their base and thin towards their top. The palpebral bones strongly stood out, forming an "antorbital buttress". The fused prefrontals did not reach the nasal boss, forming a distinctive transverse saddle-shaped groove separating the nasal boss from the supraorbital bosses. This groove extended backwards, separating the supraorbital bosses from each other and forming a T-shape in top view. These bosses were similar to those of Pachyrhinosaurus, but with taller ridges and more pronounced rugosities. The long and low supraorbital horncores of the sub-adult specimen MOR 591 were similar to those of sub-adult Einiosaurus and Pachyrhinosaurus. They had a concave surface on the inner side as with Pachyrhinosaurus; ridges on the postorbital bones were present that may indicate a beginning transition to bosses. The skull roof of Achelousaurus had a midline cavity, with an opening at the top called the frontal fontanelle, a feature found in all ceratopsids, which have a "double" skull roof formed by the frontal bones folding towards each other between the brow horn bases. This cavity formed sinuses that extended below the supraorbital bosses, which were therefore relatively thin internally, being thick from the outside to the cavity roof. This cavity appears to have partially closed over as an animal aged, with only the rear part of the fontanelle being open in the adult specimen MOR 485. Like that of all other ceratopsids, the skull of Achelousaurus had a parietosquamosal frill or "neck shield", which was formed by the parietal bones at the rear and the squamosal bones at the sides. The parietal is one of the main bones used to distinguish centrosaurine taxa from each other and resolve relationships between them, whereas the squamosal is very similar across taxa. In Achelousaurus, the squamosal bone was much shorter than the parietal. Of its inner margin the rear portion formed a step in relation to the front part, with the suture between the squamosal and the parietal showing a kink to behind at the level of the rear supratemporal fenestra, a typical centrosaurine trait. The squamosal and the jugal bone, by touching each other, excluded the quadratojugal from the edge of the lateral temporal fenestra, i.e. the opening at the rear of the skull side. The frill of Achelousaurus had two conspicuous large spikes that were directed backwards and curved to the sides away from each other. During the 1990s, it was increasingly understood that such spikes on