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

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... ticular possesses a downturned retroarticular 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 (dentarysquamosal) 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.