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Episode 230 – Pachycephalosaurs

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Among the famous groups of dinosaurs are the iconic but somehow poorly represented dome-headed species. This episode, we explore the many questions surrounding Pachycephalosaurs.

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The Thick-Headed Dinosaurs

Pachycephalosaurs are a group of bipedal herbivorous dinosaurs from the Cretaceous Period. They range from tiny (only a meter long) to fairly large (4-5 meters long), with relatively simple teeth for a diet of mostly plants, though some evidence suggests these dinosaurs might have been omnivorous. Their most iconic feature is the thickened bone on the top of their skull, often dome-shaped and surrounded by knobs and spikes.

Pachycephalosaurus skeletal replica. Image by IJReid, CC BY 4.0

The fossil record of pachycephalosaurs is very patchy, consisting mostly of skull fragments. It was a long time before paleontologists were able to determine that these thick-headed dinosaurs formed a single lineage closely related to the horned ceratopsians. Still today, only about 15 genera are known and there’s a lot we still don’t understand about the diversity and early evolution of this group.

Left: Skull of Pachycephalosaurus. Image by Jonathan Chen, CC BY-SA 4.0
Right: Skull of Prenocephale. Image by Ghedoghedo, CC BY-SA 3.0

Until recently, all definitive pachycephalosaur fossils were known from the Late Cretaceous. The discovery of Zavacephale provided our first glimpse at Early Cretaceous members of the group. In some ways, Zavacephale represents an early stage in pachycephalosaur evolution – for example, its skull dome is formed slightly differently from other species. But in many ways, it is a full-fledged dome-headed dinosaur, leaving open many questions about the early evolution of this lineage.

Replica of Zavacephale, the oldest known pachycephalosaur as of 2025.
Image by Eight heads serpent, CC BY 4.0

For many years, dome-headed forms and flat-headed forms were considered to be separate species, but more recent studies have found that this is actually how these dinosaurs changed as they aged. The famous species Dracorex, with its flat head and large skull spikes, has been revealed to be a juvenile Pachycephalosaurus. The species’ skull changed shape dramatically as it grew into an adult, going from flat and spiky to domed and knobbly.

Pachycephalosaurus adult (top), young adult (center, formerly Stygimoloch), and juvenile (right, formerly Dracorex). Image by Tim Evanson, CC BY-SA 2.0

The function of this skull dome has been the subject of much debate and discussion over the decades. The most famous interpretation is that these dinosaurs smashed their heads together like modern goats and sheep. This idea is supported by the physically reinforced structure of the skull, areas for large muscles in the neck, and the presence of injuries in the dome caused by physical trauma, all of which are features these dinosaurs share with modern head-butting mammals.

Close-up of the dome of Sphaerotholus, with arrows indicating injuries, likely caused by head-butting impacts. Image from Peterson et al, 2013.

The details of these dinosaurs’ head-butting behavior are still unclear. The fact that the domes developed dramatically as the dinosaurs reached adulthood suggests they were using their thick heads in a way that related to mating, possibly in head-to-head competition with rivals, as seen in modern goats and sheep. But many questions remain: Was it a seasonal behavior? Was it something only young adults did? Did both sexes butt heads? Did they run up and crash into each other, or was it more of a shoving contest? More research may give us the answers someday.

Left: Musk oxen head-butting. Image by Gregory “Slobirdr” Smith, CC BY-SA 2.0.
Right: Goats head-butting. Image by Mohanatnow, CC BY-SA 3.0

Learn More

The newly discovered oldest pachycephalosaur, Zavacephale
A Pachycephalosaurus skull with unexpectedly sharp teeth

Lesions in a pachycephalosaur dome (technical, open access)
Head-butting features in pachycephalosaurs and horned mammals (technical, open access)

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