New Light on the Mammalian Ear Ossicles

Abstract

In spite of many criticisms, the Reichert-Gaupp theory of the mammalian ear ossicles^{1, 2, 3, 4} has, in its main points, been confirmed by a considerable body of work on the developmental anatomy of recent mammals and reptiles, and on the structure of therapsid reptiles. There can be little question that the stapes, incus and malleus (except for its dermal component, the goniale of Gaupp) are essentially homologous respectively with the reptilian columella auris (proximal part or otostapes), quadrate and articular. The existence of vestiges of other elements of the reptilian lower jaw and extrastapes in the mammalian middle ear has been suggested by numerous investigators. Thus the goniale is usually equated with the pre-articular^{1, 2, 3, 4, etc.}, and the elements of Paauw and Spence have been regarded as parts of the extrastapes⁵. But in general there has been little agreement on the detailed homology of the parts of the stapes, of the crura of the incus, and of the various processes of the malleus; this is largely because most of our information on these points is based on the anatomy of adult and embryonic stages of living mammals and reptiles, while mammals originated from the extinct therapsid reptiles, which differ from Lacertilia and Sphenodon in important respects. Until recently, too little was known in detail about the middle ear and lower jaw of therapsids, though Broom⁶ showed the position of the tympanic membrane and the presence of an ossified portion of the extrastapes in some Therocephalia (see also ref. 7). E. C. Olson⁸ has now provided, among other interesting things, an account of the structure of the otic region of several therapsids (Anomodontia, Therocephalia, Gorgonopsia, Cynodontia) based on serial sections. The structure of the middle ear region is greatly illuminated by this work; but some of his conclusions seem to be open to question, while others can be further supported.