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In page Hornbill:

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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. Similarly, the Knobbed hornbill is more closely related to Rhyticeros, leaving the Rufous-necked hornbill the only member of the genus Aceros.[5]