View source for Tetrapod

{{Short description|Clade of the first four-limbed vertebrates and their descendants}}
{{About|four-legged vertebrates||Tetrapod (disambiguation)}}
{{Distinguish|Theropoda}}
{{Automatic taxobox
| taxon = Tetrapoda
| authority = [[Berthold Hatschek|Hatschek]] & Cori, 1896<br/>[''[[Michel Laurin|Laurin]]'']<ref>{{Cite book|last1=Hatschek|first1=B.|url=https://www.biodiversitylibrary.org/item/15974|title=Elementarcus der Zootomie in fünfzen Vorlesungen|last2=Cori|first2=C. J.|publisher=Gustav Fischer|year=1896|location=Jena|language=de|trans-title=Elementary Zootomy in Fifteen Lectures}}</ref><ref>{{cite book|title=Phylonyms: A Companion to the PhyloCode|publisher=CRC Press|year=2020|isbn=978-1-138-33293-5|editor-last=de Queiroz|editor-first=K.|location=Boca Raton|pages=759–764|language=|chapter=''Tetrapoda'' B. Hatschek and C. J. Cori 1896 [M. Laurin], converted clade name|editor-last2=Cantino|editor-first2=P. D.|editor-last3=Gauthier|editor-first3=J. A.}}</ref>
| name = Tetrapods
| fossil_range = <br/>{{fossilrange/linked|Famennian|link=[[Famennian]]? (geology)|present|ref=<ref name=":4"/>}}
| image = Tetrapoda.jpg
| image_caption = Clockwise from top left: ''[[Mercurana]] myristicapaulstris'', a shrub frog; ''[[Dermophis mexicanus]]'', a legless amphibian; ''[[Plains zebra|Equus quagga]]'', a plains zebra; ''[[Royal tern|Sterna maxima]]'', a tern (seabird); ''[[Sinai agama|Pseudotrapelus sinaitus]]'', a Sinai agama; ''[[Short-beaked echidna|Tachyglossus aculeatus]]'', a short-beaked echidna
| image_alt = a collage of six images of tetrapod animals. clockwise from top left: Mercurana myristicapaulstris, a shrub frog; Dermophis mexicanus, a legless amphibian looking like a naked snake; Equus quagga, a plains zebra; Sterna maxima, a tern (seabird); Pseudotrapelus sinaitus, a Sinai agama; Tachyglossus aculeatus, a spiny anteater
| subdivision_ranks = Subgroups
| subdivision = * †"[[Ichthyostegalia]]"?
*[[Batrachomorpha]] / [[Amphibian|Amphibia]]
** various †extinct clades
** [[Lissamphibia]]
* [[Reptiliomorpha]] / Pan-Amniota
** various †extinct clades
** [[Amniota]]
*** [[Synapsid]]a (includes [[mammal]]s)
*** [[Sauropsida]] (includes [[reptile]]s and [[bird]]s)
}}

A '''tetrapod''' ({{IPAc-en|'|t|ɛ|t|r|ə|ˌ|p|ɒ|d}};<ref>{{cite Dictionary.com|tetrapod}}</ref> {{etymology|grc|''{{wikt-lang|grc|τετρα}}'' ({{grc-transl|τετρα}})|four||''{{wikt-lang|grc|πούς}}'' ({{grc-transl|πούς}})|foot}}) is any [[vertebrate]] [[animal]] of the [[clade]] '''Tetrapoda''' ({{IPAc-en|t|ɛ|'|t|r|æ|p|ə|d|ə}}).<ref>{{Cite Merriam-Webster|tetrapoda|accessdate=2022-12-30}}</ref> Tetrapods include all [[Neontology#Extant taxa versus extinct taxa|extant]] and [[Extinction|extinct]] [[amphibian]]s and [[amniote]]s, with the latter in turn [[Evolution|evolving]] into two major clades, the [[Sauropsida|sauropsids]] ([[reptile]]s, including [[dinosaur]]s and therefore [[bird]]s) and [[synapsid]]s (extinct [[pelycosaur|"pelycosaurs"]], [[therapsid]]s and all extant [[mammal]]s, including [[Homo sapiens|humans]]). 

[[Hox gene]] mutations have resulted in some tetrapods becoming [[Limbless vertebrate|limbless]] ([[snake]]s, [[legless lizard]]s, and [[caecilian]]s) or two-limbed ([[cetacean]]s, [[sirenia]]ns, [[Bipedidae|some lizards]], [[kiwi (bird)|kiwis]], and the extinct [[moa]] and [[elephant bird]]s).<ref>{{cite journal|last1=Di-Poï|first1=Nicolas|last2=Montoya-Burgos|first2=Juan I.|last3=Miller|first3=Hilary|last4=Pourquié|first4=Olivier|last5=Milinkovitch|first5=Michel C.|last6=Duboule|first6=Denis|display-authors=3|title=Changes in Hox genes' structure and function during the evolution of the squamate body plan|url=https://www.nature.com/articles/nature08789#citeas|journal=[[Nature (journal)|Nature]]|volume=464|pages=99–103|date=2010-03-04|issue=7285|doi=10.1038/nature08789|pmid=20203609|bibcode=2010Natur.464...99D|s2cid=205219752|access-date=2023-07-06|archive-date=2023-07-05|archive-url=https://web.archive.org/web/20230705141313/https://www.nature.com/articles/nature08789#citeas|url-status=live|url-access=subscription}}</ref> Nevertheless, they still qualify as tetrapods through their ancestry, and some retain a pair of [[vestigial]] [[Pelvic spur|spurs]] that are remnants of the [[hindlimb]]s.

Tetrapods evolved from a group of [[semiaquatic]] animals within the [[Tetrapodomorpha|tetrapodomorphs]] which, in turn, evolved from ancient [[Sarcopterygii|lobe-finned fish]] ([[Sarcopterygii|sarcopterygians]]) around {{Mya|390}} in the [[Devonian#Subdivisions|Middle Devonian period]].<ref name="NarkiewiczNarkiewicz2015" /> Early tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit the body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) are [[Zachelmie trackways|trackways from the Middle Devonian]], and body fossils became common near the end of the [[Devonian#Subdivisions|Late Devonian]], around 370–360 million years ago. These Devonian species all belonged to the tetrapod [[Crown group#Stem groups|stem group]], meaning that they did not belong to any modern tetrapod group. 

Limbs evolved prior to [[terrestrial locomotion]], but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with a [[semiaquatic]] lifestyle to exploit food and shelter on land. The first [[Crown group|crown]]-tetrapods (those descended from the [[Most recent common ancestor|last common ancestors]] of extant tetrapods) appeared by the [[Tournaisian]] age of the [[Mississippian (geology)|Early Carboniferous]].<ref name=":4" />

The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The transition from a [[body plan]] for [[gill]]-based [[aquatic respiration]] and [[tail]]-propelled [[aquatic locomotion]] to one that enables the animal to survive out of water and move around on land is one of the most profound evolutionary changes known.<ref name=Gordon&Long>{{cite journal |vauthors=Long JA, Gordon MS |title=The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition |journal=Physiol. Biochem. Zool. |volume=77 |issue=5 |pages=700–19 |date=Sep–Oct 2004 |pmid=15547790 |doi=10.1086/425183 |s2cid=1260442 |url=http://www.journals.uchicago.edu/cgi-bin/resolve?PBZ040012 |access-date=2011-04-09 |archive-date=2016-04-12 |archive-url=https://web.archive.org/web/20160412011447/http://www.journals.uchicago.edu/cgi-bin/resolve?PBZ040012 |url-status=live |url-access=subscription }} [http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf as PDF] {{Webarchive|url=https://web.archive.org/web/20131029193221/http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf |date=2013-10-29 }}</ref><ref>{{cite book |last=Shubin |first=N. |author-link=Neil Shubin |title=Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body |publisher=Pantheon Books |location=New York |year=2008 |isbn=978-0-375-42447-2 |url-access=registration |url=https://archive.org/details/yourinnerfishjou00shub_0 }}</ref> Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic fish ancestors. These include distinct head and neck structures for feeding and movements, [[appendicular skeleton]]s ([[Shoulder girdle|shoulder]] and [[Pelvis|pelvic girdles]] in particular) for [[weight bearing]] and locomotion, more versatile [[eye]]s for seeing, [[middle ear]]s for hearing, and more efficient [[heart]] and [[lung]]s for oxygen circulation and exchange outside water.

Stem-tetrapods and "fish-a-pods" were primarily [[Aquatic animal|aquatic]]. [[Lissamphibia|Modern amphibians]] are generally [[semiaquatic]]; the first stages of their lives are as waterborne [[egg]]s and fish-like [[larva]]e with gills known as [[tadpole]]s, and later undergo [[metamorphosis]] to grow limbs and lungs and become partly terrestrial and partly aquatic. However, most tetrapod species today are [[amniote]]s, a group of mostly [[Terrestrial animal|terrestrial]] tetrapods that evolved early in the [[Pennsylvanian (geology)|Late Carboniferous]]. The key amniote innovation is the [[amnion]], which enables the eggs to retain their aqueous contents on land. Some tetrapods, such as [[snake]]s and [[caecilian]]s, have lost some or all of their limbs through further speciation and evolution; some only have concealed [[Vestigiality|vestigial]] bones as a remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the [[Carboniferous]] period,<ref>{{harvnb|Laurin|2010|pp=163}}</ref> while others, such as [[whales]], as recently as the [[Cenozoic]].<ref>{{cite journal |last1=Canoville |first1=Aurore |last2=Laurin |first2=Michel |date=June 2010 |title=Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences |journal=Biological Journal of the Linnean Society |volume=100 |pages=384–406 |doi=10.1111/j.1095-8312.2010.01431.x |issue=2 |doi-access=free }}</ref><ref>{{cite journal |last1=Laurin |first1=Michel |last2=Canoville |first2=Aurore |last3=Quilhac |first3=Alexandra |date=August 2009 |title=Use of paleontological and molecular data in supertrees for comparative studies: the example of lissamphibian femoral microanatomy |journal=Journal of Anatomy |volume=215 |pages=110–123 |doi=10.1111/j.1469-7580.2009.01104.x |author-link=Michel Laurin |issue=2 |pmid=19508493 |pmc=2740958 }}</ref>

Amniotes diverged into two branches: one, [[Sauropsida|sauropsids]], includes the [[reptile]]s: [[Lepidosauria|lepidosaurs]] (lizards, snakes, and the [[tuatara]]), [[archosaur]]s ([[crocodilia]]ns and [[dinosaur]]s, of which [[bird]]s are a subset), [[turtle]]s, and various other extinct forms. The other branch, [[Synapsida|synapsids]], include [[mammal]]s and their extinct relatives. 

Amniotes include the only animals apart from insects that further evolved [[Flying and gliding animals#Powered_flight|powered flight]]: the extinct [[pterosaur]]s and extant birds and [[bat]]s. In addition, tetrapods are the only terrestrial lineage among animals, excluding the various terrestrial lineages of [[Protostomia]].

