Editing Tetrapod (section)

==Evolution==

{{main|Evolution of tetrapods}}
{{See also|Evolution of fish}}

[[File:Devonianfishes ntm 1905 smit 1929.gif|thumb|right|230px|Devonian fishes, including an early shark ''[[Cladoselache]]'', ''[[Eusthenopteron]]'' and other [[lobe-finned fish]]es, and the [[placoderm]] ''[[Bothriolepis]]'' (Joseph Smit, 1905).]]
[[File:FMC08-24 Tiktaalik rosae.tif|thumb|230x230px|Fossil of ''[[Tiktaalik]]'']]

Tetrapods [[evolution|evolved]] from early [[bony fishes]] (Osteichthyes), specifically from the tetrapodomorph branch of lobe-finned fishes ([[Sarcopterygii]]), living in the early to middle [[Devonian period]].
[[File:Eusthenopteron BW.jpg|thumb|right|230px|''[[Eusthenopteron]]'', ≈385 Ma]]

<!--[[File:Panderichthys BW.jpg|thumb|right|230px|''[[Panderichthys]]'']]-->
[[File:Tiktaalik BW.jpg|thumb|right|230px|''[[Tiktaalik]]'', ≈375 Ma]]

[[File:Acanthostega BW.jpg|thumb|right|230px|''[[Acanthostega]]'', ≈365 Ma]]
The first tetrapods probably evolved in the [[Emsian]] stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments.<ref>{{harvnb|Clack|2012|pp=125–6}}</ref><ref>{{harvnb|McGhee|2013|p=92}}</ref>
The very earliest tetrapods would have been animals similar to ''[[Acanthostega]]'', with legs and lungs as well as gills, but still primarily aquatic and unsuited to life on land.<!--Probably support with Clack?-->

The earliest tetrapods inhabited saltwater, brackish-water, and freshwater environments, as well as environments of highly variable salinity. These traits were shared with many early lobed-finned fishes. As early tetrapods are found on two Devonian continents, Laurussia ([[Euramerica]]) and [[Gondwana]], as well as the island of [[North China craton|North China]], it is widely supposed that early tetrapods were capable of swimming across the shallow (and relatively narrow) continental-shelf seas that separated these landmasses.<ref>{{harvnb|Clack|2012|p=132}}</ref><ref>{{harvnb|Laurin|2010|pp=64–8}}</ref><ref>{{harvnb|Steyer|2012|pp=37–8}}</ref>

Since the early 20th century, several families of tetrapodomorph fishes have been proposed as the nearest relatives of tetrapods, among them the [[rhizodont]]s (notably ''[[Sauripterus]]''),<ref>{{harvnb|Clack|2012|p=76}}</ref><ref>{{harvnb|McGhee|2013|p=75}}</ref> the [[Osteolepidae|osteolepidids]], the [[Tristichopteridae|tristichopterids]] (notably ''[[Eusthenopteron]]''), and more recently the [[elpistostegalia]]ns (also known as Panderichthyida) notably the genus ''[[Tiktaalik]]''.<ref>{{harvnb|McGhee|2013|pp=74–75}}</ref>

A notable feature of ''Tiktaalik'' is the absence of bones covering the gills. These bones would otherwise connect the shoulder girdle with skull, making the shoulder girdle part of the skull. With the loss of the gill-covering bones, the shoulder girdle is separated from the skull, connected to the torso by muscle and other soft-tissue connections. The result is the appearance of the neck. This feature appears only in tetrapods and ''Tiktaalik'', not other tetrapodomorph fishes. ''Tiktaalik'' also had a pattern of bones in the skull roof (upper half of the skull) that is similar to the end-Devonian tetrapod ''Ichthyostega''. The two also shared a semi-rigid ribcage of overlapping ribs, which may have substituted for a rigid spine. In conjunction with robust forelimbs and shoulder girdle, both ''Tiktaalik'' and ''Ichthyostega'' may have had the ability to locomote on land in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging behind. Finally, ''Tiktaalik'' fin bones are somewhat similar to the limb bones of tetrapods.<ref>{{harvnb|Clack|2012|pp=82–4}}</ref><ref>{{harvnb|Steyer|2012|pp=17–23}}</ref>

However, there are issues with positing ''Tiktaalik'' as a tetrapod ancestor. For example, it had a long spine with far more vertebrae than any known tetrapod or other tetrapodomorph fish. Also the oldest tetrapod trace fossils (tracks and trackways) predate it by a considerable margin. Several hypotheses have been proposed to explain this date discrepancy: 1) The nearest common ancestor of tetrapods and ''Tiktaalik'' dates to the Early Devonian. By this hypothesis, the lineage is the closest to tetrapods, but ''Tiktaalik'' itself was a late-surviving relic.<ref name="JanvierClément2010">{{cite journal|last1=Janvier|first1=Philippe|last2=Clément|first2=Gaël|title=Palaeontology: Muddy tetrapod origins|journal=Nature|volume=463|issue=7277|date=7 January 2010|pages=40–41|issn=0028-0836|doi=10.1038/463040a|pmid=20054387|bibcode=2010Natur.463...40J|s2cid=447958}}</ref> 2) ''Tiktaalik'' represents a case of parallel evolution. 3) Tetrapods evolved more than once.<ref>{{harvnb|McGhee|2013|pp=79–81}}</ref><ref>{{harvnb|Clack|2012|p=126}}</ref>

{{clade| style=font-size:85%;line-height:85%;
   |label1=[[Euteleostomi]] / [[Osteichthyes]]
   |1={{clade
  |label1=[[Actinopterygii]]
  |sublabel1=(ray-finned fishes)
  |1= [[File:Common carp (white background).jpg|70px]]
  |label2=[[Sarcopterygii]] 
  |sublabel2=(fleshy-limbed vertebrates)
  |2={{clade
    |label1=[[Actinistia]]
    |1=[[Coelacanthiformes]] (coelacanths) [[File:Coelacanth flipped.png|70 px]]
    |label2=[[Rhipidistia]]
    |2={{clade
      |label1=[[Dipnomorpha]]
      |1=[[Dipnoi]] (lungfish) <span style="{{MirrorH}}">[[File:Chinle fish Arganodus cropped cropped.png|70 px]]</span>
      |label2=[[Tetrapodomorpha]]
      |2={{clade
         |1=†Tetrapodomorph fishes [[File:Tiktaalik BW white background.jpg|70 px]] |state1=double
          |2='''Tetrapoda''' [[File:Salamandra salamandra (white background).jpg|70 px]]
               }} }} }} }} |sublabel1=(bony vertebrates)}}