Is this true in the case where the horns are also rapidly enlarging? It would seem in that case that merely increasing deposition on one side would change curvature w/out any removal required.
That would result in an S-shaped horn, and that's not what we see. Instead, the horns first curve backward, then they stick straight up, and then they curve forward.
Further -- I wonder if backwards curving horns in juveniles is mentioned in lit as support (on a selective basis) for the 'extended close physical adult/offspring contact' scenario. Perhaps hunkering underneath Mom for protection from sun, weather, and predators (etc) was better tolerated by Mom if your little horns did not stick STRAIGHT up... :D
Who knows! :-)Regarding horn resorption in *Pachyrhinosaurus*, this looks to me like the horn core was resorbed while the keratinous horn stayed; counterintuitive enough, but perhaps it suggests the same for *Pachycephalosaurus*.