 As for the other Asian hornbill species, "Buceros" and "Rhinoplax" are each other's closest relatives, "Anorrhinus" is part of a clade that has "Ocyceros" and "Anthracoceros" as sister taxa, and "Aceros", "Rhyticeros", and "Penelopides" form another clade.However, according to this study, "Aceros" is polyphyletic; the Rufous-headed hornbill, Writhed hornbill, and Wrinkled hornbill form a clade with the Sulawesi hornbill, and are in turn more closely related to "Penelopides".These four species have been classified in a separate genus, "Rhabdotorrhinus".