Pachycephalosaurus păk˝ĭsĕf˝əlōsôr´əs [key] [Gr., = thick-headed lizard], bipedal herbivorous dinosaur of the late Cretaceous period, approximately 68–65 million years ago. Its distinguishing characteristic was a very heavy dome-shaped skull that was up to 10 in. (25 cm) thick; the skull was covered with bumps. Because the top vertebrae were able to lock in place, scientists have theorized that the head was used as a battering ram, perhaps to establish dominance within a herd. The mouth was beaked and the jaws held leaf-shaped cheek teeth. The eyes were placed in a way that made it capable of binocular vision, and Pachycephalosaurus is believed to have had an acute sense of smell. Because of a dearth of skeletal specimens, other characteristics of this genus can be surmised only from related dinosaurs. Fossils have been found in Montana, South Dakota, and Wyoming. Pachycephalosaurus, along with Stygimoloch and Homalocephale, belongs to the group of ornithischian dinosaurs called pachycephalosaurs, or bone-headed dinosaurs.

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