==Definitions==
The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with the earliest members of the group.<ref name=":0">{{Cite journal |last=Laurin |first=Michel |date=2002-03-01 |title=Tetrapod Phylogeny, Amphibian Origins, and the Definition of the Name Tetrapoda |journal=Systematic Biology |volume=51 |issue=2 |pages=364–369 |doi=10.1080/10635150252899815 |pmid=12028737 |issn=1076-836X}}</ref><ref name=":2">{{Cite journal |last=Anderson |first=Jason S. |date=2002-09-01 |title=Use of Well-Known Names in Phylogenetic Nomenclature: A Reply to Laurin |journal=Systematic Biology |volume=51 |issue=5 |pages=822–827 |doi=10.1080/10635150290102447 |pmid=12396594 |issn=1076-836X|doi-access=free }}</ref><ref name="RCQ03">{{cite journal |last=Ruta |first=M. |author2=Coates, M.I. |author3=Quicke, D.L.J. |year=2003 |title=Early tetrapod relationships revisited |url=http://www.amphibiatree.org/sites/amphibiatree.org/files/RutaETAL2003Tetrapod.pdf |journal=Biological Reviews |volume=78 |issue=2 |pages=251–345 |doi=10.1017/S1464793102006103 |pmid=12803423 |s2cid=31298396 |archive-date=2016-09-20 |access-date=2022-08-02 |archive-url=https://web.archive.org/web/20160920135951/http://www.amphibiatree.org/sites/amphibiatree.org/files/RutaETAL2003Tetrapod.pdf |url-status=live }}</ref><ref name=":3">{{Cite journal |last1=Laurin |first1=Michel |last2=Anderson |first2=Jason S. |date=2004-02-01 |editor-last=Simon |editor-first=Chris |title=Meaning of the Name Tetrapoda in the Scientific Literature: An Exchange |journal=Systematic Biology |language=en |volume=53 |issue=1 |pages=68–80 |doi=10.1080/10635150490264716 |pmid=14965901 |s2cid=15922260 |issn=1076-836X|doi-access=free }}</ref>

=== Apomorphy-based definitions ===
{{See also|Stegocephali}}
A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct [[Digit (anatomy)|digits]] (fingers and toes), as well as legless vertebrates with limbed ancestors.<ref name=":2" /><ref name="RCQ03" /> Limbs and digits are major [[apomorphies]] (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. The first vertebrates with limbs and digits evolved in the [[Devonian]], including the [[Late Devonian]]-age ''[[Ichthyostega]]'' and ''[[Acanthostega]]'', as well as the trackmakers of the [[Middle Devonian]]-age [[Zachelmie trackways]].<ref name="NarkiewiczNarkiewicz2015" />

Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through [[convergent evolution]]. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalent [[Cladistics|cladistic]] definition. Cladistics is a modern branch of [[Taxonomy (biology)|taxonomy]] which classifies organisms through evolutionary relationships, as reconstructed by [[phylogenetic analyses]]. A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a [[Monophyly|monophyletic]] [[clade]], a group with all of its component taxa sharing a single common ancestor.<ref name="RCQ03" /> In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates.<ref name=":3" />

=== Crown group tetrapods ===
[[File:Tetrapoda PhyloCode (en).svg|left|thumb|307x307px|A simplified cladogram demonstrating differing definitions of Tetrapoda:

<br />* Under the [[Apomorphy and synapomorphy|apomorphy]]-based definition used by many paleontologists, tetrapods originate at the orange star ("First vertebrates with tetrapod limb")
<br />* When restricted to the [[crown group]], tetrapods originate at the "last common ancestor of recent tetrapods"

]]
A portion of tetrapod scientists, led by French paleontologist [[Michel Laurin]], prefer to restrict the definition of tetrapod to the [[crown group]].<ref name=":0" /><ref name=":1">{{Cite book |last1=Queiroz |first1=Kevin de |url=https://www.taylorfrancis.com/books/mono/10.1201/9780429446276/phylonyms-kevin-de-queiroz-philip-cantino-jacques-gauthier |title=Phylonyms: A Companion to the PhyloCode |last2=Cantino |first2=Philip D. |last3=Gauthier |first3=Jacques A. |editor-first1=Kevin |editor-first2=Philip |editor-first3=Jacques |editor-last1=De Queiroz |editor-last2=Cantino |editor-last3=Gauthier |publisher=CRC Press |year=2020 |edition=1st |location=Boca Raton |chapter=Stegocephali E. D. Cope 1868 [M. Laurin], converted clade name |doi=10.1201/9780429446276 |isbn=978-0-429-44627-6 |s2cid=242704712 |archive-date=2023-04-05 |access-date=2022-08-02 |archive-url=https://web.archive.org/web/20230405085720/https://www.taylorfrancis.com/books/mono/10.1201/9780429446276/phylonyms-kevin-de-queiroz-philip-cantino-jacques-gauthier |url-status=live }}</ref> A crown group is a subset of a category of animal defined by the most recent common ancestor of living representatives. This cladistic approach defines "tetrapods" as the nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a [[Node-based taxon|node-based]] [[clade]], a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).<ref name="RCQ03" />

Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such as ''Ichthyostega'' and ''Acanthostega'', certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along the [[Crown group|stem group]], a subset of animals related to, but not within, the crown group. The stem and crown group together are combined into the [[total group]], given the name [[Tetrapodomorpha]], which refers to all animals closer to living tetrapods than to Dipnoi ([[lungfish]]es), the next closest group of living animals.<ref>{{harvnb|Clack|2012|pp=87–9}}</ref> Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as the presence of limbs and digits.

Laurin's approach to the definition of tetrapods is rooted in the belief that the term has more relevance for [[Neontology|neontologists]] (an informal term used for biologists specializing in living organisms) than paleontologists (who primarily use the apomorphy-based definition).<ref name=":3" /> In 1998, he re-established the defunct historical term '''[[Stegocephali]]''' to replace the apomorphy-based definition of tetrapod used by many authors.<ref name="LaurinGirondot2000">{{cite journal|last1=Laurin|first1=Michel|last2=Girondot|first2=Marc|last3=de Ricqlès|first3=Armand|title=Early tetrapod evolution|journal=Trends in Ecology & Evolution|volume=15|issue=3|year=2000|pages=118–123|url=http://max2.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf|issn=0169-5347|doi=10.1016/S0169-5347(99)01780-2|pmid=10675932|access-date=2015-06-08|archive-date=2012-07-22|archive-url=https://web.archive.org/web/20120722083254/http://max2.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf}}</ref> Other paleontologists use the term [[stem-tetrapod]] to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes.<ref>{{harvnb|Laurin|2010|p=9}}</ref> The term "fishapod" was popularized after the discovery and 2006 publication of ''[[Tiktaalik]]'', an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits.

The two subclades of crown tetrapods are [[Batrachomorpha]] and [[Reptiliomorpha]]. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians.<ref>{{harvnb|Benton|2009|p=99}}</ref> Gaffney (1979) provided the name '''Neotetrapoda''' to the crown group of tetrapods, though few subsequent authors followed this proposal.<ref name=":3" />

The earliest fossils attributed to crown-group tetrapods are footprints from the earliest Carboniferous ([[Tournaisian]]) of Australia, which appear to belong to early [[amniote]]s or potentially even [[Sauropsida|sauropsids]]. Prior to the discovery of these prints, the earliest evidence of crown-group tetrapods were [[Temnospondyli|temnospondyl]] footprints from slightly later in the Tournaisian, with the earliest body fossils being of the temnospondyl ''[[Balanerpeton]]'' from the [[Viséan]].<ref name=":4">{{Cite journal |last=Long |first=John A. |last2=Niedźwiedzki |first2=Grzegorz |last3=Garvey |first3=Jillian |last4=Clement |first4=Alice M. |last5=Camens |first5=Aaron B. |last6=Eury |first6=Craig A. |last7=Eason |first7=John |last8=Ahlberg |first8=Per E. |date=2025-05-14 |title=Earliest amniote tracks recalibrate the timeline of tetrapod evolution |url=https://www.nature.com/articles/s41586-025-08884-5 |journal=Nature |language=en |pages=1–8 |doi=10.1038/s41586-025-08884-5 |issn=1476-4687|doi-access=free |pmc=12119326 }}</ref>

==Biodiversity==
<!-- Deleted image removed: [[File:Biodiversity of Tetrapods by Sahney Benton and Ferry.gif|thumb|left|400px|''[[Biodiversity of Tetrapods]]'']] -->
Tetrapoda includes the four traditional living [[Class (biology)|classes]]: amphibians, reptiles, birds and mammals. Overall, the biodiversity of [[lissamphibian]]s,<ref name=M&L08>{{cite journal | last1 = Marjanović | first1 = D. | last2 = Laurin | first2 = M. | name-list-style = vanc | year = 2008 | title = Assessing confidence intervals for stratigraphic ranges of higher taxa: the case of Lissamphibia | journal = Acta Palaeontologica Polonica | volume = 53 | issue = 3 | pages = 413–432 | doi = 10.4202/app.2008.0305 | s2cid = 53592421 | url = http://app.pan.pl/archive/published/app53/app53-413.pdf | access-date = 2013-01-17 | archive-date = 2013-10-29 | archive-url = https://web.archive.org/web/20131029190341/http://app.pan.pl/archive/published/app53/app53-413.pdf | url-status = live | doi-access = free }}</ref> as well as of tetrapods generally,<ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal | last1=Sahney |first1=S. |last2=Benton |first2=M.J. |last3=Ferry |first3=P.A. | date=August 2010 | title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal=Biology Letters | doi=10.1098/rsbl.2009.1024 | volume = 6 | pages = 544–547 | pmc=2936204 | issue=4 | pmid=20106856 }}</ref> has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises, such as the [[Permian–Triassic extinction event]], which at least affected amniotes.<ref>{{cite journal |last1=Ward |first1=P.D. |last2=Botha |first2=J. |last3=Buick |first3=R. |last4=Kock |first4=M.O. |last5=Erwin |first5=D.H. |last6=Garrisson |first6=G.H. |last7=Kirschvink |first7=J.L. |last8=Smith |first8=R. |date=4 February 2005 |title=Abrupt and gradual extinction among late Permian land vertebrates in the Karoo Basin, South Africa |journal=Science |volume=307 |pages=709–714 |doi=10.1126/science.1107068 |pmid=15661973 |issue=5710 |bibcode=2005Sci...307..709W |url=http://www.gps.caltech.edu/users/jkirschvink/pdfs/WardKarooScienceFinal.pdf |citeseerx=10.1.1.503.2065 |s2cid=46198018 |access-date=28 October 2017 |archive-date=13 August 2012 |archive-url=https://web.archive.org/web/20120813061414/http://www.gps.caltech.edu/users/jkirschvink/pdfs/WardKarooScienceFinal.pdf }}</ref> 

The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of species and the number of niches that tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.<ref name=SahneyBentonFerry2010LinksDiversityVertebrates/> The following table shows summary estimates for each tetrapod class from the ''[[IUCN Red List of Threatened Species]]'', 2014.3, for the number of [[Extant taxa|extant species]] that have been described in the literature, as well as the number of [[threatened species]].<ref name=IUCN2023>{{cite web |website=[[IUCN Red List of Threatened Species]] |version=2023.1 |title=Summary Statistics |url=https://www.iucnredlist.org/resources/summary-statistics |access-date=5 February 2024 |archive-date=12 August 2023 |archive-url=https://web.archive.org/web/20230812030519/https://www.iucnredlist.org/resources/summary-statistics |url-status=live }} Table 1a: [https://nc.iucnredlist.org/redlist/content/attachment_files/2023-1_RL_Table_1a.pdf Number of species evaluated in relation to the overall number of described species, and numbers of threatened species by major groups of organisms]</ref>

{| class="wikitable"
|+[[IUCN]] global summary estimates for extant tetrapod species as of 2023<ref name=IUCN2023 />
|-
! Tetrapod group
! Image
! [[Class (biology)|Class]]
! Estimated number of<br />described species<ref name=IUCN2023 />{{Efn|The estimates for amphibians, reptiles, birds and mammals were respectively taken from [[Amphibian Species of the World|Amphibian Species of the World: An Online Reference]] (version 6.2, 1 December 2023), the [[Reptile Database]] (accessed: 0 December 2023), [[Handbook of the Birds of the World]] and [[BirdLife International]] digital checklist of the birds of the world (version 8; accessed: 11 December 2023) and the [[American Society of Mammalogists#Mammal Diversity Database|Mammal Diversity Database]] (version 1.11, released 15 April 2023; accessed 01 December 2023).<ref name=IUCN2023 />}}
! Number of species <br /> evaluated for Red List<ref name=IUCN2023 />
! Share of described<br />species evaluated<br /> for Red List<ref name=IUCN2023 />
! Threatened species<br />in [[IUCN Red List|Red List]]<ref name=IUCN2023 />
! Best estimate<br />of percent of<br />threatened species<ref name=IUCN2023 />
|-
! [[Anamniote]]s<br /><small>lay eggs in water</small>
| [[File:Lithobates pipiens.jpg|100px]] 
| [[Amphibian]]s
| align=center | 8,707
| align=center | 8,020
| align=center | 92%
| align=center | 2,876
| align=center | 41%
|-
! rowspan="3" | [[Amniote]]s<br /><small>adapted to lay eggs<br />on land</small>
| [[File:Florida Box Turtle Digon3.jpg|100px]] 
| [[Reptile]]s
| align=center | 12,060
| align=center | 10,254
| align=center | 85%
| align=center | 1,848
| align=center | 21%
|-
| [[File:Cuvier-33-Moineau_domestique.jpg|100px]]
| [[Bird]]s
| align=center | 11,197
| align=center | 11,197
| align=center | 100%
| align=center | 1,354
| align="center" | 12%
|-
| [[File:Squirrel (PSF).png|100px]] 
| [[Mammal]]
| align=center | 6,631
| align=center | 5,980
| align=center | 90%
| align=center | 1,339
| align=center | 26%
|-
! colspan=3 style="background:#ddf8f8;" | Overall 
! align=center style="background:#ddf8f8;" | 38,595
! align=center style="background:#ddf8f8;" | 35,451
! align=center style="background:#ddf8f8;" | 92%
! align=center style="background:#ddf8f8;" | 7,417
! align=center style="background:#ddf8f8;" | 
|}
{{notelist}}

==Classification==
{{see also|List of chordate orders|List of tetrapod families}}
[[File:Linnaeus - Regnum Animale (1735).png|thumb|upright=1.4|[[Carl Linnaeus]]'s 1735 classification of animals, with tetrapods occupying the first three classes]]
The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits.<ref name=Romer>{{cite book |author=Romer, A.S. |title=The Vertebrate Body |publisher=W.B. Saunders |location=Philadelphia |year=1949 }} (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)</ref> [[Snake]]s and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to [[caecilians]] and [[aquatic mammals]]. Newer taxonomy is frequently based on [[cladistics]] instead, giving a variable number of major "branches" ([[clade]]s) of the tetrapod [[phylogenetic tree|family tree]].

