Pachycephalosaurus was a small to medium sized dinosaur famous for its thick, rounded skull dome. It lived near the end of the Cretaceous Period, about 70 to 66 million years ago, in what is now western North America. Its name means thick headed lizard, a reference to the dense bone on top of its skull.
Studying this dinosaur helps paleontologists understand behavior, growth, and survival in ecosystems just before the mass extinction that ended the age of nonavian dinosaurs.
The skull dome of Pachycephalosaurus could be more than 20 centimeters thick, but scientists still debate exactly how it was used. Some evidence suggests individuals may have pushed, shoved, or struck each other during contests, while other researchers argue the dome was better suited for display than direct head butting. Fossils also show spikes and knobs around the back of the skull, which may have helped with species recognition or defense.
Because complete skeletons are rare, paleontologists compare skulls, related species, and bone tissue to reconstruct its body, movement, and behavior.
Key Facts
- Time period: Late Cretaceous, about 70 to 66 million years ago.
- Location: Fossils are mainly known from North America, especially the Hell Creek and Lance formations.
- Skull feature: The dome could reach over 20 cm thick in some adults.
- Estimated length: Pachycephalosaurus was about 4 to 5 m long.
- Estimated mass: Many estimates place its mass around 300 to 450 kg.
- Speed relationship: average speed = distance ÷ time, so a 40 m dash in 5 s gives v = 40 m ÷ 5 s = 8 m/s.
Vocabulary
- Pachycephalosaur
- A member of a group of bipedal herbivorous or omnivorous dinosaurs known for thickened skull roofs.
- Skull dome
- The thick, rounded mass of bone on top of the skull in dinosaurs such as Pachycephalosaurus.
- Cretaceous Period
- The geologic period from about 145 to 66 million years ago, ending with the extinction of nonavian dinosaurs.
- Fossil formation
- A layer or group of rock layers that preserves fossils from a particular time and environment.
- Ontogeny
- The growth and development of an organism from youth to adulthood.
Common Mistakes to Avoid
- Assuming Pachycephalosaurus definitely rammed head first like a bighorn sheep is wrong because the skull, neck, and impact evidence are still debated.
- Drawing it as a giant dinosaur is wrong because Pachycephalosaurus was much smaller than animals like Tyrannosaurus, usually estimated at about 4 to 5 m long.
- Treating every skull shape as a separate species is risky because some differences may come from age, growth stage, or individual variation.
- Calling it a Jurassic dinosaur is wrong because Pachycephalosaurus lived in the Late Cretaceous, tens of millions of years after the Jurassic ended.
Practice Questions
- 1 A Pachycephalosaurus is estimated to be 4.5 m long. If a scale drawing shows it as 15 cm long, what scale factor in meters per centimeter is being used?
- 2 If the skull dome is 22 cm thick and the total skull length is 60 cm, what percentage of the skull length is the dome thickness?
- 3 Explain why paleontologists might disagree about whether Pachycephalosaurus used its dome for combat, display, or both. Use at least two kinds of evidence that could support an interpretation.