The Inner Ear of Two Late Cretaceous Multituberculate Mammals, and Its Implications for Multituberculate Hearing

The inner ear of the Late Cretaceous multituberculates Nemegtbaatar gobiensis and Chulsan-baatar vulgaris is described from serial sections and enlarged models. The size and proportions of the inner ear as a whole are as expected for extant small mammals. The lengths of the cochlea (Nemegtbaatar gobiensis, 3.0 mm, Chulsanbaatar vulgaris, 2.0 mm) are comparable to those of other multituberculates, when ratios of length of the cochlea to skull length are calculated. The vestibule is not as expanded in the two taxa as in Lambdopsalis, ?Meniscoessus, and ?Catopsalis; the estimated volume for Nemegtbaatar gobiensis is 9 mm³. A slightly laterally curved, anteriomedially directed cochlea, relatively robust ear ossicles, and the estimations of the area of the tympanic membrane and stapedial footplate in Chulsanbaatar suggest high-frequency hearing but a relatively low sensitivity to low-decibel sounds. The semicircular canals of Nemegtbaatar and Chulsanbaatar are fully developed; the size of the anterior, posterior, and lateral canals and their angles and proportions are comparable to those of extant mammals of similar size. The anterior semicircular canal of Nemegtbaatar forms a smooth half-circle and thus is more derived than the angular canal of Ornithorhynchus. The notable differences between the ratio of the width of the lateral semicircular canal to skull length and the size of the vestibule in Nemegtbaatar and the Paleocene multituberculate Lambdopsalis bulla are probably related to different modes of life.