As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In [[phylogenetic nomenclature]], in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type ''of'' dinosaurs.

===History of classification===
The tetrapods, including all large- and medium-sized land animals, have been among the best understood animals since earliest times. By [[Aristotle]]'s time, the basic division between mammals, birds and egg-laying tetrapods (the "[[Herpetology|herptiles]]") was well known, and the inclusion of the legless snakes into this group was likewise recognized.<ref>{{cite journal|last=Lloyd|first=G.E.R.|title=The Development of Aristotle's Theory of the Classification of Animals|journal=Phronesis|year=1961|volume=6|issue=1|pages=59–81|jstor=4181685|doi=10.1163/156852861X00080}}</ref> With the birth of modern [[biological classification]] in the 18th century, [[Carl Linnaeus|Linnaeus]] used the same division, with the tetrapods occupying the first three of his six classes of animals.<ref name="Linn1758" >{{cite book |first=Carolus |last=Linnaeus |title=Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis |publisher=Laurentius Salvius |location=Stockholm |year=1758 |url=https://www.biodiversitylibrary.org/bibliography/542 |language=la |edition=[[10th edition of Systema Naturae|10th edition]] |access-date=2018-01-13 |archive-date=2008-10-10 |archive-url=https://web.archive.org/web/20081010032456/http://www.biodiversitylibrary.org/bibliography/542 |url-status=live }}</ref> While reptiles and amphibians can be quite similar externally, the French zoologist [[Pierre André Latreille]] recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.<ref>Latreielle, P.A. (1804): Nouveau Dictionnaire à Histoire Naturelle, xxiv., cited in Latreille's ''Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique'', 1825</ref>

===Modern classification===
With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominantly done by experts working within classes. In the early 1930s, American [[Vertebrate paleontology|vertebrate palaeontologist]] [[Alfred Romer]] (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his ''[[Vertebrate Paleontology (Romer)|Vertebrate Paleontology]]''.<ref>Smith, C. H. (2005). [http://www.wku.edu/~smithch/chronob/ROME1894.htm "Romer, Alfred Sherwood (United States 1894-1973)"]. {{Webarchive|url=https://web.archive.org/web/20081012014018/http://www.wku.edu/~smithch/chronob/ROME1894.htm |date=2008-10-12 }}. [[Western Kentucky University]]</ref> This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. [[Michael Benton|Benton]] (1998) and Knobill and Neill (2006).<ref>Benton, M. J. (1998). "The quality of the fossil record of vertebrates". pp. 269–303 in Donovan, S. K. and Paul, C. R. C. (eds), ''The adequacy of the fossil record'', Fig. 2. New York: Wiley.</ref><ref>Neill, J. D. (ed.) (2006). ''Knobil and Neill's Physiology of Reproduction'' (3rd ed.). Vol 2. [[Academic Press]]. p. 2177.</ref> While mostly seen in general works, it is also still used in some specialist works like Fortuny et al. (2011).<ref>{{cite journal | last1 = Fortuny | first1 = J. | last2 = Bolet | first2 = A. | last3 = Sellés | first3 = A. G. | last4 = Cartanyà | first4 = J. | last5 = Galobart | first5 = À. | year = 2011 | title = New insights on the Permian and Triassic vertebrates from the Iberian Peninsula with emphasis on the Pyrenean and Catalonian basins | url = http://www.ucm.es/info/estratig/JIG/vol_content/vol_37_1/JIG_37_1_Fortuny_65-86.pdf | journal = [[Journal of Iberian Geology]] | volume = 37 | issue = 1 | pages = 65–86 | doi = 10.5209/rev_JIGE.2011.v37.n1.5 | access-date = 2012-12-04 | archive-date = 2011-05-17 | archive-url = https://web.archive.org/web/20110517050605/http://www.ucm.es/info/estratig/JIG/vol_content/vol_37_1/JIG_37_1_Fortuny_65-86.pdf | url-status = live | doi-access = free }}</ref> The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):<ref name="hilde">{{cite book|page=429|isbn=978-0-471-29505-1|last1=Hildebrand |first1=M. |author2=G. E. Goslow Jr |others=ill. Viola Hildebrand |year=2001 |publisher=Wiley |location=New York |title=Analysis of vertebrate structure}}</ref>

*'''Superclass [[Tetrapoda]]''' – four-limbed vertebrates
**'''Class [[Amphibia]]''' – amphibians
***'''Subclass''' '''[[Ichthyostegalia]]''' – early fish-like amphibians ([[Paraphyly|paraphyletic]] group outside leading to the crown-clade Neotetrapoda)
***'''Subclass''' '''[[Anthracosauria]]''' – reptile-like amphibians (often thought to be the ancestors of the [[amniote]]s)
***'''Subclass''' '''[[Temnospondyli]]''' – large-headed Paleozoic and Mesozoic amphibians
***'''Subclass''' '''[[Lissamphibia]]''' – modern amphibians
**'''Class [[Reptilia]]''' – reptiles
***'''Subclass''' '''[[Diapsida]]''' – diapsids, including crocodiles, dinosaurs, birds, lizards, snakes and turtles
***'''Subclass [[Euryapsida]]''' – euryapsids
***'''Subclass [[Synapsida]]''' – synapsids, including mammal-like reptiles-now a separate group (often thought to be the ancestors of mammals)
***'''Subclass [[Anapsida]]''' – anapsids
**'''Class [[Mammalia]]''' – mammals
***'''Subclass [[Prototheria]]''' – egg-laying mammals, including monotremes
***'''Subclass [[Allotheria]]''' – multituberculates
***'''Subclass [[Theria]]''' – live-bearing mammals, including marsupials and placentals

This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique from [[cladistics]]. The earliest tetrapods are grouped under class Amphibia, although several of the groups are more closely related to [[amniote]]s than to [[Lissamphibia|modern day amphibians]]. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendants of [[Theropoda|theropod dinosaurs]]. [[Basal (phylogenetics)|Basal]] non-mammalian [[synapsid]]s ("mammal-like reptiles") traditionally also sort under class Reptilia as a separate subclass,<ref name=Romer/> but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on [[phylogeny]], disregarding the anatomy and physiology.

==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)}}

== History ==

=== Palaeozoic ===

==== Devonian stem-tetrapods ====
The oldest evidence for the existence of tetrapods comes from [[trace fossil]]s: tracks (footprints) and [[Zachelmie trackways|trackways]] found in [[Zachełmie, Świętokrzyskie Voivodeship|Zachełmie]], Poland, dated to the [[Devonian#Subdivisions|Eifelian]] stage of the Middle Devonian, {{Ma|390}},<ref name="NarkiewiczNarkiewicz2015">{{cite journal|last1=Narkiewicz|first1=Katarzyna|last2=Narkiewicz|first2=Marek|title=The age of the oldest tetrapod tracks from Zachełmie, Poland|journal=Lethaia|volume=48|issue=1|date=January 2015|pages=10–12|issn=0024-1164|doi=10.1111/let.12083|bibcode=2015Letha..48...10N }}</ref> although these traces have also been interpreted as the ichnogenus ''[[Piscichnus]]'' (fish nests/feeding traces).<ref>{{Cite journal |doi = 10.1080/10420940.2015.1063491|title = Thinopusand a Critical Review of Devonian Tetrapod Footprints|year = 2015|last1 = Lucas|first1 = Spencer G.|journal = Ichnos|volume = 22|issue = 3–4|pages = 136–154| bibcode=2015Ichno..22..136L |s2cid = 130053031}}</ref> The adult tetrapods had an estimated length of 2.5 m (8 feet), and lived in a lagoon with an average depth of 1–2 m, although it is not known at what depth the underwater tracks were made. The lagoon was inhabited by a variety of marine organisms and was apparently salt water. The average water temperature was 30 degrees C (86 F).<ref name="NiedźwiedzkiSzrek2010">{{cite journal|last1=Niedźwiedzki|first1=Grzegorz|last2=Szrek|first2=Piotr|last3=Narkiewicz|first3=Katarzyna|last4=Narkiewicz|first4=Marek|last5=Ahlberg|first5=Per E.|title=Tetrapod trackways from the early Middle Devonian period of Poland|journal=Nature|volume=463|issue=7277|date=7 January 2010|pages=43–48|issn=0028-0836|doi=10.1038/nature08623|pmid=20054388|bibcode=2010Natur.463...43N|s2cid=4428903}}</ref><ref name="NarkiewiczGrabowski2015">{{cite journal|last1=Narkiewicz|first1=Marek|last2=Grabowski|first2=Jacek|last3=Narkiewicz|first3=Katarzyna|last4=Niedźwiedzki|first4=Grzegorz|last5=Retallack|first5=Gregory J.|last6=Szrek|first6=Piotr|last7=De Vleeschouwer|first7=David|title=Palaeoenvironments of the Eifelian dolomites with earliest tetrapod trackways (Holy Cross Mountains, Poland)|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=420|date=15 February 2015|pages=173–192|issn=0031-0182|doi=10.1016/j.palaeo.2014.12.013|bibcode=2015PPP...420..173N}}</ref> The second oldest evidence for tetrapods, also tracks and trackways, date from ca. 385 Mya ([[Valentia Island]], Ireland).<ref>Stossel, I. (1995) The discovery of a new Devonian tetrapod trackway in SW Ireland. Journal of the Geological Society, London, 152, 407–413.</ref><ref>Stossel, I., Williams, E.A. & Higgs, K.T. (2016) Ichnology and depositional environment of the Middle Devonian Valentia Island tetrapod trackways, south-west Ireland. Palaeogeography, Palaeoclimatology, Palaeoecology, 462, 16–40.</ref>

The oldest partial fossils of tetrapods date from the [[Frasnian]] beginning ≈380 mya. These include ''[[Elginerpeton]]'' and ''[[Obruchevichthys]]''.<ref>{{harvnb|Clack|2012|pp=117–8}}</ref> Some paleontologists dispute their status as true (digit-bearing) tetrapods.<ref>{{harvnb|Laurin|2010|p=85}}</ref>

All known forms of Frasnian tetrapods became extinct in the [[Late Devonian extinction]], also known as the end-Frasnian extinction.<ref name="McGhee 2013 103–4">{{harvnb|McGhee|2013|pp=103–4}}</ref> This marked the beginning of a gap in the tetrapod fossil record known as the [[Famennian]] gap, occupying roughly the first half of the Famennian stage.<ref name="McGhee 2013 103–4"/>

The oldest near-complete tetrapod fossils, ''Acanthostega'' and ''Ichthyostega'', date from the second half of the Fammennian.<ref name="CallierClack2009">{{cite journal|last1=Callier|first1=V.|last2=Clack|first2=J. A.|last3=Ahlberg|first3=P. E.|title=Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs|journal=Science|volume=324|issue=5925|year=2009|pages=364–367|issn=0036-8075|doi=10.1126/science.1167542|pmid=19372425|bibcode=2009Sci...324..364C|s2cid=28461841}}</ref><ref>{{harvnb|Clack|2012|pp=147}}</ref> Although both were essentially four-footed fish, ''Ichthyostega'' is the earliest known tetrapod that may have had the ability to pull itself onto land and drag itself forward with its forelimbs. There is no evidence that it did so, only that it may have been anatomically capable of doing so.<ref>{{harvnb|Clack|2012|pp=159}}</ref><ref name="PierceClack2012">{{cite journal|last1=Pierce|first1=Stephanie E.|last2=Clack|first2=Jennifer A.|last3=Hutchinson|first3=John R.|title=Three-dimensional limb joint mobility in the early tetrapod Ichthyostega|journal=Nature|year=2012|issn=0028-0836|doi=10.1038/nature11124|pmid=22722854|volume=486|issue=7404|pages=523–6|bibcode=2012Natur.486..523P|s2cid=3127857|url=http://researchonline.rvc.ac.uk/id/eprint/6182/|archive-date=2022-10-05|access-date=2023-12-09|archive-url=https://web.archive.org/web/20221005194423/https://researchonline.rvc.ac.uk/id/eprint/6182/|url-status=live}}</ref>

