The taxonomic revision of Isocetus depauwi Van Beneden, 1880 was carried out through the description of a number of specimens assigned to this species by Van Beneden and Abel in the last decades of the 19th and the early decades of the 20th centuries. After detailed comparisons with the published record of archaic mysticetes, the genus and species are considered valid despite the incompleteness and poor preservation of the material. Diagnostic features are found in the morphology of the mandibular condyle, the angular process of the dentary and the thoracic vertebrae. The new species Parietobalaena campiniana is established based on a partial skeleton, including most of the skull and ear bones, previously assigned to Isocetus depauwi by Abel. Parietobalaena campiniana is described and compared in detail, and its phylogenetic relationships are assessed by a new large-scale cladistic analysis of 246 morphological characters scored for 46 taxa. The results support a basal position of the genus Parietobalaena in the radiation of Miocene ‘cetotheres’, the monophyly of Balaenoidea (Neobalaenidae and Balaenidae), Balaenopteroidea (Eschrichtiidae and Balaenopteridae), and Cetotheriidae and the paraphyly of ‘cetotheres’s.l. The new superfamily Thalassotherii is established based on high bootstrap support and a number of morphological characters; Thalassotherii includes Balaenopteridae, Eschrichtiidae, Cetotheriidae and the basal ‘cetothere’ mysticetes with the exclusion of Balaenoidea, Eomysticetoidea and toothed mysticetes.