MONOTREME AFFINITIES AND LOW-FREQUENCY HEARING SUGGESTED BY MULTITUBERCULATE EAR

MULTITUBERCULATES are an extinct, dentally distinctive group of Mesozo ic/early Cenozoic mammals of uncertain affinities(1). We report here t he discovery of a multituberculate ectotympanic bone, associated with the malleus in original life position, from two exquisitely preserved auditory regions, This documents, to our knowledge for the first time, incorporation of the angular and prearticular bones (jaw components i n non-mammalian tetrapods) into the middle ear of multituberculates, f avouring the hypothesized single origin of the ossicular chain in mamm als(2,3). Morphology and orientation of these elements are strikingly similar to those of the extant egg-laying platypus and echidnas, sugge sting a unique common ancestry of these forms(4), an affiliation once generally discredited(5-8) but regaining some recent support(9,10). Th e structure of these new multituberculate auditory ossicles, in conjuc tion with a greatly inflated vestibule and an uncoiled cochlea, implie s an ear inefficient for reception of high-frequency airborne vibratio ns but well suited for bone-conducted hearing,