The publication in 2018 of ''[[Tutusius]] umlambo'' and ''[[Umzantsia]] amazana'' from high latitude Gondwana setting indicate that the tetrapods enjoyed a global distribution by the end of the Devonian and even extended into the high latitudes.<ref>{{Cite journal |title=A tetrapod fauna from within the Devonian Antarctic Circle |journal=Science |date=8 June 2018 |last1=Gess |first1=Robert |last2=Ahlberg |first2=Per Erik |volume=360 |issue=6393 |pages=1120–1124 |doi=10.1126/science.aaq1645 |pmid=29880689 |bibcode=2018Sci...360.1120G |s2cid=46965541 |doi-access=free }}</ref>
[[File:Ichthyostega stensioei.png|thumb|230x230px|''[[Ichthyostega]]'' (a four-limbed stem-tetrapod, Late Devonian)]]
The end-Fammenian marked another extinction, known as the end-Fammenian extinction or the [[Hangenberg event]], which is followed by another gap in the tetrapod fossil record, [[Romer's gap]], also known as the [[Tournaisian]] gap.<ref>{{harvnb|McGhee|2013|pp=214–5}}</ref> This gap, which was initially 30 million years, but has been gradually reduced over time, currently occupies much of the 13.9-million year Tournaisian, the first stage of the Carboniferous period.<ref name="ClaessensAnderson2015">{{cite journal|last1=Claessens|first1=Leon|last2=Anderson|first2=Jason S.|last3=Smithson|first3=Tim|last4=Mansky|first4=Chris F.|last5=Meyer|first5=Taran|last6=Clack|first6=Jennifer|title=A Diverse Tetrapod Fauna at the Base of 'Romer's Gap'|journal=PLOS ONE|volume=10|issue=4|date=27 April 2015|article-number=e0125446|issn=1932-6203|doi=10.1371/journal.pone.0125446|pmid=25915639|pmc=4411152|bibcode=2015PLoSO..1025446A|doi-access=free}}</ref><!--[[File:Hynerpeton BW.jpg|thumb|right|230px|''[[Hynerpeton]]'']]-->
<!--[[File:Tulerpeton12DB.jpg|thumb|right|230px|''[[Tulerpeton]]'']]-->
Tetrapod-like vertebrates first appeared in the Early Devonian period, and species with limbs and digits were around by the Late Devonian.<ref name="McGhee 2013 78">{{harvnb|McGhee|2013|p=78}}</ref> These early "stem-tetrapods" included animals such as ''[[Ichthyostega]]'',<ref name="NiedźwiedzkiSzrek2010"/> with legs and lungs as well as gills, but still primarily aquatic and poorly adapted for life on land. The Devonian stem-tetrapods went through two major [[population bottleneck]]s during the [[Late Devonian extinction]]s, also known as the [[Kellwasser event|end-Frasnian]] and [[Hangenberg event|end-Fammenian]] extinctions. These extinction events led to the disappearance of stem-tetrapods with fish-like features.<ref>{{harvnb|McGhee|2013|pp=263–4}}</ref> When stem-tetrapods reappear in the fossil record in early [[Carboniferous]] deposits, some 10 million years later, the adult forms of some are somewhat adapted to a terrestrial existence.<ref name="ClaessensAnderson2015" /><ref>{{Cite web |url=http://www.southampton.ac.uk/oes/research/projects/the_mid_palaeozoic_biotic_crisis.page#overview |title=Research project: The Mid-Palaeozoic biotic crisis: Setting the trajectory of Tetrapod evolution |access-date=2014-04-06 |archive-date=2013-12-12 |archive-url=https://web.archive.org/web/20131212234030/http://www.southampton.ac.uk/oes/research/projects/the_mid_palaeozoic_biotic_crisis.page#overview |url-status=live }}</ref> Why they went to land in the first place is still debated.

====Carboniferous ====
{{See also|List of Carboniferous tetrapods}}
<!--[[File:Pederpes22small.jpg|thumb|right|230px|''[[Pederpes]]'', 359–345 Ma]]-->
<!--[[File:Crassigyrinus BW.jpg|thumb|right|230px|''[[Crassigyrinus]]'']]-->
<!--[[File:Eryops1DB.jpg|thumb|right|230px|''[[Eryops]]'', ≈295 Ma]]-->[[File:Edops craigi12DB.jpg|thumb|right|230px|''[[Edops]]'' (an early temnospondyl, Late Carboniferous - Early Permian)]]During the early Carboniferous, the number of digits on [[hand]]s and feet of stem-tetrapods became standardized at no more than five, as lineages with more digits died out (exceptions within crown-group tetrapods arose among some secondarily aquatic members). By the very beginning of the Carboniferous,<ref name=":4" /> the stem-tetrapods had radiated into two branches of true ("crown group") tetrapods, one ancestral to modern amphibians and the other ancestral to amniotes. 

[[Lissamphibia|Modern amphibians]] are most likely derived from the [[temnospondyl]]s, a particularly diverse and long-lasting group of tetrapods. A less popular proposal draws comparisons to the "[[lepospondyl]]s", an eclectic mixture of various small tetrapods, including burrowing, limbless, and other bizarrely-shaped forms. The [[Reptiliomorpha|reptiliomorphs]] (sometimes known as "[[anthracosaur]]s") were the relatives and ancestors of the [[amniote]]s (reptiles, mammals, and kin). The first amniotes are known from the early part of the [[Pennsylvanian (geology)|Late Carboniferous]]. All basal amniotes had a small body size, like many of their contemporaries, though some Carboniferous tetrapods evolved into large crocodile-like predators, informally known as "[[Labyrinthodontia|labyrinthodonts]]".<ref>{{Cite book |url=https://books.google.com/books?id=68urAgAAQBAJ&q=%22Like+the+basal+batrachomorphs%2C+the+basal+reptiliomorphs+were+surprisingly+small%22&pg=PA231 |title=When the Invasion of Land Failed: The Legacy of the Devonian Extinctions |isbn=978-0-231-16056-8 |access-date=2020-04-25 |archive-date=2020-08-08 |archive-url=https://web.archive.org/web/20200808121720/https://books.google.no/books?id=68urAgAAQBAJ&pg=PA231&dq=%22Like+the+basal+batrachomorphs,+the+basal+reptiliomorphs+were+surprisingly+small%22&hl=no&sa=X&ved=0ahUKEwjD1vvpiIPpAhUNuIsKHVoPCngQ6AEIKjAA#v=onepage&q=%22Like%20the%20basal%20batrachomorphs%2C%20the%20basal%20reptiliomorphs%20were%20surprisingly%20small%22&f=false |url-status=live |last1=George r. Mcghee |first1=Jr |date=12 November 2013 |publisher=Columbia University Press }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=Z0YWn5F9sWkC&q=%22As+in+the+oldest+known+amniotes%2C+it+is+both+very+small+and+highly+ossified%22&pg=PA209 |title=Fins into Limbs: Evolution, Development, and Transformation |isbn=978-0-226-31340-5 |access-date=2020-04-25 |archive-date=2020-08-09 |archive-url=https://web.archive.org/web/20200809023449/https://books.google.no/books?id=Z0YWn5F9sWkC&pg=PA209&dq=%22As+in+the+oldest+known+amniotes,+it+is+both+very+small+and+highly+ossified%22&hl=no&sa=X&ved=0ahUKEwjh9ZXcjoPpAhVlpIsKHQAlDCQQ6AEIKDAA#v=onepage&q=%22As%20in%20the%20oldest%20known%20amniotes%2C%20it%20is%20both%20very%20small%20and%20highly%20ossified%22&f=false |url-status=live |last1=Hall |first1=Brian K. |date=15 September 2008 |publisher=University of Chicago Press }}</ref> Amphibians must return to water to lay eggs; in contrast, amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land.

Amphibians and amniotes were affected by the [[Carboniferous rainforest collapse]] (CRC), an extinction event that occurred around 307 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Amniotes and temnospondyls in particular were more suited to the new conditions. They invaded new ecological niches and began diversifying their diets to include plants and other tetrapods, previously having been limited to insects and fish.<ref name="SahneyBentonFalconLang2010RainforestCollapse">{{cite journal | author= Sahney, S., Benton, M.J. & Falcon-Lang, H.J. | year=2010 | title= Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica | journal=Geology | doi=10.1130/G31182.1 | volume = 38 | pages = 1079–1082 | issue=12 | bibcode=2010Geo....38.1079S}}</ref>

====Permian ====
{{See also|List of Permian tetrapods}}
[[File:Diadectes1DB.jpg|thumb|right|230px|''[[Diadectes]]'' (a terrestrial diadectomorph, Early Permian)]]
In the [[Permian]] period, amniotes became particularly well-established, and two important clades filled in most terrestrial niches: the [[sauropsid]]s and the [[synapsid]]s. The latter were the most important and successful Permian land animals, establishing complex terrestrial ecosystems of predators and prey while acquiring various adaptations retained by their modern descendants, the mammals. Sauropsid diversity was more subdued during the Permian, but they did begin to fracture into several lineages ancestral to modern reptiles. Amniotes were not the only tetrapods to experiment with prolonged life on land. Some temnospondyls, [[Seymouriamorpha|seymouriamorphs]], and [[Diadectomorpha|diadectomorphs]] also successfully filled terrestrial niches in the earlier part of the Permian. Non-amniote tetrapods declined in the later part of the Permian.

The end of the Permian saw a major turnover in fauna during the [[Permian–Triassic extinction event]]. There was a protracted loss of species, due to multiple extinction pulses.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |last1=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |name-list-style=amp | year=2008 | title=Recovery from the most profound mass extinction of all time | journal=Proceedings of the Royal Society B: Biological Sciences | doi=10.1098/rspb.2007.1370 | volume=275 | pages=759–765 | pmid=18198148 | issue=1636 | pmc=2596898 }}</ref> Many of the once large and diverse groups died out or were greatly reduced.

===Mesozoic ===
The [[diapsid]] reptiles (a subgroup of the sauropsids) strongly diversified during the [[Triassic]], giving rise to the [[turtle]]s, [[pseudosuchia]]ns (crocodilian ancestors), [[dinosaur]]s, [[pterosaur]]s, and [[lepidosaurs]], along with many other reptile groups on land and sea. Some of the new Triassic reptiles would not survive into the [[Jurassic]], but others would flourish during the Jurassic. [[Lizard]]s, turtles, dinosaurs, pterosaurs, [[crocodylomorphs]], and [[plesiosaur]]s were particular beneficiaries of the Triassic-Jurassic transition. [[Bird]]s, a particular subset of [[Theropoda|theropod]] dinosaurs capable of flight via feathered wings, evolved in the Late Jurassic. In the [[Cretaceous]], [[snake]]s developed from lizards, [[rhynchocephalia]]ns (tuataras and kin) declined, and modern birds and crocodilians started to establish themselves.

Among the characteristic Paleozoic non-amniote tetrapods, few survived into the Mesozoic. [[Temnospondyl]]s briefly recovered in the Triassic, spawning the large aquatic [[Stereospondyli|stereospondyls]] and the small terrestrial lissamphibians (the earliest frogs, salamanders, and caecilians). However, stereospondyl diversity would crash at the end of the Triassic. By the Late Cretaceous, the only surviving amphibians were lissamphibians. Many groups of synapsids, such as [[anomodont]]s and [[therocephalia]]ns, that once comprised the dominant terrestrial fauna of the Permian, also became extinct during the Triassic. During the Jurassic, one synapsid group ([[Cynodontia]]) gave rise to the modern [[mammal]]s, which survived through the rest of the Mesozoic to later diversify during the Cenozoic. The [[Cretaceous-Paleogene extinction event]] at the end of the Mesozoic killed off many organisms, including all the non-avian dinosaurs and nearly all marine reptiles. Birds survived and diversified during the Cenozoic, similar to mammals.

