MULTITUBERCULATES are an extinct, dentally distinctive group of Mesozoic/early Cenozoic mammals of uncertain affinities(1). We report here the 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 in non-mammalian tetrapods) into the middle ear of multituberculates, favouring the hypothesized single origin of the ossicular chain in mammals(2,3). Morphology and orientation of these elements are strikingly similar to those of the extant egg-laying platypus and echidnas, suggesting a unique common ancestry of these forms(4), an affiliation once generally discredited(5-8) but regaining some recent support(9,10). The structure of these new multituberculate auditory ossicles, in conjuction with a greatly inflated vestibule and an uncoiled cochlea, implies an ear inefficient for reception of high-frequency airborne vibrations but well suited for bone-conducted hearing,