Pachyrhinosaurus

Pachyrhinosaurus was probably the most unusual and distinctive ceratopsid. It did not have brow or nasal horns; instead it had a thick, bumpy, spongy pad of bone along the upper surface of its flattened face. This bony pad ran from the front of its nose back to above its eyes. The skull of Pachyrhinosaurus was massive; only Triceratops, Pentaceratops, and Torosaurus had larger skulls. Pachyrhinosaurus was the largest centrosaurine ceratopsian.

Charles Sternberg named Pachyrhinosaurus canadensis from three partial skulls found in southern Alberta. Pachyrhinosaurus means "thick-nose reptile"; the name refers to the bony facial pad, called the boss. Because skulls of this dinosaur are rare, and because of their odd, gnarled facial boss, some paleontologists thought these specimens were "pathological." That is, they thought the facial pad was formed because a nasal horn broke off and then healed over. So they thought the facial boss was a "scar." Paleontologists recently uncovered a large bone bed with many Pachyrhinosaurus specimens in north central Alberta. This proves that the facial pad is a normal feature of Pachyrhinosaurus and not a scar.

The rest of the skull of Pachyrhinosaurus looked very much like that of other centrosaurine ceratopsians such as Centrosaurus, Monoclonius, and Styracosaurus. Like them, Pachyrhinosaurus had a short frill, a deep face, and a short beak. Pachyrhinosaurus was also closely related to Brachyceratops and Avaceratops.