Placodermi

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The placoderms are among the most ancient of fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct.  The name "Placoderm" is from the Greek and means "tablet + skin", referring to the heavy armoured bony plates that completely covered the head and thorax of these curious prehistoric fish.   
The placoderms are among the most ancient of fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct.  The name "Placoderm" is from the Greek and means "tablet + skin", referring to the heavy armoured bony plates that completely covered the head and thorax of these curious prehistoric fish.   
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The [[Class]] Placodermi is composed of a group of armoured prehistoric fishes known from [[fossil]]s dating from the Mid or Late [[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 scaled or naked. Placoderms were one of the first [[Gnathostomata|jawed fish]], their jaws likely evolving from the first of their [[gill arch]]es. Starting with the the studies of [[Erik Stensio|Dr Erik Stensio]], and supported by uncrushed fossils that preserve their 3-dimensional structures from the [[Gogo Reef Formation]] in [[Australia]], it is presumed that sharks share a common ancestry with placoderms.  
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The [[Class]] Placodermi is composed of a group of armoured prehistoric fishes known from [[fossil]]s dating from the Mid or Late [[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 scaled or naked. Placoderms were one of the first [[Gnathostomata|jawed fish]], their jaws likely evolving from the first of their [[gill arch]]es. Starting with the the studies of [[Erik Stensio|Dr Erik Stensio]], and supported by uncrushed fossils that preserve their 3-dimensional structures from the [[Gogo Reef Formation]] in [[Australia]], it is presumed that sharks share a common ancestry with placoderms. Unlike extant Gnathostomes, Placodermi didn't have [[teeth]], but rather bony plates on the ridges of their jaws.[1] Furthermore, in 1997 a Placoderm fossil found in Anarctica had color pigments, thus we found out that (some) placoderms may have had color vison.[1]
Although they [[Placodermi fossil record|first appeared]] during the early [[Silurian]] period, the placoderms did not become common or widespread until the early Devonian, when they underwent an extraordinary [[evolutionary radiation]].  Or, at the very least, until the begining of the [[Devonian]], they did not live in enough environments conductive to fossilizing their remains.  By the start of the Devonian periond, they soon came to dominate most [[brackish water ecosystem|brackish]] and [[ near-shore ecosystem|near-shore]] [[ecosystem]]s, and also spread to [[marine environment|marine]] and [[freshwater environment|freshwater]] environments.  More than 250 genera are known in all, making them the most diverse and important of early vertebrates.  
Although they [[Placodermi fossil record|first appeared]] during the early [[Silurian]] period, the placoderms did not become common or widespread until the early Devonian, when they underwent an extraordinary [[evolutionary radiation]].  Or, at the very least, until the begining of the [[Devonian]], they did not live in enough environments conductive to fossilizing their remains.  By the start of the Devonian periond, they soon came to dominate most [[brackish water ecosystem|brackish]] and [[ near-shore ecosystem|near-shore]] [[ecosystem]]s, and also spread to [[marine environment|marine]] and [[freshwater environment|freshwater]] environments.  More than 250 genera are known in all, making them the most diverse and important of early vertebrates.  
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===Earliest Appearance in the Fossil Record===
===Earliest Appearance in the Fossil Record===
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The first identifiable placoderm fossils are known from the Middle [[Silurian]] ([[Wenlock]]).  The first appearance of [[Silurian]] placoderm fossils, in [[China]], show the fishes already differentiated into [[Antiarchi|Antiarchs]] and [[Arthrodira|Arthrodires]], along with the other, more primitive groups; apparently Placoderm diversity originated long before the Devonian, somewhere in the middle Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata.  At least, that's what scientists assume so far.  The Silurian placoderm fossil record is literally fragmented, as they are known only from fragments of plates and armor.  Some of these fragments bear a strong enough resemblence (due to ornamental and or histological details) to certain groups (i.e., the Antiarchs and Arthrodires) to be identified as being members of those groups, though none of these fragments have been officially described, or even named.  Ironically, the best known, or at least, most commonly cited example of a Silurian placoderm, ''[[Wangolepis]]'', is known from fragments that currently defy attempts at placement in any of the known orders of placoderms.
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The first identifiable placoderm fossils are known from the Middle [[Silurian]] ([[Wenlock]]).  The first appearance of [[Silurian]] placoderm fossils, in [[China]], show the fishes already differentiated into [[Antiarchi|Antiarchs]] and [[Arthrodira|Arthrodires]], along with the other, more primitive groups; apparently Placoderm diversity originated long before the Devonian, somewhere in the middle Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata.  At least, that's what scientists assume so far.  The Silurian placoderm fossil record is literally fragmented, as they are known only from fragments of plates and armor.  Some of these fragments bear a strong enough resemblence (due to ornamental and or histological details) to certain groups (i.e., the Antiarchs and Arthrodires) to be identified as being members of those groups, though none of these fragments have been officially described, or even named.  Ironically, the best known, or at least, most commonly cited example of a Silurian placoderm, ''[[Wangolepis]]'', is known from fragments that currently defy attempts at placement in any of the known orders of placoderms. By the Early [[Devonian]], Placoderms were well established, found in nearly every marine habitat, and soon fresh water habitats as well.[1]
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'''Credits:''' [[User:Stanton|Stanton]] 060924, MAK060926, MAK060930
'''Credits:''' [[User:Stanton|Stanton]] 060924, MAK060926, MAK060930
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== Footnotes ==
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1. [http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html Introduction to the Placodermi, UCMP]
[[Category:Placodermi]]
[[Category:Placodermi]]

Revision as of 07:11, 10 January 2014

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