Head horns shown for 15 species of Onthophagus. Common changes in horn shape include the splitting of a single horn into two or more horns, the addition of forks or branches, and transitions from straight to curved. Top row: O. victoriensis; O. rupicapra; O. sugillatus; O. capella; O. ouratta. Middle row: O. aeruginosus; O. pentacanthus; O. fuliginosus; O. taurus; O. leai. Bottom row: O. gazella; O. nuchicornis; O. rangifer, and two Paleonthophagus species. Specied morphologies for O. victoriensis, O. rupicapra, O. sugillatus, O. capella, O. ourata, O. pentacanthus, O. fuliginosus and O. leai were derived from Matthews (1972); Paleonthophagus species were derived from Fig. 11 of Kabakov (1990).