Dinosaurs A to Z
Pachycephalosauridae

Pachycephalosaurus wyomingensis PAK-ee-SEF-uh-loh-SOR-us

"Thick-headed lizard"

Pachycephalosaurus silhouette
You 1.8 m (5.9 ft) tall
Pachycephalosaurus 4.5 m (14.8 ft) long
as long as 3 people standing fingertip to fingertip
silhouette · Gareth Monger (CC-BY) via PhyloPic
Length
4.5 m (14.8 ft) — About 4.5 m long, ~450 kg — with a dome of skull bone 25 cm thick.
Period
Late Cretaceous (70–66 Mya)
Place
North America · Montana · Wyoming · South Dakota
Food
Omnivore — Plants mostly, but the teeth suggest some animal matter too — fruit, insects, small lizards.
Clade
Pachycephalosauridae Dinosauria Ornithischia Cerapoda Marginocephalia Dome-headed bipedal herbivores.

A thick-domed plant-eater that may have head-butted like a bighorn sheep, debated to this day.

Pachycephalosaurus reconstruction by Fred Wierum
Reconstruction · Fred Wierum (CC-BY)

Pachycephalosaurus had a head built like a battering ram. The top of its skull was solid bone 25 cm thick, surrounded by a circle of small bony bumps. For decades scientists thought this meant head-butting fights like modern bighorn sheep. Newer studies actually found healed injuries on dome surfaces — proof that the heads were used in combat after all.

More about Pachycephalosaurus

Discovery

· Western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta.

For a long time it was known mainly from a single skull and a few extremely thick skull roofs. Its history is tangled, involving confusion with other genera and a lack of complete remains, with more complete fossils found in later years.

The name means "thick-headed lizard", from the Greek pachys ("thickness"), kephalon ("head"), and sauros ("lizard").

Did young Pachycephalosaurus look different from adults?
Younger individuals may have had flatter skulls and larger horns projecting from the back of the head. As the animal grew, those horns appear to have shrunk and rounded off while the dome grew. This growth pattern is part of why some specimens once named as separate genera, like Dracorex, are now read as growth stages.
What did it actually eat?
Scientists do not yet know. Its tiny, ridged, leaf-shaped teeth could not have chewed tough fibrous plants as well as other dinosaurs, so a mixed diet of leaves, seeds, and fruits is assumed. Blade-like serrated front teeth on the most complete jaw have even led to suspicion that it may have eaten some meat.

What we know

  • Skull dome up to 25 cm of solid bone — thickest of any known animal.
  • Healed injuries on the domes confirm they were used in combat.
  • Lived alongside T. rex, Triceratops, and Edmontosaurus in the last million years of the Cretaceous.
  • Mostly known from skulls — body skeletons are rare.

What we guess

  • Whether the fights were head-to-head ramming or flank-butting like giraffes.
  • Whether males and females both fought, or only males.
  • Whether dome shape changed with age — Stygimoloch and Dracorex may be juvenile Pachycephalosaurus.