My PhD project encompasses functional-morphological investigations of the bony labyrinth as well as phylogenetic examinations of the auditory ossicles of the Diprotodontia (Marsupialia, Australidelphia). The members of this subfamily of Australian marsupials differ significantly in locomotion and life style (e.g. arboreal, fossorial). The mammalian inner ear comprises not only the organ for hearing, the cochlea, but also the three semicircular canals which are essential for orientation in 3D space. The study of these structures by geometric morphometric analyses shall give information about the type of movement and hereafter the reconstruction of the paleoenvironment. The mammalian middle ear consists of stapes, incus and malleus and bears information about the evolutionary development. Scrutiny and comparison of the three ossicles of different taxa will help to understand the origination and separate development of the sister group of placental mammals.