaw like that found in modern [[Osteichthyes|bony fishes]]. This includes a [[Mandible|dentary bone]], which is found in humans and other [[tetrapod]]s,.ref Placoderms were also the first fish to develop [[pelvic fin]]s, the precursor to hindlimbs in tetrapods, as well as true teeth.ref 380-million-year-old fossils of three other genera, Incistoscutum, Materpiscis and [[Austroptyctodus]], represent the oldest known example of live birth.ref
The first identifiable placoderms appear in the fossil record during the late [[Llandovery]] epoch of the early Silurian.ref The various groups of placoderms were diverse and abundant during the [[Devonian]], but became extinct at the end-Devonian [[Hangenberg event]] 360 million years ago ref
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! [[Pseudopetalichthyida|Pseudo-<br />petalichthyida]]<br />?
| [[File:Pseudopetalichthys problematica.jpg|80px|center]]<center>[[Pseudopetalicthys problematica]]</center>
| | [[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 consider then they are a group more advanced than the [[ptyctodont]]s. As such, pro-placoderm experts consider Pseudopetalichthyida to be the sister group of the [[Arthrodires]] + [[Phyllolepida]] + [[Antiarchi]] and the [[Acanthothoraci]] + [[Rhenanida]] [[dichotomy]].
! [[Stensioellida]]<br />?
| [[File:Stensioella heintzi.jpg|80px|center]]<center>[[Stensioella heintzi]]</center>