as large as its close relative Einiosaurus, but with a much heavier build. Achelousaurus approached the robustness of one of the largest and most heavily built horned dinosaurs known: Triceratops. As a ceratopsid, Achelousaurus would have been a quadrupedal animal with hoofed digits and a shortened, downwards swept tail. Its very large head, which would have rested on a straight neck, had a hooked upper beak, very large nasal openings, and long tooth rows developed into dental batteries that contained hundreds of appressed and stacked individual teeth. In the tooth sockets, new teeth grew under the old ones, each position housing a column of teeth posed on top of each other. Achelousaurus had 25 to 28 such tooth positions in each maxilla (upper jaw bone). Distinguishing traits In 1995, when describing the species, Sampson gave a formal list of four traits that distinguish Achelousaurus from its centrosaurine relatives. Firstly, adult individuals have nasal bones with a boss on top that is relatively small and thin, and heavily covered with pits; secondly, adult individuals do not have true horns above the eye sockets but relatively large bosses with high ridges; thirdly, not yet fully grown individuals, or subadults, have true horncores (the bony part of the horns) above the eye sockets with the inward facing surface being concave; and fourthly, the parietal bones of the neck shield have a single pair of curved spikes sticking out from the rear margin to behind and to the outside. Besides these unique characteristics, Sampson pointed out additional differences with two very closely related forms. The frill spikes of Achelousaurus are more outwards oriented than the spikes of Einiosaurus, which are medially curved; the spikes of Achelousaurus are nevertheless less directed to the outside than the comparable spikes of Pachyrhinosaurus. Achelousaurus also differs from Pachyrhinosaurus in its smaller nasal boss that does not reach the frontal bones at its rear. Apart from the skull, no features of the skeleton are known that distinguish Achelousaurus from other members of the Centrosaurinae. Skull Horned dinosaurs mainly differ from each other in their horns, which are located on the snout and above the eyes, and in the large skull frill, which covers the neck like a shield. Achelousaurus exhibited the build of derived ("advanced") centrosaurines, which are typified by short brow horns or bosses, combined with elaborate frill spikes. The general frill proportions are typically centrosaurine, with a