Editing Tetrapod (section)

=== 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]].