Achelousaurus (“Achelous lizard”) seems to have been a transitional form among the Pachyrhinosaurini. While slightly more basal members like Einiosaurus had well-developed nose horns, and the later Pachyrhinosaurus had enormous flattened nasal bosses, Achelousaurus was somewhere in-between.
Living in Montana, USA, about 74 million years ago, it reached sizes of about 6m long (19′8″). Instead of horns, rough patches of bone on its skull supported bosses over its snout and eyes, which in life would have been covered by thick layers of keratinous skin.
Unlike adults, juveniles had small brow horns with bony cores which gradually developed into full bosses as they grew. It’s possible that the bosses of mature individuals were used for headbutting or ramming behaviors.