New dome-headed dinosaur species is the most complete pachycephalosaur ever found

A well-preserved skeleton is revealing new details about some of the world’s most distinctive dinosaurs.

Pachycephalosaurs are famous for their thick, bony domed skulls that were likely used in combat between individuals. The highly distinctive fossils have helped them to capture the public imagination through appearances in films, television series and video games.

But while these dinosaurs might be popular with the public, very little is actually known about where they came from. This is because thick skulls often survived the fossilisation process, but the rest of the body didn’t.

This has now changed with the discovery of a new species from Mongolia, known as Zavacephale rinpoche. Dr Lindsay Zanno, a scientist who co-authored new research into the pachycephalosaur, says that it’s helping to reveal key details about these “rare and mysterious” dinosaurs.

“Before Zavacephale, our record of pachycephalosaurs was almost exclusively limited to their indestructible domes,” Lindsay explains. “With such scanty skeletons, we were left to wonder about basic aspects of their anatomy, like what their arms would have looked like and how their digestive system functioned.”

“The well-preserved skeleton of Zavacephale is helping us fill in the blanks with its reduced forelimbs, tiny hands and stomach stones for grinding food.”

Dr Tsogtbaatar Chinzorig, the study’s lead author, adds that Zavacephale predates all other known pachycephalosaur fossils by around 15 million years. This means it can also shed light on how these dinosaurs evolved.

“Zavacephale was a small animal, measuring less than one metre long, but is the most skeletally complete pachycephalosaur yet found. This makes it an important specimen for understanding how the cranial dome of pachycephalosaurs developed.”

The new study was published in the journal Nature.

What does Zavacephale reveal about pachycephalosaurs?

Pachycephalosaurs are part of a major group known as the bird-hipped dinosaurs, or ornithischians, that evolved during the Late Triassic more than 230 million years ago. Other bird-hipped dinosaurs include ankylosaurs, iguanodontians and ceratopsians.

Even in such a large and diverse group, pachycephalosaurs stand out. By the time they first appear in the fossil record, they already have their distinctive skulls and other features that set them apart from their relatives. This means they must have been evolving for millions of years beforehand.

But evidence of these earlier species has been hard to find, as pachycephalosaur remains are rare and generally fragmentary. Our dinosaur expert Professor Paul Barrett says it’s not entirely clear why.

“On the one hand, pachycephalosaurs might have been genuinely rare in their ecosystems so there weren’t many opportunities for them to fossilise,” Paul explains. “Alternatively, they might have lived mostly in inland environments where fossilisation occurred less frequently.”

“We also know that pachycephalosaurs were quite small, and it’s well known that the skeletons of small-bodied dinosaurs are more easily scattered and destroyed by predators, scavengers and erosion than large bones. Generally, the solid domes are the only fossils we find.”

This wasn’t the case for Zavacephale. Over half of its skeleton has been found, including features such as a hand and full tail, neither of which have never been found in pachycephalosaurs before.

The fossil also contained clusters of rocks where the dinosaur’s digestive system would have been. Known as gastroliths, these are stones that have been swallowed by an animal to help grind down food, such as tough vegetation.

How did pachycephalosaurs use their heads?

Zavacephale is also revealing more about how the iconic pachycephalosaur skull evolved. Similar to the armour of ankylosaurs or the frills of Triceratops, the skull domes of pachycephalosaurs are thought to have played a role in sexual selection.

It’s historically been thought that pachycephalosaurs would butt their heads together at high speed, similar to bighorn sheep. Analysis of their domes has shown that many fossils have evidence of wounds caused by a heavy impact, which suggests this is the case.

Other scientists have argued that pachycephalosaurs instead used their domes to shove each other, or perhaps to headbutt the flanks of competitors and predators. The domes could also have been used for display as well, similar to how deer use their antlers to fight and to show off to rivals.

Analysis of Zavacephale’s skeleton reveals that most of its bones hadn’t fused, meaning that it was still growing when it died. As its skull was already developed, it suggests that the dome formed more rapidly than the rest of the animal’s skeleton.

“This young Zavacephale specimen had a fully formed cranial dome, which means that these animals were at least practicing their combat skills before reaching their maximum size,” adds Lindsay. “If they weren’t already fighting over mates and territory, they were at least honing their skills for the future.”

While these domes are thought of as an exclusive feature of pachycephalosaurs, Zavacephale’s position as the oldest and earliest diverging member of these dinosaurs questions that. It raises the possibility that the domes predate the pachycephalosaurs themselves.

Finding the origins of these domes will mean uncovering more fossils of early pachycephalosaurs and their ancestors. More fossils will also help to answer other outstanding mysteries about these dinosaurs.

“At the moment, we know relatively little about the biology of pachycephalosaurs because only a few near complete skeletons are known,” Paul explains. “If we can find more skeletons, it will help to reveal more about how these dinosaurs moved and behaved.”

“The relationships of pachycephalosaurs to the bird-hipped dinosaurs are also contentious, so we need to find fossils of a really primitive pachycephalosaur to help link the group more closely into the dinosaur family tree. This will help us get a better idea of how they evolved into such unusual animals.”