Pachycephalosaurus, the “thick-headed lizard,” is the archetypal pachycephalosaurid. It was described by Charles W. Gilmore in 1931, and lived during the Late Cretaceous period from 70 million years ago until the Cretaceous-Tertiary extinction event 65.5 million years ago. The true anatomy of Pachycephalosaurus is poorly known, as only the remains of its skull have been discovered, though it is believed that it was about 4 m (13 ft) long, 1.5 m (5 ft) tall, and about 365 kg (800 lbs) in weight. It is known from the Lance Formation in Wyoming, the Hell Creek Formation in Wyoming and the Dakotas, and the Judith River Formation of Montana, all in the United States. It should be noted, though, that the genera Stygimoloch and Dracorex are thought to be specimens of Pachycephalosaurus by Dr. Jack Horner, which would indicate that they also lived in southern Canada, though this theory is still under serious debate. Pachycephalosaurus and several other members of Pachycephalosauria are best known for their dome-shaped heads, which are exceptionally thick. Though initial speculation led many people, including the creative minds behind Jurassic Park, to conclude that such pachycephalosaurs used their heads to ram each others’ heads much like bighorn sheep, more recent evaluations have proven this to be impossible. It is, however, possible that they may have rammed each others’ flanks.
Pachycephalosaurus live on Isla Sorna, herding with Gallimimus and Parasaurolophus. It was originally suggested that the pachycephalosaurs created by InGen had undergone some changes, based on the amount of smaller specimens seen in The Lost World: Jurassic Park. This seemed to warrant a new taxonomic subspecies, Pachycephalosaurus wyomingensis “vegrandis,” but more accurate paleontological research in recent times have proven such assignment unnecessary and incorrect. As it turned out, the pachycephalosaurs were indeed the correct size, according to the Making of Book animatronic picture seen here and here. A size comparison was also done of animals in the round-up sequence, seen here. The mistake on our part was merely a matter of perception, which just goes to show that the Jurassic Park Encyclopedia is under constant revision and peer analysis.
During the roundup from the InGen Hunters on Isla Sorna, Pachycephalosaurus was given the nickname “Friar Tuck” by Roland Tembo. One specimen was captured by the Snagger after sending Carter through the side of his Jeep.
The Pachycephalosaurus were an attraction on the Jurassic World theme park on Isla Nublar. They were predominately implied to be exhibited in the Pachy Arena attraction, although dialogue from other characters implied that they were housed in other areas of the park. They are listed as being of a ‘medium’ aggression level on the in-universe Jurassic World website.
In the events of Jurassic World, Claire Dearing is informed by Lowery Cruthers that a Pachy has breached containment during a routine morning briefing. This is implied to be a regular occurrence. The animal was shown on a monitor being sedated by the Asset Containment unit, with a medical van nearby for transportation of the animal.