The Evolution of the Tetrapod Middle Ear in the Rhipidistian-Amphibian Transition

From a survey ot the structure of the skull in rhipidistianfishes and early labylinthodont Amphibia and of the mechanismof hearing in these two groups, an account of the evolutionof the tetrapod middle ear is presented. The overall modificationof the otic region of the skull during the rhipidistian-amphibiantransition is analyzed in terms of changes in different organsystems in response to different selective pressures (affecting,for example, the feeding, respiratory, and locomotory mechanisms).These changes are seen to occur in a completely integrated pattern.Considerations of the different requirements for sound receptionunder water and in air, in connection with this correlated progressionof evolutionary change in the otic region of the head, revealthe manner in which the hyomandibular, spiracular diverticulum,and operculum of rhipidistian fishes became modified to formthe stapes, the tympanic cavity, and the outer portion of thetympanum, respectively, of tetrapods.