Editing Tetrapod (section)

=== 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>