Horopeta can be distinguished from other balaenomorphs in the following characters: base of frontal sloping moderately from skull midline; parietal considerably exposed at sagittal crest; parietal largely exposed at sagittal crest; unfused and short posterior process of periotic and tympanic bulla; prominent superior process of periotic; presence of elliptical foramen and sigmoidal cavity in tympanic bulla; unfused and short posterior processes of periotic and tympanic bulla; a distinct pedicle plate situated in fovea epitubaria; presence of horizontal cleft, elliptical foramen, and sigmoidal cavity in tympanic bulla.[1]
Initial cladistic analysis recovered Horopeta as either a basal balaenomorph or a basal cetotheriid, although the describers noted that the juvenile nature of the holotype might influence its cladistic position within Chaeomysticeti. Later cladistic analyses corroborated the placement of Horopeta outside crown Mysticeti.[1][2][3]
^ abcCheng-Hsiu Tsai; R. Ewan Fordyce (2015). "The Earliest Gulp-Feeding Mysticete (Cetacea: Mysticeti) from the Oligocene of New Zealand". Journal of Mammalian Evolution 22 (4): 535–560. doi:10.1007/s10914-015-9290-0.
^Tsai, C.H., Fordyce R.E., 2016. Archaic baleen whale from the Kokoamu Greensand: earbones distinguish a new late Oligocene mysticete (Cetacea: Mysticeti) from New Zealand. Journal of the Royal Society of New Zealand. doi:10.1080/03036758.2016.1156552
^Carlos Mauricio Peredo; Mark D. Uhen (2016). "A new basal chaeomysticete (Mammalia: Cetacea) from the Late Oligocene Pysht Formation of Washington, USA". Papers in Palaeontology. Online edition. doi:10.1002/spp2.1051