EVOLUTIONARY ORIGINS OF THE MAMMALIAN PROMONTORIUM AND COCHLEA

Mammals have two apomorphies in the ear region: an elongated cochlear canal and an eminence on the tympanic side of the cochlear housing, kn own as the petrosal promontorium. In nonmammalian cynodonts, the cochl ear recess is shorter, smaller, and oriented more medially than the ma mmalian cochlear canal; their cochlear housing is formed by the prooti c, opisthotic, and basisphenoid. New fossil materials of two early mam mals, Adelobasileus and Sinoconodon, reveal evidence on the evolutiona ry transformation of the mammalian cochlear canal and bony cochlear ho using. Adelobasileus is more derived than any known non-mammalian cyno dont in possessing an incipient promontorium, but more primitive than other mammals in retaining a vestigial basisphenoid wing. Sinoconodon has a fully developed petrosal promontorium but a short cochlea that e xtends only one third of the promontorium length. The cochlear canal o f Morganucodon is twice as long as that of Sinoconodon, relative to bo th skull and promontorium. More derived mammals, such as Haldanodon an d multituberculates, have an even longer cochlear canal. We propose th at the promontorium transformed by expansion of the petrosal bone at t he expense of the basisphenoid (probably also the basioccipital) throu gh the transition from non-mammalian cynodonts to mammals. This change in ossification of the cochlear housing. resulted in a re-orientation of the cochlea and a better acoustic insulation of the inner ear. The mosaic of a fully developed promontorium and a short cochlear canal i n Sinoconodon suggests that the formation of the promontorium preceded (and thus may be necessary for) the elongation of the mammalian cochl ea.