Hell Creek Formation
Ornithischians are abundant in the Hell Creek Formation. The main groups of ornithischians are ankylosaurians, ornithopods, ceratopsians, and pachycephalosaurians. One ankylosaurian and two nodosaurians are known, "Ankylosaurus", "Denversaurus" and possibly "Edmontonia". Multiple genera of ceratopsians are known from the formation, the leptoceratopsid "Leptoceratops" and the chasmosaurines "Nedoceratops", "Torosaurus", "Triceratops", and "Tatankaceratops". Hadrosaurs are common in the Hell Creek Formation, and are known from multiple species of the ornithopod "Thescelosaurus", and the hadrosaurids "Edmontosaurus", and an undescribed genus similar to "Parasaurolophus". Five pachycephalosaurians have been found in the Hell Creek Formation. Among them are the derived pachycephalosaurids "Sphaerotholus", "Stygimoloch", "Dracorex", "Pachycephalosaurus", and an undescribed specimen from North Dakota.