===Cenozoic ===
Following the great extinction event at the end of the Mesozoic, representatives of seven major groups of tetrapods persisted into the [[Cenozoic]] era. One of them, a group of semiaquatic reptiles known as the [[Choristodera]], became extinct 11 million years ago for unclear reasons.<ref>{{cite journal|display-authors=6|vauthors=Böhme M, Spassov N, Fuss J, Tröscher A, Deane AS, Prieto J, Kirscher U, Lechner T, Begun DR|date=November 2019|title=Supplementary Information: A new Miocene ape and locomotion in the ancestor of great apes and humans|url=https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-019-1731-0/MediaObjects/41586_2019_1731_MOESM1_ESM.pdf|journal=Nature|volume=575|issue=7783|pages=489–493|doi=10.1038/s41586-019-1731-0|pmid=31695194|bibcode=2019Natur.575..489B|s2cid=207888156|via=|archive-date=2021-06-26|access-date=2022-01-18|archive-url=https://web.archive.org/web/20210626035025/https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-019-1731-0/MediaObjects/41586_2019_1731_MOESM1_ESM.pdf|url-status=live}}</ref> The seven Cenozoic tetrapods groups are:

* [[Lissamphibia]]: [[frog]]s, [[salamander]]s, and [[caecilian]]s
* [[Mammalia]]: [[monotreme]]s, [[marsupial]]s, [[placental]]s, and †[[multituberculates]]
* [[Lepidosauria]]: [[tuatara]]s and [[lizard]]s (including [[amphisbaenian]]s and [[snake]]s)
* [[Testudines]]: [[turtle]]s
* [[Crocodilia]]: [[crocodile]]s, [[alligator]]s, [[caimans]] and [[gharial]]s
* [[Aves]]: [[bird]]s
* †[[Choristodera]] (extinct)

== Phylogeny ==

=== Stem group ===
[[Stem tetrapoda|Stem tetrapods]] are all animals more closely related to tetrapods than to lungfish, but excluding the tetrapod crown group. The cladogram below illustrates the relationships of stem-tetrapods. All these lineages are extinct except for Dipnomorpha and Tetrapoda; from Swartz, 2012:<ref name=SB12>{{cite journal | last = Swartz | first = B. | year = 2012 | title = A marine stem-tetrapod from the Devonian of Western North America | journal = PLOS ONE | pmid = 22448265 | volume = 7 | issue = 3 | pmc = 3308997 | article-number = e33683 | doi = 10.1371/journal.pone.0033683 | bibcode = 2012PLoSO...733683S | doi-access = free }}</ref>

{{clade| style=font-size:85%;line-height:85%
|label1=[[Rhipidistia]]
|1={{clade
|1=[[Dipnomorpha]] (lungfishes and relatives) [[File:Protopterus dolloi Boulenger2.jpg|70 px]]
|label2=[[Tetrapodomorpha]]
|2={{clade
   |1={{extinct}}''[[Kenichthys]]''
   |2={{clade
      |1={{extinct}}[[Rhizodontidae]] [[File:Gooloogongia loomesi reconstruction.jpg|70 px]]
      |2={{clade
         |label1={{extinct}}[[Canowindridae]]
         |1={{clade
            |1={{extinct}}''[[Marsdenichthys]]''
            |2={{clade
               |1={{extinct}}''[[Canowindra (fish)|Canowindra]]''
               |2={{clade
                  |1={{extinct}}''[[Koharalepis]]''
                  |2={{extinct}}''[[Beelarongia]]''}} }} }}
         |2={{clade
            |label1={{extinct}}[[Megalichthyiformes]]
            |1={{clade
               |1={{extinct}}''[[Gogonasus]]'' [[File:Gogonasus BW.jpg|70 px]]
               |2={{clade
                  |1={{extinct}}''[[Gyroptychius]]''
                  |2={{clade
                     |1={{extinct}}''[[Osteolepis]]'' [[File:Osteolepis BW.jpg|70 px]]
                     |2={{clade
                        |1={{extinct}}''[[Medoevia]]''
                        |2={{extinct}}[[Megalichthyidae]]}} }} }} }}
            |label2=[[Eotetrapodiformes]]
            |2={{clade
               |label1={{extinct}}[[Tristichopteridae]]
               |1={{clade
                  |1={{extinct}}''[[Spodichthys]]''
                  |2={{clade
                     |1={{extinct}}''[[Tristichopterus]]''
                     |2={{clade
                        |1={{extinct}}''[[Eusthenopteron]]'' [[File:Eusthenopteron BW.jpg|70 px]]
                        |2={{clade
                           |1={{extinct}}''[[Jarvikina]]''
                           |2={{clade
                              |1={{extinct}}''[[Cabbonichthys]]''
                              |2={{clade
                                 |1={{extinct}}''[[Mandageria]]''
                                 |2={{extinct}}''[[Eusthenodon]]''}} }} }} }} }} }}
               |2={{clade
                  |1={{extinct}}''[[Tinirau (fish)|Tinirau]]''
                  |2={{clade
                     |1={{extinct}}''[[Platycephalichthys]]''
                     |label2=[[Elpistostegalia]]
                     |2={{clade
                        |1={{extinct}}''[[Panderichthys]]'' [[File:Panderichthys BW.jpg|70 px]]
                        |label2=[[Stegocephalia]]
                        |2={{clade
                           |1={{clade
                              |1={{extinct}}''[[Tiktaalik]]'' [[File:Tiktaalik BW.jpg|70 px]]
                              |2={{extinct}}''[[Elpistostege]]''}}
                           |2={{clade
                              |1={{extinct}}''[[Elginerpeton]]'' [[File:Elginerpeton BW.jpg|70 px]]
                              |2={{clade
                                 |1={{extinct}}''[[Ventastega]]''
                                 |2={{clade
                                    |1={{extinct}}''[[Acanthostega]]'' [[File:Acanthostega BW.jpg|70 px]]
                                    |2={{clade
                                       |1={{extinct}}''[[Ichthyostega]]'' [[File:Ichthyostega BW.jpg|70 px]]
                                       |2={{clade
                                          |1={{extinct}}[[Whatcheeriidae]] [[File:Pederpes22small.jpg|70 px]]
                                          |2={{clade
                                             |1={{extinct}}[[Colosteidae]] [[File:Greererpeton BW.jpg|70 px]]
                                             |2={{clade
                                                |1={{extinct}}''[[Crassigyrinus]]'' [[File:Crassigyrinus BW.jpg|70 px]]
                                                |2={{clade
                                                   |1={{extinct}}[[Baphetidae]]
                                                   |2=Tetrapoda [[File:Seymouria BW.jpg|70 px]]
}} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }}

=== Crown group ===
Crown tetrapods are defined as the nearest common ancestor of all living tetrapods (amphibians, reptiles, birds, and mammals) along with all of the descendants of that ancestor.

The inclusion of certain extinct groups in the crown Tetrapoda depends on the relationships of modern amphibians, or [[lissamphibia]]ns. There are currently three major hypotheses on the origins of lissamphibians. In the temnospondyl hypothesis (TH), lissamphibians are most closely related to dissorophoid [[temnospondyl]]s, which would make temnospondyls tetrapods. In the lepospondyl hypothesis (LH), lissamphibians are the sister taxon of lysorophian [[lepospondyl]]s, making lepospondyls tetrapods and temnospondyls stem-tetrapods. In the polyphyletic hypothesis (PH), frogs and salamanders evolved from dissorophoid temnospondyls while caecilians come out of microsaur lepospondyls, making both lepospondyls and temnospondyls true tetrapods.<ref name="SigurdsenGreen2011">{{cite journal|last1=Sigurdsen|first1=Trond|last2=Green|first2=David M.|title=The origin of modern amphibians: a re-evaluation|journal=Zoological Journal of the Linnean Society|volume=162|issue=2|date=June 2011|pages=457–469|issn=0024-4082|doi=10.1111/j.1096-3642.2010.00683.x|doi-access=free}}</ref><ref name="Benton2014">{{cite book|last=Benton|first=Michael|author-link=Michael Benton|title=Vertebrate Palaeontology|url=https://books.google.com/books?id=qak-BAAAQBAJ&pg=PT398|access-date=2 July 2015|date=4 August 2014|publisher=Wiley|isbn=978-1-118-40764-6|page=398|archive-date=19 August 2020|archive-url=https://web.archive.org/web/20200819201052/https://books.google.com/books?id=qak-BAAAQBAJ&pg=PT398|url-status=live}}</ref>

=== Origins of modern amphibians ===
==== Temnospondyl hypothesis (TH) ====
This hypothesis comes in a number of variants, most of which have lissamphibians coming out of the dissorophoid temnospondyls, usually with the focus on amphibamids and branchiosaurids.<ref name="NarinsFeng2006">{{cite book|last1=Narins|first1=Peter M.|last2=Feng|first2=Albert S.|last3=Fay|first3=Richard R.|title=Hearing and Sound Communication in Amphibians|url=https://books.google.com/books?id=UPJcyY7TuiIC&pg=PA16|access-date=2 July 2015|date=11 December 2006|publisher=Springer Science & Business Media|isbn=978-0-387-47796-1|page=16|archive-date=20 August 2020|archive-url=https://web.archive.org/web/20200820000649/https://books.google.com/books?id=UPJcyY7TuiIC&pg=PA16|url-status=live}}</ref>

The temnospondyl hypothesis is the currently favored or majority view, supported by Ruta ''et al'' (2003a,b), Ruta and Coates (2007), Coates ''et al'' (2008), Sigurdsen and Green (2011), and Schoch (2013, 2014).<ref name="Benton2014"/><ref name="HedgesKumar2009">{{cite book|author1=S. Blair Hedges|author2=Sudhir Kumar|title=The Timetree of Life|url=https://books.google.com/books?id=9rt1c1hl49MC&pg=PA354|date=23 April 2009|publisher=OUP Oxford|isbn=978-0-19-156015-6|pages=354–|access-date=13 March 2016|archive-date=18 August 2020|archive-url=https://web.archive.org/web/20200818213648/https://books.google.com/books?id=9rt1c1hl49MC&pg=PA354|url-status=live}}</ref>

[[Cladogram]] modified after Coates, Ruta and Friedman (2008).<ref name="CoatesRuta2008">{{cite journal|last1=Coates|first1=Michael I.|last2=Ruta|first2=Marcello|last3=Friedman|first3=Matt|title=Ever Since Owen: Changing Perspectives on the Early Evolution of Tetrapods|journal=Annual Review of Ecology, Evolution, and Systematics|volume=39|issue=1|year=2008|pages=571–592|issn=1543-592X|doi=10.1146/annurev.ecolsys.38.091206.095546|jstor=30245177}}</ref>
{{clade
|label1='''{{nowrap|Crown-group Tetrapoda}}'''
|1={{clade
  |grouplabel1=total group Lissamphibia
  |1={{clade
    |label1=[[Temnospondyli]]
    |1=<div style="border-top: 2px solid green; padding: 0.1em;">'''Crown group [[Lissamphibia]]'''[[File:The tailless batrachians of Europe (Page 194) (Pelobates fuscus).jpg|x40px]]</div>
    |barend1=green
  }}
  |2=|state2=none
  |grouplabel3=total group Amniota|grouplabelstyle3=vertical-align:middle;
  |3={{clade
    |1={{extinct}} [[Embolomeri]] [[File:Archeria BW (white background).jpg|x30px]]|barbegin1=darkred
    |2={{clade
      |1={{extinct}} [[Gephyrostegidae]] [[File:Gefyrostegus22DB.jpg|x40px]]|bar1=darkred
      |2={{clade
        |1={{extinct}} [[Seymouriamorpha]] [[File:Karpinskiosaurus1DB.jpg|x30px]]|bar1=darkred
        |2={{clade

          |1={{clade
            |1={{extinct}} [[Microsauria]] [[File:Hyloplesion.jpg|x30px]]|bar1=darkred
            |2={{clade
              |1={{extinct}} [[Lysorophia]] [[File:Lysorophus.jpg|x40px]]|bar1=darkred
              |2={{clade
                |1={{extinct}} [[Adelospondyli]]|bar1=darkred
                |2={{clade
                  |1={{extinct}} [[Aistopoda]]|bar1=darkred
                  |2={{extinct}} [[Nectridea]] [[File:Diplocaulus Underside (Updated).png|x30px]]|bar2=darkred
 }} }} }} }}
          |2={{clade
            |1={{extinct}} [[Diadectomorpha]] [[File:Diadectes1DB (flipped).jpg|x25px]]|bar1=darkred
            |2='''Crown-group [[Amniota]]'''[[File:British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg|x30px]]|barend2=darkred
          }}
 }} }} }} }} }} }}

====Lepospondyl hypothesis (LH) ====
[[Cladogram]] modified after Laurin, ''How Vertebrates Left the Water'' (2010).<ref>{{harvnb|Laurin|2010|pp=133}}</ref>

