Evolutionary Origins of The Mammalian Promontorium and Cochlea

Abstract

ABSTRACT Mammals have two apomorphies in the ear region: an elongated cochlear canal and an eminence on the tympanic side of the cochlear housing, known as the petrosal promontorium. In nonmammalian cynodonts, the cochlear recess is shorter, smaller, and oriented more medially than the mammalian cochlear canal; their cochlear housing is formed by the prootic, opisthotic, and basisphenoid. New fossil materials of two early mammals, Adelobasileus and Sinoconodon, reveal evidence on the evolutionary transformation of the mammalian cochlear canal and bony cochlear housing. Adelobasileus is more derived than any known non-mammalian cynodont 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 extends only one third of the promontorium length. The cochlear canal of Morganucodon is twice as long as that of Sinoconodon, relative to both skull and promontorium. More derived mammals, such as Haldanodon and multituberculates, have an even longer cochlear canal. We propose that the promontorium transformed by expansion of the petrosal bone at the expense of the basisphenoid (probably also the basioccipital) through 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 in Sinoconodon suggests that the formation of the promontorium preceded (and thus may be necessary for) the elongation of the mammalian cochlea.