Osseous inner ear structures and hearing in early marsupials and placentals

Based on the internal anatomy of petrosal bones as shown in radiographs and scanning electron microscopy, the inner ear structures of Late Cretaceous marsupials and placentals (about 65 Myr ago) from the Bug Creek Anthills locality of Montana, USA, are described. The inner ears of Late Cretaceous marsupials and placentals are similar to each other in having the following tribosphenic therian synapomorphies: a fully coiled cochlea, primary and secondary osseous spiral laminae, the perilymphatic recess merging with the scala tympani of the cochlea, an aqueductus cochleae, a true fenestra cochleae, a radial pattern of the cochlear nerve and an elongate basilar membrane extending to the region between the fenestra vestibuli and fenestra cochleae. The inner ear structures of living therians differ from those of their Late Cretaceous relatives mainly in having a greater number of spiral turns of the cochlea and a longer basilar membrane. Functionally, a coiled cochlea not only permits the development of an elongate basilar membrane within a restricted space in the skull but also allows a centralized nerve system to innervate the elongate basilar membrane. Qualitative and quantitative analyses show that, with a typical therian inner ear, Late Cretaceous marsupials and placentals were probably capable of high-frequency hearing.