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Evolution of the mammalian middle ear.
J Morphol. 1975 Dec; 147(4):403-37.JM

Abstract

The structure and evolution of the mandible, suspensorium, and stapes of mammal-like reptiles and early mammals are examined in an attempt to determine how, why, and when in phylogeny the precursors of the mammalian tympanic bone, malleus, and incus (postdentary jaw elements and quadrate) came to function in the reception of air-borne sound. The following conclusions are reached: It is possible that at no stage in mammalian phylogeny was there a middle ear similar to that of "typical" living reptiles, with a postquadrate tympanic membrane contracted by an extrastapes. The aquamosal sulcus of cynodonts and other therapsids, usually thought to have housed a long external acoustic meatus, possibly held a depressor mandibulae muscle. In therapsids an air-filled chamber (recessus mandibularis of Westoll) extended deep to the reflected lamina and into the depression (external fossa) on the outer aspect of the angular element. A similar chamber was present in sphenacodontids but pterygoideus musculature occupied the small external fossa. The thin tissues superficial to the recessus mandibularis served as eardrum. Primitively, vibrations reached the stapes mainly via the anterior hyoid cornu, but in dicynodonts, therocephalians, and cynodants vibrations passed mainly or exclusively from mandible to quadrate to stapes and the reflected lamina was a component of the eardrum. In the therapsid phase of mammalian phylogeny, auditory adaptation was an important aspect of jaw evolution. Auditory efficiency, and sensitivity to higher sound frequencies were enhanced by diminution and loosening of the postdentary elements and quadrate, along with transference of musculature from postdentary elements to the dentary. These changes were made possible by associated modifications, including posterior expansion of the dentary. Establishment of a dentary-squamosal articulation permitted continuation of these trends, leading to the definitive mammalian condition, with no major change in auditory mechanism except that in most mammals (not monotremes) the angular, as tympanic, eventually bcame a non-vibrating structure.

Authors

No affiliation info available

Pub Type(s)

Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

1202224

Citation

Allin, E F.. "Evolution of the Mammalian Middle Ear." Journal of Morphology, vol. 147, no. 4, 1975, pp. 403-37.
Allin EF. Evolution of the mammalian middle ear. J Morphol. 1975;147(4):403-37.
Allin, E. F. (1975). Evolution of the mammalian middle ear. Journal of Morphology, 147(4), 403-37.
Allin EF. Evolution of the Mammalian Middle Ear. J Morphol. 1975;147(4):403-37. PubMed PMID: 1202224.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Evolution of the mammalian middle ear. A1 - Allin,E F, PY - 1975/12/1/pubmed PY - 1975/12/1/medline PY - 1975/12/1/entrez SP - 403 EP - 37 JF - Journal of morphology JO - J. Morphol. VL - 147 IS - 4 N2 - The structure and evolution of the mandible, suspensorium, and stapes of mammal-like reptiles and early mammals are examined in an attempt to determine how, why, and when in phylogeny the precursors of the mammalian tympanic bone, malleus, and incus (postdentary jaw elements and quadrate) came to function in the reception of air-borne sound. The following conclusions are reached: It is possible that at no stage in mammalian phylogeny was there a middle ear similar to that of "typical" living reptiles, with a postquadrate tympanic membrane contracted by an extrastapes. The aquamosal sulcus of cynodonts and other therapsids, usually thought to have housed a long external acoustic meatus, possibly held a depressor mandibulae muscle. In therapsids an air-filled chamber (recessus mandibularis of Westoll) extended deep to the reflected lamina and into the depression (external fossa) on the outer aspect of the angular element. A similar chamber was present in sphenacodontids but pterygoideus musculature occupied the small external fossa. The thin tissues superficial to the recessus mandibularis served as eardrum. Primitively, vibrations reached the stapes mainly via the anterior hyoid cornu, but in dicynodonts, therocephalians, and cynodants vibrations passed mainly or exclusively from mandible to quadrate to stapes and the reflected lamina was a component of the eardrum. In the therapsid phase of mammalian phylogeny, auditory adaptation was an important aspect of jaw evolution. Auditory efficiency, and sensitivity to higher sound frequencies were enhanced by diminution and loosening of the postdentary elements and quadrate, along with transference of musculature from postdentary elements to the dentary. These changes were made possible by associated modifications, including posterior expansion of the dentary. Establishment of a dentary-squamosal articulation permitted continuation of these trends, leading to the definitive mammalian condition, with no major change in auditory mechanism except that in most mammals (not monotremes) the angular, as tympanic, eventually bcame a non-vibrating structure. SN - 0362-2525 UR - https://www.unboundmedicine.com/medline/citation/1202224/Evolution_of_the_mammalian_middle_ear_ L2 - https://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0362-2525&date=1975&volume=147&issue=4&spage=403 DB - PRIME DP - Unbound Medicine ER -