Pachycephalosaurus, whose name means thick-headed reptile, had an incredibly thick skull (25 cm), a tiny brain, and large eyes, suggesting good vision. When it lowered its head, its body remained completely horizontal, giving it the ability to charge like a battering ram. It was a herding dinosaur that lived in small groups.

 

Pachycephalosaurus is a genus of pachycephalosaurid ornithischian dinosaur that lived between 70 and 66 million years ago, during the Late Cretaceous in what is now North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta.

 

This dinosaur is known primarily from a single skull, along with some extremely thick skull roofs (measuring 22 cm). More complete fossils would be found in subsequent years.

Pachycephalosaurus was one of the last non-avian dinosaur species on Earth before the Cretaceous–Paleogene extinction event. The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex, in recent studies.

 

         

 

Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore, possessing long, strong legs and somewhat small arms with five-fingered hands. Pachycephalosaurus is the largest known pachycephalosaur, known for having an extremely thick, slightly domed cranial roof; visually, the skull structure suggests a "battering ram" function in life, evolved for use as a defensive mechanism or in interspecies combat, similar to what is seen today with bighorn sheep or musk oxen (with male animals routinely charging and butting heads, colliding with each other for dominance). In fact, this hypothesis has been highly controversial in recent years.

 

The skull was short and had large, rounded, forward-facing eye sockets, suggesting the animal had binocular vision. Pachycephalosaurus had a small snout ending in a pointed beak. The teeth were tiny, with leaf-like crowns. The head was supported by an S- or U-shaped neck. Younger Pachycephalosaurus individuals may have had flatter skulls and larger horns protruding from the back of the skull. As the animal grew, the horns shrank and became rounded as the dome grew.

 

Pachycephalosaurus was bipedal and possibly the largest of all pachycephalosaurids. It has been estimated that Pachycephalosaurus measured about 4.5 meters in length and weighed between 370 and 450 kg.

Based on other pachycephalosaurids, it probably had a fairly short and thick neck, short arms, a bulky body, long legs, and a heavy tail that was probably held rigid by ossified tendons.