{{clade| style=width:auto;font-size:90%;line-height:100%
|grouplabel1={{clade labels  |label1={{extinct}} stem tetrapods |top1=18%
                             |label2=total group Lissamphibia |top2=61%
                             |label3=total group Amniota |top3=92%}}
|label1='''Stegocephalia'''
|sublabel1=("Tetrapoda")
|1={{clade
   |1={{clade|1={{clade|1= {{extinct}} ''[[Acanthostega]]'' [[File:Acanthostega BW (flipped).jpg|70 px]] }} }} |barbegin1=green
   |2={{clade|1={{clade|1= {{extinct}} ''[[Ichthyostega]]'' [[File:Ichthyostega BW (flipped).jpg|70 px]] }} }} |bar2=green
   |3={{clade
      |1={{clade|1={{extinct}} [[Temnospondyli]] [[File:Eryops1DB.jpg|70 px]] }} |bar1=green
      |2={{clade
         |1={{extinct}} [[Embolomeri]] [[File:Archeria BW (white background).jpg|70 px]]|bar1=green
         |2={{clade
            |1={{extinct}} [[Seymouriamorpha]] [[File:Karpinskiosaurus1DB.jpg|70 px]]|barend1=green
            |2=|state2=none
            |3={{clade
               |label1=[[Amphibia]]
               |1={{clade
                  |1={{clade
                     |1={{extinct}} [[Adelogyrinidae]] |barbegin1=midnightblue
                     |2={{extinct}} [[Aistopoda]] |bar2=midnightblue
                     }}
                  |2={{clade
                     |1={{clade|1= {{extinct}} [[Nectridea]] [[File:Diplocaulus Underside (Updated).png|70 px]] }} |bar1=midnightblue
                     |2={{clade
                        |1={{extinct}} [[Lysorophia]] [[File:Lysorophus.jpg|70 px]]|bar1=midnightblue
                        |2={{clade
                           |1='''[[Lissamphibia]]''' [[File:The tailless batrachians of Europe (Page 194) (Pelobates fuscus).jpg|50 px]]   |barend1=midnightblue
                           }} 
                        }} 
                     }} 
                  }} 
               |2=|state2=none
               |label3=[[Reptiliomorpha]]
               |3={{clade
                  |1={{extinct}} [[Diadectomorpha]] [[File:Diadectes1DB (flipped).jpg|70 px]] |barbegin1=darkred
                  |2='''[[Amniota]]'''  [[File:British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg|70 px]] |barend2=darkred
                  }}
               }} 
            }} 
         }} 
      }} 
   }} 
}}

==== Polyphyly hypothesis (PH) ====
This hypothesis has batrachians (frogs and salamanders) coming out of dissorophoid temnospondyls, with caecilians out of microsaur lepospondyls. There are two variants, one developed by [[Robert L. Carroll|Carroll]],<ref name="Carroll2007">{{cite journal |last1=Carroll |first1=Robert L. |title=The Palaeozoic Ancestry of Salamanders, Frogs and Caecilians |journal=Zoological Journal of the Linnean Society |volume=150 |issue=s1 |year=2007 |pages=1–140 |issn=0024-4082 |doi=10.1111/j.1096-3642.2007.00246.x |doi-access=free}}</ref> the other by Anderson.<ref name="Anderson2008">{{cite journal |last1=Anderson |first1=Jason S. |title=Focal Review: The Origin(s) of Modern Amphibians |journal=Evolutionary Biology |volume=35 |issue=4 |date=December 2008 |pages=231–247 |issn=0071-3260 |doi=10.1007/s11692-008-9044-5 |bibcode=2008EvBio..35..231A |s2cid=44050103}}</ref>

[[Cladogram]] modified after Schoch, Frobisch, (2009).<ref name="FrobischSchoch2009">{{cite journal |last1=Frobisch |first1=N. B. |last2=Schoch |first2=R.R. |title=Testing the Impact of Miniaturization on Phylogeny: Paleozoic Dissorophoid Amphibians |journal=Systematic Biology |volume=58 |issue=3 |year=2009 |pages=312–327 |issn=1063-5157 |doi=10.1093/sysbio/syp029 |pmid=20525586 |doi-access=free}}</ref>
{{clade| style=width:auto;font-size:100%;line-height:100%
|label1='''Tetrapoda'''
|1={{clade
  |1=stem tetrapods  |state1=double
  |2={{clade
  |1={{clade
    |label1=[[Temnospondyli]]
    |1={{clade
      |1=basal temnospondyls |state1=double
      |label2=[[Dissorophoidea]]
      |2={{clade
        |1={{clade
          |1={{extinct}} [[Amphibamidae]]
          |2='''[[Frog]]s''' [[File:The tailless batrachians of Europe (Page 194) (Pelobates fuscus).jpg|45 px]]
}}
      |2={{clade
          |1={{extinct}} [[Branchiosauridae]] [[File:Branchiosaurus_BW.jpg|70 px]]
          |2='''[[Salamander]]s''' [[File:Salamandra salamandra (white background).jpg|70 px]]
}} }} }}
      |2={{clade
      |label1=[[Lepospondyli]]
      |1={{clade
            |1={{extinct}} [[Lysorophia]] [[File:Lysorophus.jpg|70 px]]
      |2={{clade
          |1={{extinct}} [[Microsauria]] [[File:Hyloplesion.jpg|70 px]]
          |2='''[[Caecilians]]''' [[File:Syphonops annulatus cropped.jpg|70 px]]
}} }} }} }}
      |2={{clade
        |1={{clade
          |1={{extinct}} [[Seymouriamorpha]] [[File:Karpinskiosaurus1DB.jpg|70 px]]
          |2={{clade
              |1={{extinct}} [[Diadectomorpha]] [[File:Diadectes1DB (flipped).jpg|70 px]]
              |2='''[[Amniota]]''' [[File:British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg|70 px]]
}} }} }} }} }} }}

==Anatomy and physiology==
{{More citations needed section|date=July 2015}}
The tetrapod's ancestral fish, tetrapodomorph, possessed similar traits to those inherited by the early tetrapods, including internal nostrils and large, fleshy [[pectoral fin|pectoral]] and [[pelvic fin]]s built on bones that could give rise to the tetrapod limb. To propagate in the terrestrial [[natural environment|environment]], animals had to overcome certain challenges. Their bodies needed additional support, because [[buoyancy]] was no longer a factor. Water retention was now important, since it was no longer the living [[matrix (biology)|matrix]], and could be lost easily to the environment. Finally, animals needed new sensory input systems to have any ability to function reasonably on land.

===Skull===
[[File:MAC TAV 0001 Tavola parietale cranio schematizzato di un tetrapode.jpg|thumb|Schematic skull of a tetrapod]]
The brain only filled half of the skull in the early tetrapods. The rest was filled with fatty tissue or fluid, which gave the brain space for growth as they adapted to a life on land.<ref>[https://www.discovermagazine.com/the-sciences/the-rise-of-the-tetrapods-how-our-early-ancestors-left-water-to-walk-on-land The Rise of the Tetrapods: How Our Early Ancestors Left Water to Walk on Land]</ref> The [[palate|palatal]] and jaw structures of tetramorphs were similar to those of early tetrapods, and their [[dentition]] was similar too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. 

A major difference between early tetrapodomorph fishes and early tetrapods was in the relative development of the front and back [[skull]] portions; the snout is much less developed than in most early tetrapods and the post-orbital skull is exceptionally longer than an amphibian's. A notable characteristic that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the [[orbit (anatomy)|orbital vacuities]] were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull.

=== Neck ===
In tetrapodomorph fishes such as ''[[Eusthenopteron]]'', the part of the body that would later become the neck was covered by a number of gill-covering bones known as the [[Operculum (fish)|opercular series]]. These bones functioned as part of pump mechanism for forcing water through the mouth and past the gills. When the mouth opened to take in water, the gill flaps closed (including the gill-covering bones), thus ensuring that water entered only through the mouth. When the mouth closed, the gill flaps opened and water was forced through the gills.

In ''Acanthostega'', a basal tetrapod, the gill-covering bones have disappeared, although the underlying gill arches are still present. Besides the opercular series, ''Acanthostega'' also lost the throat-covering bones (gular series). The opercular series and gular series combined are sometimes known as the operculo-gular or operculogular series. Other bones in the neck region lost in ''Acanthostega'' (and later tetrapods) include the extrascapular series and the supracleithral series. Both sets of bones connect the shoulder girdle to the skull. With the loss of these bones, tetrapods acquired a neck, allowing the head to rotate somewhat independently of the torso. This, in turn, required stronger soft-tissue connections between head and torso, including muscles and ligaments connecting the skull with the spine and shoulder girdle. Bones and groups of bones were also consolidated and strengthened.<ref>{{harvnb|Clack|2012|pp=29,45–6}}</ref>

In Carboniferous tetrapods, the neck joint (occiput) provided a pivot point for the spine against the back of the skull. In tetrapodomorph fishes such as ''Eusthenopteron'', no such neck joint existed. Instead, the [[notochord]] (a rod made of proto-cartilage) entered a hole in the back of the braincase and continued to the middle of the braincase. ''Acanthostega'' had the same arrangement as ''Eusthenopteron'', and thus no neck joint. The neck joint evolved independently in different lineages of early tetrapods.<ref>{{harvnb|Clack|2012|pp=207,416}}</ref>

All tetrapods appear to hold their necks at the maximum possible vertical extension when in a normal, alert posture.<ref name="taylor14">{{Cite journal | doi = 10.7717/peerj.712| title = Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs| journal = PeerJ| volume = 2| article-number = e712| year = 2014| last1 = Taylor | first1 = M. P.| pmid=25551027| pmc=4277489| doi-access = free}}</ref>

=== Dentition ===
[[File:Labyrinthodon Mivart.png|thumb|right|230px|Cross-section of a labyrinthodont tooth]]
Tetrapods had a tooth structure known as "plicidentine" characterized by infolding of the enamel as seen in cross-section. The more extreme version found in early tetrapods is known as "labyrinthodont" or "labyrinthodont plicidentine". 

This type of tooth structure has evolved independently in several types of bony fishes, both ray-finned and lobe finned, some modern lizards, and in a number of tetrapodomorph fishes. The infolding appears to evolve when a fang or large tooth grows in a small jaw, erupting when it is still weak and immature. The infolding provides added strength to the young tooth, but offers little advantage when the tooth is mature. Such teeth are associated with feeding on soft prey in juveniles.<ref>{{harvnb|Clack|2012|pp=373–4}}</ref><ref name="Schmidt-KittlerVogel2012">{{cite book|last1=Schmidt-Kittler|first1=Norbert|last2=Vogel|first2=Klaus|title=Constructional Morphology and Evolution|url=https://books.google.com/books?id=CiL0CAAAQBAJ&pg=PA151|access-date=15 July 2015|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-3-642-76156-0|pages=151–172|archive-date=19 August 2020|archive-url=https://web.archive.org/web/20200819024553/https://books.google.com/books?id=CiL0CAAAQBAJ&pg=PA151|url-status=live}}</ref>

=== Axial skeleton ===
With the move from water to land, the spine had to resist the bending caused by body weight and had to provide mobility where needed. Previously, it could bend along its entire length. Likewise, the paired appendages had not been formerly connected to the spine, but the slowly strengthening limbs now transmitted their support to the axis of the body.

=== Girdles ===
The shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The early sarcopterygians' [[cleithrum]] was retained as the [[clavicle]], and the [[interclavicle]] was well-developed, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could have grown ventrally in such a way as to form a broad chest plate. The upper portion of the girdle had a flat, [[Scapula|scapular blade (shoulder bone)]], with the [[glenoid cavity]] situated below performing as the [[Joint|articulation]] surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.

The [[pelvis|pelvic]] girdle also was much larger than the simple plate found in fishes, accommodating more muscles. It extended far dorsally and was joined to the backbone by one or more specialized sacral [[rib]]s. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was the [[ilium (bone)|ilium]], while the broad ventral plate was composed of the [[pubis (bone)|pubis]] in front and the [[ischium]] in behind. The three bones met at a single point in the center of the pelvic triangle called the [[acetabulum]], providing a surface of articulation for the femur.

=== Limbs ===
Fleshy lobe-fins supported on bones seem to have been an ancestral trait of all bony fishes ([[Osteichthyes]]). The ancestors of the ray-finned fishes ([[Actinopterygii]]) evolved their fins in a different direction. The [[tetrapodomorph]] ancestors of the tetrapods further developed their lobe fins. The paired fins had bones distinctly [[homology (biology)|homologous]] to the [[humerus]], [[ulna]], and [[Radius (bone)|radius]] in the fore-fins and to the [[femur]], [[tibia]], and [[fibula]] in the pelvic fins.<ref>{{cite journal |last1=Meunier |first1=François J. |last2=Laurin |first2=Michel |author2-link=Michel Laurin |title=A microanatomical and histological study of the fin long bones of the Devonian sarcopterygian ''Eusthenopteron foordi'' |journal=Acta Zoologica |date=January 2012 |volume=93 |issue=1 |pages=88–97 |doi=10.1111/j.1463-6395.2010.00489.x }}</ref>

The paired fins of the early sarcopterygians were smaller than tetrapod limbs, but the skeletal structure was very similar in that the early sarcopterygians had a single proximal bone (analogous to the [[humerus]] or [[femur]]), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the [[Carpal bones|carpus]]/[[tarsus (skeleton)|tarsus]] and [[hand|phalanges]] of a hand.

