Here we present two new Miocene basal balaenomorph mysticetes recognized by periotic bones with a unique combination of characters. The fossil periotics originated from former shallow marine sediments and were discovered in a suburban quarry of Bocholt in the German Münsterland holding a number of remains of a multispecies community of mysticetes. A detailed accompanying analysis of the internal and external structures of mysticete periotics resulted in a critical review and revision of two important characteristics of the pars cochlearis. The partition between the openings for the facial nerve and the vestibulocochlear nerve, named here ‘bony septum’ for baleen whales, has been developed so prominently during mysticete evolution that the crista transversa is no longer recognizable in derived forms, as it lies deep inside the inner ear canal and within the fundus of the internal acoustic meatus where it separates the cochlear nerve from the inferior vestibular nerve. We show that the designation of the opening for the superior vestibular nerve as ‘foramen singulare’ is misleading since this term describes the opening for the nerve to the posterior semicircular canal within the fundus of the internal acoustic meatus.
