an evolutionary tree. The cladogram below shows the phylogenetic position of Achelousaurus in a cladogram from Wilson and colleagues, 2020. Paleobiology Function of skull ornamentation In 1995, Sampson noted that earlier studies had found that the horns and frills of ceratopsians most likely had a function in intraspecific display and combat, and that these features would therefore have resulted from sexual selection for successful mating. Likewise, in 1997 Horner concluded that such ornamentation was used by males to establish dominance and that females would have preferred well-equipped males as their offspring would then inherit these traits, conferring a reproduction benefit. Dodson thought that in the Centrosaurinae in general the display value of the frill had been reduced compared to the nasal and supraorbital ornamentation. Sampson in 1995 rejected the possibility that the difference in skull ornamentation between Einiosaurus and Achelousaurus represented sexual dimorphism, for three reasons. Firstly, the extensive Einiosaurus bone beds did not contain any specimens with bosses, as would have been expected if one of the sexes had them. Secondly, Einiosaurus and Achelousaurus are found in strata of a different age. Thirdly, in a situation of sexual dimorphism usually only one of the sexes shows exaggerated secondary sexual characters. Einiosaurus and Achelousaurus however, each have developed a distinct set of such traits. Hieronymus, in 2009, concluded that the nasal and supraorbital bosses were used for butting or ramming the head or the flank of a rival. The bone structure indicates that the bosses were covered by cornified pads as in modern muskoxen, suggesting dominance fights similar to those of members of the Caprinae subfamily. In the latter group, an evolutionary transition can be observed, where the originally straight horns become more robust, padded, and increasingly curved downwards. The evolution from horncores into bosses in Centrosaurinae would likewise have reflected a change in fighting technique, from clashing to high-energy head-butting. Head-butting would have been an expensive and risky behavior. Opponents would have engaged this way only after assessing each other's strengths visually. For this reason, Hieronymus considered it unlikely that the bosses served for species recognition as this was already guaranteed by the innate species-specific display rituals preceding a real – instead of a ritual – fight. The bosses would have evolved for actual combat, part of a social selection in which individuals competed for scarce resources such as mates, food and breeding grounds. Previously it had been suggested