Editing Tetrapod (section)

== 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/ }}</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 extend 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|pages=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|population bottlenecks]] 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=9780231160568 |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=9780226313405 |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|pseudosuchians]] (crocodilian ancestors), [[dinosaur]]s, [[Pterosaur|pterosaurs]], 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|Lizards]], turtles, dinosaurs, pterosaurs, [[crocodylomorphs]], and [[Plesiosaur|plesiosaurs]] 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|rhynchocephalians]] (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=}}</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)