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The mammalian stapes is the subject of considerable investigation, but ambiguity remains with respect to the primitive condition in higher-level mammal clades and the pattern of subsequent modifications. We question the widely held belief that the strongly bicrurate, stirrup-like stapes represents the ancestral mammalian and therian state (Goodrich, 1930). The primitive stapes in the common ancestor of mammals and therapsids was probably columelliform and had a stapedial foramen for the passage of the stapedial artery. However, additional modifications are required to produce the bicrurate structure characteristic of many eutherians. Moreover, distributional evidence does not rule out the possibility that the columelliform-imperforate stapes seen in adult monotremes was ancestral for therians or a group comprising therians and monotremes.

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