CT scanning and computerized reconstructions of the inner ear of multituberculate mammals

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Abstract
The inner-ear structure of four multituberculate petrosals from the Late Cretaceous and Early Paleocene of North America was examined with computerized tomography (CT). Transverse CT scan images of these petrosals were digitized to produce a three-dimensional reconstruction of the acousto-vestibular spaces of multituberculates. Our investigation shows that the acousto-vestibular spaces of multituberculates are characterized by a straight cochlear canal and an extraordinarily enlarged vestibular cavity. Comparison of multituberculate inner-ear structures with those in other major mammalian clades suggests that the cochlea of multituberculates, which is similar to that of Morganucodon, is more derived than the cochlea of non-mammalian therapsids but more primitive than the coiled cochlea of therian and monotreme mammals. The extraordinary inflation of the vestibule of multituberculates is a uniquely derived character of most Cretaceous and Tertiary multituberculates, and it may be the synapomorphy of the order Multituberculata. Inflation of the vestibule may be correlated with low frequency hearing.

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