Editing Tetrapod (section)

==== 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]].