The origin of the mammalian middle ear.

SYNOPSIS: A functional explanation is presented for the shift of the reptilian articular and quadrate into the mammalian middle ear to become the malleus and incus. Modification of the masticatory apparatus of therapsids results in reduction of stresses on the jaw joint and consequently in reduction of posterior elements of the jaw. In the late therapsid, Jiieno-Iherium, the quadrate and post-dentary jaw bones resemble the mammalian malleus and incus which together form a lever. The therapsid articular possesses a downturned retro-articular process (for insertion of M. depressor mandibulae) homologous with the manubriuni (force lever arm) of the malleus. About the time of origin of the mammalian (dentary-squamosal) jaw joint and following the origin of the mammalian depressor, the reptilian depressor is lost. This allows the enlarging reptilian tympanum to become attached to the retroarticular process. The new lever system thus formed by articular and quadrate increases the sensitivity of the ear and the reptilian one-bone system is replaced. In early mammals the reflected lamina of the angular migrates posteriorly with the angle of the dentary so that it contacts and assumes support of the tympanum. Non-homology of the monotreme and therian depressors indicates a multiple origin of the mammalian middle ear. Comparative anatomical studies of the nineteenth and early twentieth centuries demonstrated the surprising fact that two of the three tiny bones of the sound-conducting mechanism of the mammalian middle ear could be identified in reptiles as the bones forming the joint between the skull and lower jaw, having no part in the conduction of sound. Studies of living reptiles and mammals have not yielded explanations as to how and why these bones were transferred from the jaw to the ear, but paleontological evidence is now available which allows the probable sequence of events leading to the mammalian middle ear to be worked out in some detail. The following is a brief presentation of the main points of this story, with emphasis on the stages immediately preceding the actual transition from a reptilian to a mammalian type of middle ear. 1 The sound-conducting mechanism of the middle ear of reptiles and other non-mam-malian tetrapods consists of a relatively large superficial tympanum connected by a rod-like stapes to the small fenestra ovalis in the otic capsule (Fig. 1A). The system functions as a piston in which weak pressure changes (i.e., sound vibrations) in the 1 A more detailed study of the …