It was a cloudy morning in southeast Mongolia. Paleontologist Chinzorig Tsogtbaatar and several colleagues set out by foot from their campsite to a rocky outcrop dating back some 110 million years to the early Cretaceous.
"Then, after 15 to 20 minutes, I saw something [on the] other side of the hill," says Tsogtbaatar. It was a bright object of some sort. "It [didn't] look like a rock," he recalls. "It [was] very unusual."
Once he got closer, Tsogtbaatar — who now works at North Carolina State University — knew exactly what it was. A dome-shaped skull. It turned out that Tsogtbaatar had just discovered a new species of pachycephalosaur, a unique group of dinosaurs defined by their thick, bony, hemispherical skulls but about which little else is known.
In that moment, "we just stopped breathing," he says.
"This is the first definitive pachycephalosaur that's ever been found in the early Cretaceous," says Lindsay Zanno, also a paleontologist at North Carolina State University in addition to serving as the head of paleontology at the North Carolina Museum of Natural Sciences. "It just pops out of the fossil record with a fully developed dome, and bells and whistles on its head."
Tsogtbaatar, Zanno, and their colleagues have introduced the world to this new species in a paper published in the journal Nature. They gave it the scientific name Zavacephale rinpoche. The word rinpoche is Tibetan for "precious one" and refers to the domed skull, which appeared out of the rockface to Tsogtbaatar that June morning like a perfectly polished jewel.
Bones of enchantment
The new specimen is 15 million years older than what had previously been the oldest pachycephalosaur ever found. "So this dinosaur fills in a critical gap in the early evolution of this famous group of dome-headed dinosaurs," says David Evans, a dinosaur paleontologist at the Royal Ontario Museum in Toronto who wasn't involved in the research.
After its excavation in 2019, the fossil stayed in Mongolia during the pandemic. Finally, in 2022, it was shipped to North Carolina for additional analysis. Zanno says the moment she first laid eyes on it, she was "enchanted."
"Everyone just stood around the lab just holding this precious, incredible skull," she says. "It's just the most beautiful specimen. It's magical."
That domed head — made of solid bone and edged in spikes — is any pachycephalosaur's most distinguishing feature.
"Their domes are essentially indestructible," says Zanno. "It would have been able to protect the skull against pressure or impact. And so we think that these animals were battling it out with their heads."
Still, researchers have known relatively little about these plant-eaters because most of the rest of their bodies haven't been found. So when the team in Mongolia went on to excavate dozens more bones from Zavacephale rinpoche, it became the most complete pachycephalosaur skeleton ever to be unearthed.
"It's about the size of a German shepherd," says Zanno, "but you have to remember 75% of the body is a neck and a tail, so they're very light. It would have only weighed about 12 pounds."
A juvenile jewel
This little fossil has already revealed a few new insights. The stomach contained small stones that would have helped the animal digest its food. The tail is shot through with bony tendons, which would have made it more rigid.
Plus, this was the first time a pachycephalosaur's hand bones had ever been found. "At first, we thought it was something it ingested instead of parts of its own body. That's how tiny its hands were," says Zanno.
In addition, the limb bones allowed the researchers to determine the age of the dinosaur. It was at least two years old, making it a juvenile. This shows that young pachycephalosaurs like this one already had fully developed domes.
"Whether they were battling it out for territory or mates is something we're not entirely sure of," says Zanno, "but what's clear is that whatever they were doing with those domes, they started practicing at a very young age."
The specimen is now back home at the Institute of Paleontology of the Mongolian Academy of Sciences as part of the country's mission to protect its natural and cultural heritage.
Cary Woodruff, a curator of vertebrate paleontology at the Frost Museum of Science in Miami who wasn't involved in the study, said he was floored by the discovery — and that his surprise was mixed with a dash of "very friendly jealousy."
"How he even found the specimen is just bonkers," Woodruff said of Tsogtbaatar. "He doesn't just find a new one. He doesn't just find the geologically oldest one. He finds [what] everyone who works on pachycephalosaurs has always wanted to find."
Evans was similarly impressed. "I was just stunned by the beauty and completeness of this particular fossil," he says. "This is a specimen that we'll be learning from for many, many years to come."
Woodruff is also confident that this fossil will spawn new ideas about how pachycephalosaurs once lived. He says usually paleontologists have to work with very incomplete skeletons. "In our mind, we can see, we can imagine what the rest of it looked like," Woodruff says. "But you didn't have to imagine anything with a specimen like this. It was [all] there. The teeth are literally smiling at you."
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