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

Abstract

From a survey ot the structure of the skull in rhipidistian fishes and early labylinthodont Amphibia and of the mechanism of hearing in these two groups, an account of the evolution of the tetrapod middle ear is presented. The overall modification of the otic region of the skull during the rhipidistian-amphibian transition is analyzed in terms of changes in different organ systems 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 reception under water and in air, in connection with this correlated progression of evolutionary change in the otic region of the head, reveal the manner in which the hyomandibular, spiracular diverticulum, and operculum of rhipidistian fishes became modified to form the stapes, the tympanic cavity, and the outer portion of the tympanum, respectively, of tetrapods.