=== Locomotion ===
In typical early tetrapod posture, the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near [[right angle]]. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal's [[physical strength|strength]] was used to just lift its body off the ground for walking, which was probably slow and difficult. With this sort of posture, it could only make short broad strides. This has been confirmed by fossilized footprints found in Carboniferous [[rock (geology)|rock]]s.

=== Feeding ===
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were moderate-sized palatal and vomerine (upper) and coronoid (lower) fangs, as well rows of smaller teeth. This was in contrast to the larger fangs and small marginal teeth of earlier tetrapodomorph fishes such as ''[[Eusthenopteron]]''. Although this indicates a change in feeding habits, the exact nature of the change in unknown. Some scholars have suggested a change to bottom-feeding or feeding in shallower waters (Ahlberg and Milner 1994). Others have suggesting a mode of feeding comparable to that of the Japanese giant salamander, which uses both suction feeding and direct biting to eat small crustaceans and fish. A study of these jaws shows that they were used for feeding underwater, not on land.<ref name="NeenanRuta2014">{{cite journal|last1=Neenan|first1=J. M.|last2=Ruta|first2=M.|last3=Clack|first3=J. A.|last4=Rayfield|first4=E. J.|title=Feeding biomechanics in ''Acanthostega'' and across the fish-tetrapod transition|journal=Proceedings of the Royal Society B: Biological Sciences|volume=281|issue=1781|date=22 April 2014|article-number=20132689|issn=0962-8452|doi=10.1098/rspb.2013.2689|pmid=24573844|pmc=3953833}}</ref>

In later terrestrial tetrapods, two methods of jaw closure emerge: static and kinetic inertial (also known as snapping). In the static system, the jaw muscles are arranged in such a way that the jaws have maximum force when shut or nearly shut. In the kinetic inertial system, maximum force is applied when the jaws are wide open, resulting in the jaws snapping shut with great velocity and momentum. Although the kinetic inertial system is occasionally found in fish, it requires special adaptations (such as very narrow jaws) to deal with the high viscosity and density of water, which would otherwise impede rapid jaw closure.

The tetrapod [[tongue]] is built from muscles that once controlled gill openings. The tongue is anchored to the [[hyoid bone]], which was once the lower half of a pair of gill bars (the second pair after the ones that evolved into jaws).<ref>{{harvnb|Clack|2012|p=49,212}}</ref><ref name="ButlerHodos2005">{{cite book|last1=Butler|first1=Ann B.|last2=Hodos|first2=William|title=Comparative Vertebrate Neuroanatomy: Evolution and Adaptation|url=https://books.google.com/books?id=6kGARvykJKMC&pg=PA38|access-date=11 July 2015|date=2 September 2005|publisher=John Wiley & Sons|isbn=978-0-471-73383-6|page=38|archive-date=18 August 2020|archive-url=https://web.archive.org/web/20200818183857/https://books.google.com/books?id=6kGARvykJKMC&pg=PA38|url-status=live}}</ref><ref name="Cloudsley-Thompson2012">{{cite book|last=Cloudsley-Thompson|first=John L.|title=The Diversity of Amphibians and Reptiles: An Introduction|url=https://books.google.com/books?id=8i_vCAAAQBAJ&pg=PA117|access-date=11 July 2015|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-3-642-60005-0|page=117|archive-date=18 August 2020|archive-url=https://web.archive.org/web/20200818204146/https://books.google.com/books?id=8i_vCAAAQBAJ&pg=PA117|url-status=live}}</ref> The tongue did not evolve until the gills began to disappear. ''Acanthostega'' still had gills, so this would have been a later development. In an aquatically feeding animals, the food is supported by water and can literally float (or get sucked in) to the mouth. On land, the tongue becomes important.

=== Respiration ===
The evolution of early tetrapod respiration was influenced by an event known as the "charcoal gap", a period of more than 20 million years, in the middle and late Devonian, when atmospheric oxygen levels were too low to sustain wildfires.<ref name="Clack2007">{{cite journal|last1=Clack|first1=J. A.|title=D Comparative Biology|journal=Integrative and Comparative Biology |volume=47|issue=4|year=2007|pages=510–523|issn=1540-7063|doi=10.1093/icb/icm055|pmid=21672860|doi-access=free}}</ref> During this time, fish inhabiting [[anoxic waters]] (very low in oxygen) would have been under evolutionary pressure to develop their air-breathing ability.<ref>{{harvnb|McGhee|2013|pp=111,139–41}}</ref><ref name="ScottGlasspool2006">{{cite journal|last1=Scott|first1=A. C.|last2=Glasspool|first2=I. J.|title=The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration|journal=Proceedings of the National Academy of Sciences|volume=103|issue=29|date=18 July 2006|pages=10861–10865|issn=0027-8424|doi=10.1073/pnas.0604090103|pmid=16832054|pmc=1544139|bibcode=2006PNAS..10310861S|doi-access=free}}</ref><ref>{{harvnb|Clack|2012|pp=140}}</ref>

Early tetrapods probably relied on four methods of [[Respiration (physiology)|respiration]]: with [[lungs]], with [[gills]], [[cutaneous respiration]] (skin breathing), and breathing through the lining of the digestive tract, especially the mouth.

==== Gills ====
The early tetrapod ''Acanthostega'' had at least three and probably four pairs of gill bars, each containing deep grooves in the place where one would expect to find the afferent branchial artery. This strongly suggests that functional gills were present.<ref>{{harvnb|Clack|2012|pp=166}}</ref> Some aquatic temnospondyls retained internal gills at least into the early Jurassic.<ref name="SuesFraser2013">{{cite book|last1=Sues|first1=Hans-Dieter|author-link1=Hans-Dieter Sues|last2=Fraser|first2=Nicholas C.|title=Triassic Life on Land: The Great Transition|url=https://books.google.com/books?id=wVtxqddQKgwC&pg=PA85|access-date=21 July 2015|date=13 August 2013|publisher=Columbia University Press|isbn=978-0-231-50941-1|page=85|archive-date=20 August 2020|archive-url=https://web.archive.org/web/20200820014809/https://books.google.com/books?id=wVtxqddQKgwC&pg=PA85|url-status=live}}</ref> Evidence of clear fish-like internal gills is present in ''[[Archegosaurus]]''.<ref>{{cite journal | last1 = Witzmann | first1 = Florian | last2 = Brainerd | first2 = Elizabeth | year = 2017 | title = Modeling the physiology of the aquatic temnospondyl ''Archegosaurus decheni'' from the early Permian of Germany | journal = Fossil Record | volume = 20 | issue = 2| pages = 105–127 | doi = 10.5194/fr-20-105-2017 | doi-access = free | bibcode = 2017FossR..20..105W }}</ref>

==== Lungs ====
Lungs originated as an extra pair of pouches in the throat, behind the gill pouches.<ref>{{harvnb|Clack|2012|pp=23}}</ref> They were probably present in the last common ancestor of bony fishes. In some fishes they evolved into swim bladders for maintaining [[buoyancy]].<ref>{{harvnb|Laurin|2010|pp=36–7}}</ref><ref>{{harvnb|McGhee|2013|pp=68–70}}</ref> Lungs and swim bladders are homologous (descended from a common ancestral form) as is the case for the pulmonary artery (which delivers de-oxygenated blood from the heart to the lungs) and the arteries that supply swim bladders.<ref name="WebsterWebster2013">{{cite book|last1=Webster|first1=Douglas|last2=Webster|first2=Molly|title=Comparative Vertebrate Morphology|url=https://books.google.com/books?id=l7HfBAAAQBAJ&pg=PA372|access-date=22 May 2015|date=22 October 2013|publisher=Elsevier Science|isbn=978-1-4832-7259-7|pages=372–5|archive-date=19 August 2020|archive-url=https://web.archive.org/web/20200819141052/https://books.google.com/books?id=l7HfBAAAQBAJ&pg=PA372|url-status=live}}</ref> Air was introduced into the lungs by a process known as [[buccal pumping]].<ref>{{harvnb|Benton|2009|p=78}}</ref><ref>{{harvnb|Clack|2012|pp=238}}</ref>

In the earliest tetrapods, exhalation was probably accomplished with the aid of the muscles of the torso (the thoracoabdominal region). Inhaling with the ribs was either primitive for amniotes, or evolved independently in at least two different lineages of amniotes. It is not found in amphibians.<ref>{{harvnb|Clack|2012|pp=73–4}}</ref><ref name="BrainerdOwerkowicz2006">{{cite journal|last1=Brainerd|first1=Elizabeth L.|last2=Owerkowicz|first2=Tomasz|title=Functional morphology and evolution of aspiration breathing in tetrapods|journal=Respiratory Physiology & Neurobiology|volume=154|issue=1–2|year=2006|pages=73–88|url=https://www.researchgate.net/publication/6925157|issn=1569-9048|doi=10.1016/j.resp.2006.06.003|pmid=16861059|s2cid=16841094|access-date=2018-11-24|archive-date=2020-09-04|archive-url=https://web.archive.org/web/20200904221941/https://www.researchgate.net/publication/6925157_Functional_morphology_and_evolution_of_aspiration_breathing_in_tetrapods|url-status=live}}</ref> The muscularized diaphragm is unique to mammals.<ref name="MerrellKardon2013">{{cite journal|last1=Merrell|first1=Allyson J.|last2=Kardon|first2=Gabrielle|title=Development of the diaphragm - a skeletal muscle essential for mammalian respiration|journal=FEBS Journal|volume=280|issue=17|year=2013|pages=4026–4035|issn=1742-464X|doi=10.1111/febs.12274|pmid=23586979|pmc=3879042}}</ref>

==== Recoil aspiration ====
Although tetrapods are widely thought to have inhaled through buccal pumping (mouth pumping), according to an alternative hypothesis, aspiration (inhalation) occurred through passive recoil of the [[exoskeleton]] in a manner similar to the contemporary primitive ray-finned fish ''[[Polypterus]]''. This fish inhales through its [[Spiracle (vertebrates)|spiracle]] (blowhole), an anatomical feature present in early tetrapods. Exhalation is powered by muscles in the torso. During exhalation, the bony scales in the upper chest region become indented. When the muscles are relaxed, the bony scales spring back into position, generating considerable negative pressure within the torso, resulting in a very rapid intake of air through the spiracle.<ref name="EvansClaiborne2005">{{cite book|last1=Evans|first1=David H.|last2=Claiborne|first2=James B.|title=The Physiology of Fishes, Third Edition|url=https://books.google.com/books?id=lBltoKDaBVEC&pg=PA107|access-date=28 July 2015|date=15 December 2005|publisher=CRC Press|isbn=978-0-8493-2022-4|page=107|archive-date=19 August 2020|archive-url=https://web.archive.org/web/20200819141304/https://books.google.com/books?id=lBltoKDaBVEC&pg=PA107|url-status=live}}</ref><ref name="GrahamWegner2014">{{cite journal|last1=Graham|first1=Jeffrey B.|last2=Wegner|first2=Nicholas C.|last3=Miller|first3=Lauren A.|last4=Jew|first4=Corey J.|last5=Lai|first5=N Chin|last6=Berquist|first6=Rachel M.|last7=Frank|first7=Lawrence R.|last8=Long|first8=John A.|title=Spiracular air breathing in polypterid fishes and its implications for aerial respiration in stem tetrapods|journal=Nature Communications|volume=5|date=January 2014|url=https://www.researchgate.net/publication/259875906|issn=2041-1723|doi=10.1038/ncomms4022|pmid=24451680|page=3022|bibcode=2014NatCo...5.3022G|doi-access=free|access-date=2018-11-24|archive-date=2020-09-04|archive-url=https://web.archive.org/web/20200904221941/https://www.researchgate.net/publication/259875906_Spiracular_air_breathing_in_polypterid_fishes_and_its_implications_for_aerial_respiration_in_stem_tetrapods|url-status=live}}</ref><ref name="VickaryousSire2009">{{cite journal|last1=Vickaryous|first1=Matthew K.|last2=Sire|first2=Jean-Yves|title=The integumentary skeleton of tetrapods: origin, evolution, and development|journal=Journal of Anatomy|volume=214|issue=4|date=April 2009|pages=441–464|issn=0021-8782|doi=10.1111/j.1469-7580.2008.01043.x|pmid=19422424|pmc=2736118}}</ref>

==== Cutaneous respiration ====
Skin breathing, known as [[cutaneous respiration]], is common in fish and amphibians, and occur both in and out of water. In some animals waterproof barriers impede the exchange of gases through the skin. For example, keratin in human skin, the scales of reptiles, and modern proteinaceous fish scales impede the exchange of gases. However, early tetrapods had scales made of highly vascularized bone covered with skin. For this reason, it is thought that early tetrapods could engage some significant amount of skin breathing.<ref>{{harvnb|Clack|2012|pp=233–7}}</ref>

