Editing Middle ear (section)

==Diversity and evolution==

The middle ear of [[tetrapod]]s is [[analogy (biology)|analogous]] with the [[Spiracle (vertebrates)|spiracle]] of fishes, an opening from the [[pharynx]] to the side of the head in front of the main gill slits. In fish embryos, the spiracle forms as a pouch in the pharynx, which grows outward and breaches the skin to form an opening; in most tetrapods, this breach is never quite completed, and the final vestige of tissue separating it from the outside world becomes the eardrum. The inner part of the spiracle, still connected to the pharynx, forms the eustachian tube.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood |author2=Parsons, Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location=Philadelphia, PA |pages=480–488 |isbn=978-0-03-910284-5}}</ref>

In [[reptile]]s, [[bird]]s, and early fossil tetrapods, there is a single auditory ossicle, the [[Columella (auditory system)|columella]] which is homologous with the stapes, or "stirrup" of mammals. This is connected indirectly with the eardrum via a mostly cartilaginous extracolumella and medially to the inner-ear spaces via a widened footplate in the fenestra ovalis.<ref name=VB/> The columella is an evolutionary derivative of the bone known as the hyomandibula in fish ancestors, a bone that supported the skull and braincase.

=== Amphibians ===
The structure of the middle ear in living [[amphibian]]s varies considerably and is often degenerate. In most [[anurans|frogs and toads]], it is similar to that of reptiles, but in other amphibians, the middle ear cavity is often absent. In these cases, the stapes either is also missing or, in the absence of an eardrum, connects to the [[quadrate bone]] in the skull, although, it is presumed, it still has some ability to transmit vibrations to the inner ear. In many amphibians, there is also a second auditory ossicle, the ''operculum'' (not to be confused with the [[operculum (fish)|structure of the same name]] in fishes). This is a flat, plate-like bone, overlying the fenestra ovalis, and connecting it either to the stapes or, via a special muscle, to the [[scapula]]. It is not found in any other vertebrates.<ref name=VB/>

=== Mammals ===
Mammals are unique in having [[evolution of mammalian auditory ossicles|evolved a three-ossicle middle-ear]] independently of the various single-ossicle middle ears of other land vertebrates, all during the Triassic period of geological history. Functionally, the mammalian middle ear is very similar to the single-ossicle ear of non-mammals, except that it responds to sounds of higher frequency, because these are better taken up by the inner ear (which also responds to higher frequencies than those of non-mammals). The malleus, or "hammer", evolved from the [[articular]] bone of the lower jaw, and the incus, or "anvil", from the quadrate. In other vertebrates, these bones form the primary jaw joint, but the expansion of the [[mandible|dentary]] bone in mammals led to the evolution of an entirely new jaw joint, freeing up the old joint to become part of the ear. For a period of time, both jaw joints existed together, one medially and one laterally. The evolutionary process leading to a three-ossicle middle ear was thus an "accidental" byproduct of the simultaneous evolution of the new, secondary jaw joint. In many mammals, the middle ear also becomes protected within a cavity, the [[auditory bulla]], not found in other vertebrates. A bulla evolved late in time and independently numerous times in different mammalian clades, and it can be surrounded by membranes, cartilage or bone. The bulla in humans is part of the [[temporal bone]].<ref name="VB" />

Recently found fossils such as [[Morganucodon]] show intermediary steps of middle ear evolution. A new morganucodontan-like species, ''[[Dianoconodon youngi]]'', shows parts of the [[mandible]] (= dentary) that permit an auditory function, although these bones are still attached to the mandible.<ref>{{Cite journal |last1=Mao |first1=Fangyuan |last2=Zhang |first2=Chi |last3=Ren |first3=Jicheng |last4=Wang |first4=Tao |last5=Wang |first5=Guofu |last6=Zhang |first6=Fakui |last7=Rich |first7=Thomas |last8=Vickers-Rich |first8=Patricia |last9=Meng |first9=Jin |date=2024-04-03 |title=Fossils document evolutionary changes of jaw joint to mammalian middle ear |url=https://www.nature.com/articles/s41586-024-07235-0 |journal=Nature |volume=628 |issue=8008 |language=en |pages=576–581 |doi=10.1038/s41586-024-07235-0 |pmid=38570677 |bibcode=2024Natur.628..576M |issn=1476-4687|url-access=subscription }}</ref>