Rotation of middle ear ossicles during cetacean development

Cetacean middle ears are unique among mammals in that they have an elongate d tympanic membrane, a greatly reduced manubrium mallei, and an incudal cru s longum that is shorter than the crus breve. Elongation of the tympanic me mbrane and reduction of the manubrium is thought to be related to an evolut ionary rotation of the incus and malleus out of the plane of the tympanic m embrane. We examined if rotation also occurs during ontogeny by comparing t he middle ears of two species of dolphins (Delphinus delphis, Stenella atte nuata) at different stages of development. We observed that: the incus has the body and crural proportions as in terrestrial mammals early in developm ent; the incudomallear complex rotates approximately 90 degrees following o ssification; the tympanic membrane is not elongated until relatively late i n development. Therefore, some of the unique characteristics of the cetacea n middle ear develop as modifications of an initially terrestrial-like morp hology. J. Morphol. 249: 126-131, 2001. (C) 2001 Wiley-Liss, Inc.