Means: Thick-headed Lizard 

Period: Late Cretaceous, 68-66 Million Years Ago

Region: Montana and Wyoming (USA), Alberta (Canada)

Pachycephalosaurus is the largest of the bone-headed dinosaurs known to science.  It measured up to 4.6 metres in length and weighed approximately 500 kilograms.  Named and scientifically described in 1943 from fragmentary fossil finds, these dinosaurs are famous for their robust dome-shaped skulls that in the very largest specimens were up to 25 cm thick.

It has been suggested that male pachycephalosaurs used their thick skulls in head butting contests to settle disputes within the herd.  Studies have demonstrated that the skulls may have been poorly suited to direct headlong ramming.  If pachycephalosaurs did ram each other to settle disputes or to defend themselves, they probably aimed for the flanks.

Palaeontologists have noted a great deal of variation in the number and size of the lumps and bumps on the skull domes of these dinosaurs.  It has been suggested that those dinosaurs with larger and more complex cranial ornamentation represented the more dominant members of the herd. If this is the case, then the skulls played a role in visual communication between pachycephalosaurus.

No complete skeleton of Pachycephalosaurus has been found and museum exhibits featuring Pachycephalosaurus have largely been based on scaling up the fossil bones of smaller bone-headed dinosaurs.  In 2018, a nearly complete skull and jaws of a juvenile Pachycephalosaurus was found in eastern Montana.  This discovery permitted palaeontologists to study the front teeth of Pachycephalosaurus for the first time.  These teeth resemble the teeth of a meat-eating dinosaur which suggests that Pachycephalosaurus was probably an omnivore, browsing on leaves, flowers and fruit but also feeding on insects and small animals.