Pachycephalosaurus wyomingensis PAK-ee-SEF-uh-loh-SOR-us
"Thick-headed lizard"
You 1.8 m (5.9 ft) tall
Pachycephalosaurus 4.5 m (14.8 ft) long
3 people holding hands
- 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.
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.
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.