==== Carbon dioxide metabolism ====
Although air-breathing fish can absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the ability of lungs to discharge CO<sub>2</sub> came about gradually, and was not fully attained until the evolution of amniotes. The same limitation applies to gut air breathing (GUT), i.e., breathing with the lining of the digestive tract.<ref name="Nelson2014">{{cite journal|last1=Nelson|first1=J. A.|title=Breaking wind to survive: fishes that breathe air with their gut|journal=Journal of Fish Biology|volume=84|issue=3|date=March 2014|pages=554–576|issn=0022-1112|doi=10.1111/jfb.12323|pmid=24502287|bibcode=2014JFBio..84..554N }}</ref> Tetrapod skin would have been effective for both absorbing oxygen and discharging CO<sub>2</sub>, but only up to a point. For this reason, early tetrapods may have experienced chronic [[hypercapnia]] (high levels of blood CO<sub>2</sub>). This is not uncommon in fish that inhabit waters high in CO<sub>2</sub>.<ref>{{harvnb|Clack|2012|p=235}}</ref>
According to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a mechanism for relieving [[respiratory acidosis]] (acidic blood caused by excess CO<sub>2</sub>) through compensatory [[metabolic alkalosis]].<ref name="JanisDevlin2012">{{cite journal|last1=Janis|first1=C. M.|last2=Devlin|first2=K.|last3=Warren|first3=D. E.|last4=Witzmann|first4=F.|title=Dermal bone in early tetrapods: a palaeophysiological hypothesis of adaptation for terrestrial acidosis|journal=Proceedings of the Royal Society B: Biological Sciences|volume=279|issue=1740|date=August 2012|pages=3035–3040|issn=0962-8452|doi=10.1098/rspb.2012.0558|pmid=22535781|pmc=3385491}}</ref>

=== Circulation ===
Early tetrapods probably had a three-chambered [[heart]], as do modern amphibians and lepidosaurian and chelonian reptiles, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.<ref>{{harvnb|Clack|2012|pp=235–7}}</ref>

=== Senses ===

==== Olfaction ====
The difference in [[density]] between air and water causes [[odor|smells]] (certain chemical compounds detectable by [[chemoreceptor]]s) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory apparatus had evolved in the context of aquatic detection. The [[vomeronasal organ]] also evolved in the nasal cavity for the first time, for detecting pheromones from biological substrates on land, though it was subsequently lost or reduced to vestigial in some lineages, like [[archosaurs]] and [[catarrhines]], but expanded in others like [[lepidosaurs]].<ref>{{cite journal | last1=Poncelet | first1=Guillaume | last2=Shimeld | first2=Sebastian M. | title=The evolutionary origins of the vertebrate olfactory system | journal=Open Biology | date=2020 | volume=10 | issue=12 | article-number=200330 | doi=10.1098/rsob.200330 | doi-access=free | pmid=33352063 | pmc=7776563 }}</ref>

==== Lateral line system ====
Fish have a [[lateral line]] system that detects [[pressure]] fluctuations in the water. Such pressure is non-detectable in air, but grooves for the lateral line sense organs were found on the skull of early tetrapods, suggesting either an aquatic or largely aquatic [[habitat (ecology)|habitat]]. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the [[higher vertebrates]].

==== Vision ====
Changes in the eye came about because the behavior of light at the surface of the eye differs between an air and water environment due to the difference in [[refractive index]], so the [[focal length]] of the [[lens (anatomy)|lens]] altered to function in air. The [[eye]] was now exposed to a relatively dry environment rather than being bathed by water, so [[eyelid]]s developed and [[tear duct]]s evolved to produce a liquid to moisten the eyeball.

Early tetrapods inherited a set of five [[rod cell|rod]] and [[cone cell|cone]] opsins known as the vertebrate [[opsin]]s.<ref name="HuntHankins2014">{{cite book|author1=David M. Hunt|author2=Mark W. Hankins|author3=Shaun P Collin|author4=N. Justin Marshall|title=Evolution of Visual and Non-visual Pigments|url=https://books.google.com/books?id=APWwBAAAQBAJ&pg=PA165|date=4 October 2014|publisher=Springer|isbn=978-1-4614-4355-1|pages=165–|access-date=13 March 2016|archive-date=18 August 2020|archive-url=https://web.archive.org/web/20200818220721/https://books.google.com/books?id=APWwBAAAQBAJ&pg=PA165|url-status=live}}</ref><ref name="StavengaGrip2000">{{cite book|last1=Stavenga|first1=D.G.|last2=de Grip|first2=W.J.|last3=Pugh|first3=E.N.|title=Molecular Mechanisms in Visual Transduction|url=https://books.google.com/books?id=ZbWim1qiifgC&pg=PA269|access-date=14 June 2015|date=30 November 2000|publisher=Elsevier|isbn=978-0-08-053677-4|page=269|archive-date=20 August 2020|archive-url=https://web.archive.org/web/20200820040132/https://books.google.com/books?id=ZbWim1qiifgC&pg=PA269|url-status=live}}</ref><ref name="LazarevaShimizu2012">{{cite book|last1=Lazareva|first1=Olga F.|last2=Shimizu|first2=Toru|author3=Edward A. Wasserman|author-link3=Edward Wasserman|title=How Animals See the World: Comparative Behavior, Biology, and Evolution of Vision|url=https://books.google.com/books?id=KOv6cHWdjG8C&pg=PA459|access-date=14 June 2015|date=19 April 2012|publisher=OUP USA|isbn=978-0-19-533465-4|page=459|archive-date=19 August 2020|archive-url=https://web.archive.org/web/20200819135654/https://books.google.com/books?id=KOv6cHWdjG8C&pg=PA459|url-status=live}}</ref>

Four cone opsins were present in the first vertebrate, inherited from invertebrate ancestors:

*[[OPN1LW|LWS]]/[[OPN1MW|MWS]] (long- to medium-wave sensitive) - green, yellow, or red 
*[[OPN1SW|SWS1]] (short-wave sensitive) - ultraviolet or violet - lost in monotremes (platypus, echidna)
*SWS2 (short-wave sensitive) - violet or blue - lost in therians (placental mammals and marsupials)
*RH2 (rhodopsin-like cone opsin) - green - lost separately in amphibians and mammals, retained in reptiles and birds

A single rod opsin, rhodopsin, was present in the first jawed vertebrate, inherited from a jawless vertebrate ancestor:

*[[rhodopsin|RH1]] (rhodopsin) - blue-green - used night vision and color correction in low-light environments

==== Balance ====
Tetrapods retained the balancing function of the inner ear from fish ancestry.

==== Hearing ====
Air [[oscillation|vibrations]] could not set up [[pulse (signal processing)|pulsation]]s through the skull as in a proper auditory [[Organ (anatomy)|organ]]. The [[Spiracle (vertebrates)|spiracle]] was retained as the [[otic notch]], eventually closed in by the [[Tympanal organ|tympanum]], a thin, tight [[biological membrane|membrane]] of connective tissue also called the eardrum (however this and the otic notch were lost in the ancestral [[amniotes]], and later eardrums were obtained independently).

The [[hyomandibula]] of fish migrated upwards from its jaw supporting position, and was reduced in size to form the [[columella (auditory system)|columella]]. Situated between the tympanum and braincase in an air-filled cavity, the columella was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the columella became an important element in an [[Impedance matching#Acoustics|impedance matching]] system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibian [[Lineage (evolution)|lineages]].

The impedance matching ear had to meet certain conditions to work. The columella had to be perpendicular to the tympanum, small and light enough to reduce its [[inertia]], and suspended in an air-filled cavity. In modern species that are sensitive to over 1&nbsp;kHz [[frequency|frequencies]], the footplate of the columella is 1/20th the area of the tympanum. However, in early amphibians the columella was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the columella more probably just supported the brain case against the cheek.

Only in the early Triassic, about a hundred million years after they conquered land, did the tympanic [[middle ear]] evolve (independently) in all the tetrapod lineages.<ref>{{Cite journal |url=http://rspb.royalsocietypublishing.org/content/282/1802/20141943 |title=Better than fish on land? Hearing across metamorphosis in salamanders |year=2015 |doi=10.1098/rspb.2014.1943 |access-date=2016-01-20 |archive-date=2016-04-22 |archive-url=https://web.archive.org/web/20160422173533/http://rspb.royalsocietypublishing.org/content/282/1802/20141943 |url-status=live |last1=Christensen |first1=Christian Bech |last2=Lauridsen |first2=Henrik |last3=Christensen-Dalsgaard |first3=Jakob |last4=Pedersen |first4=Michael |last5=Madsen |first5=Peter Teglberg |journal=Proceedings of the Royal Society B: Biological Sciences |volume=282 |issue=1802 |pmid=25652830 |pmc=4344139 }}</ref> About fifty million years later (late Triassic), in mammals, the columella was reduced even further to become the [[stapes]].

==See also==
* [[Body form]]
* [[Geologic timescale]]
* [[Hexapoda]]
* [[Marine tetrapods]]
* [[Octopod]]
* [[Prehistoric life]]
* {{section link|Quadrupedalism|Quadrupeds vs. tetrapods}}

==References==
{{Reflist|30em}}

==Further reading==

{{Commons category |Tetrapoda}}

* {{cite book |last=Benton |first=Michael |author-link=Michael Benton |title=Vertebrate Palaeontology |url=https://books.google.com/books?id=VThUUUtM8A4C&pg=PA1 |access-date=10 June 2015 |edition=3 |date=5 February 2009 |publisher=John Wiley & Sons |isbn=978-1-4051-4449-0 |page=1}}
* {{cite book |last=Clack |first=J.A. |author-link=Jenny Clack |title=Gaining ground: the origin and evolution of tetrapods |publisher=Indiana University Press |location=Bloomington, Indiana, US. |year=2012 |edition=2nd |url=https://books.google.com/books?id=6Ztrhm8uLQ0C&pg=PA1 |isbn=978-0-253-35675-8 }}
* {{cite book |last=Laurin |first=Michel |author-link=Michel Laurin |title=How Vertebrates Left the Water |url=https://books.google.com/books?id=fa6gOvRdl9sC&pg=PA163 |access-date=26 May 2015 |year=2010 |publisher=University of California Press |isbn=978-0-520-26647-6}}
* {{cite book |last=McGhee |first=George R. Jr. |title=When the Invasion of Land Failed: The Legacy of the Devonian Extinctions |url=https://books.google.com/books?id=wFqrAgAAQBAJ&pg=PA92 |access-date=2 May 2015 |year=2013 |publisher=Columbia University Press |isbn=978-0-231-16057-5}}
* {{cite book |last=Steyer |first=Sebastien |title=Earth Before the Dinosaurs |url=https://books.google.com/books?id=9cjTW7FVBXgC&pg=PA59 |access-date=1 June 2015 |year=2012 |publisher=Indiana University Press |isbn=978-0-253-22380-7 |page=59}}
* {{cite journal |last=Clack |first=Jennifer A. |title=The Fin to Limb Transition: New Data, Interpretations, and Hypotheses from Paleontology and Developmental Biology |journal=Annual Review of Earth and Planetary Sciences |year=2009 |volume=37 |issue=1 |pages=163–179 |doi=10.1146/annurev.earth.36.031207.124146 |bibcode=2009AREPS..37..163C |ref=none}}
* {{cite book |editor-last=Hall |editor-first=Brian K. |title=Fins Into Limbs: Evolution, Development, and Transformation |year=2007 |publisher=University of Chicago Press |location=Chicago |isbn=978-0-226-31340-5 |url=https://books.google.com/books?id=Z0YWn5F9sWkC |ref=none}}
* {{cite journal |last=Long |first=John A. |last2=Young |first2=Gavin C. |last3=Holland |first3=Tim |last4=Senden |first4=Tim J. |last5=Fitzgerald |first5=Erich M. G. |title=An exceptional Devonian fish from Australia sheds light on tetrapod origins |journal=Nature |volume=444 |issue=7116 |date=2006 |issn=0028-0836 |doi=10.1038/nature05243 |pages=199–202 |ref=none}}
* {{cite book |last=Benton |first=Michael |author-link=Michael Benton |title=Vertebrate Palaeontology |publisher=Blackwell Publishing |edition=3rd |year=2005 |title-link=Vertebrate Palaeontology (Benton) |ref=none}}

{{Chordata}}
{{Tetrapodomorpha |St. |state=autocollapse}}
{{evolution}}
{{fins, limbs and wings}}
{{Taxonbar |from=Q19159}}
{{Authority control}}

[[Category:Tetrapods| ]]
[[Category:Body plans]]
[[Category:Middle Devonian first appearances]]
[[Category:Extant Devonian first appearances]]