Editing Tetrapod (section)

==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 a large fleshy [[fin]] 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===
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| pages = 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|pages=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.|author-link=|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>Poncelet, G., and Shimeld, S. M. (2020). The evolutionary origin of the vertebrate olfactory system. Open Biol. 10:200330. doi: 10.1098/rsob.200330</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]].