What Is a Pachycephalosaur? Thick Skulls and an Unsettled Argument
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A dome of solid bone up to twenty-five centimetres thick sat on top of these animals' heads, and whether they rammed each other with it has been argued for fifty years. The evidence has accumulated on both sides and the answer has moved more than once.
What the dome is
The skull roof in these bipedal plant eaters is enormously thickened, in the largest species into a solid dome, and is frequently surrounded by bony knobs and spikes around the rear margin and the snout. The internal structure is not uniform bone but a complex arrangement of dense outer layers over a fibrous interior with the fibres oriented perpendicular to the surface, which is a construction that absorbs and distributes impact energy rather than transmitting it, and that arrangement is itself part of the evidence in the argument since it has no obvious purpose otherwise. The animals were bipedal, around one to five metres long depending on species, with small leaf-shaped teeth suited to soft vegetation and possibly to an omnivorous diet, and a broad body indicating a substantial gut. They are known largely from skulls, since the domes are dense and survive when the rest of the skeleton does not, which is why complete specimens are unusual and why much of the debate rests on isolated heads.
The case for and against head-butting
Both positions have specific evidence and the argument has been genuinely productive:
- •For: the internal fibre arrangement absorbs impact in a way that serves no other identified function
- •For: healed lesions on the domes of numerous specimens are consistent with repeated impact injury, documented in a substantial statistical sample
- •For: the neck and spine articulate to align the head, neck and body in a straight line, which is what transmitting an impact without injury requires
- •Against: the domes are rounded rather than flat, so a head-on collision would glance off rather than transmit force, which was the original objection
- •Against: the neck vertebrae in some specimens appear insufficient to withstand the forces, according to some biomechanical analyses
- •A middle position now widely held is flank-butting, with animals striking each other's sides rather than colliding head-on, which avoids the glancing problem and fits the injuries
The growth argument
A separate dispute concerns how many species existed. Domes change dramatically with age, becoming taller and thicker while the spikes around the margin shorten and in some cases are resorbed entirely, which is documented in growth series. On that basis a proposal was made that two flat-headed spiky forms are juveniles of the well-known domed species, which would eliminate two genera from the list and substantially reduce the apparent diversity of the group at the end of the Cretaceous. That proposal has been contested for over a decade, with arguments about whether the specimens come from the right layers, whether the histology supports the ages claimed, and whether the transformation required is plausible. The dispute matters beyond nomenclature, since the number of distinct species present in the final stages of the Cretaceous bears on whether dinosaur diversity was declining before the impact, which is one of the substantive questions about the extinction.
What the argument illustrates
The head-butting debate is a useful case study in how behaviour is inferred from anatomy. The original interpretation was proposed in the 1950s on the analogy of bighorn sheep, which is reasoning from a living animal with a superficially similar structure, and it was widely accepted and illustrated before being tested. The objection from dome curvature came from thinking about the mechanics rather than the appearance. The lesion evidence came from examining many specimens statistically rather than describing individual ones. The biomechanical modelling came later still. Each stage tested a claim that had been accepted on weaker grounds, and the current position is more qualified and better supported than either of the earlier ones. That sequence is what progress in the field looks like, and it is worth noting that the popular image lags behind it by several decades, since the head-on collision remains the standard depiction.
The takeaway
The dome is dense bone over a fibrous interior with fibres running perpendicular to the surface, which absorbs impact and has no other identified purpose. Healed lesions on many specimens support repeated impact, while the rounded shape argues against head-on collisions, and flank-butting is the position that fits both. Domes change so much with age that two named genera may be juveniles of a third.