On a cloudy June morning in 2019, deep in the rocky outcrops of southeast Mongolia, paleontologist Chinzorig Tsogtbaatar stumbled upon something extraordinary. As he and his colleagues set out from their campsite, the glint of a bright, unusual object caught his eye on the other side of a hill. "It didn’t look like a rock," he recalled to NPR. Getting closer, Tsogtbaatar realized he was staring at a dome-shaped skull—a discovery that would soon rewrite the story of a remarkable group of dinosaurs.
That fossil turned out to be the first definitive pachycephalosaur ever found from the early Cretaceous period, a group of bipedal, dome-headed dinosaurs whose thick, bony skulls have long puzzled scientists. The new species, named Zavacephale rinpoche—with "rinpoche" meaning "precious one" in Tibetan—was introduced to the world in a 2025 paper published in Nature. According to NPR, this fossil is not just another addition to the dinosaur family tree; it’s the most complete pachycephalosaur skeleton ever unearthed and is about 15 million years older than any previously known member of its lineage.
Meanwhile, halfway across the world in Montana, another scientific team was making its own headlines. According to ABC News, researchers uncovered five fossil specimens in the Two Medicine Formation in Glacier County, Montana. These remains belonged to a new species, Brontotholus harmoni, which roamed the earth about 75 million years ago during the Late Cretaceous period. Published in the Zoological Journal of the Linnean Society earlier this month, this discovery marks the first time a pachycephalosaurid from the Late Cretaceous has been found in that region.
Both discoveries have sent ripples through the paleontological community, shining a spotlight on the enigmatic pachycephalosaurids. These dinosaurs, known for their thickened domes and ornamented skulls, were exclusively found in the Late Cretaceous of Asia and western North America. Their fossils are rare, and most of what scientists know comes from cranial remains—until now.
For Tsogtbaatar and his team, the Mongolia find was nothing short of magical. As Lindsay Zanno, a paleontologist at North Carolina State University and the North Carolina Museum of Natural Sciences, told NPR, "It just pops out of the fossil record with a fully developed dome, and bells and whistles on its head." The team was able to excavate dozens more bones from Zavacephale rinpoche, making it the most complete skeleton of its kind. Zanno described the moment she first saw the fossil: "Everyone just stood around the lab just holding this precious, incredible skull. It’s just the most beautiful specimen. It’s magical."
At roughly the size of a German shepherd and weighing only about 12 pounds, Zavacephale rinpoche was a lightweight among dinosaurs. But don’t let its size fool you—its skull, made of solid bone and edged with spikes, was built for battle. "Their domes are essentially indestructible," Zanno explained. "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." The fossil’s stomach even contained small stones, likely used to help digest tough plant matter, and its tail was stiffened with bony tendons for extra support.
This remarkable completeness offered scientists new insights: for the first time, hand bones of a pachycephalosaur were found, and they were so tiny that researchers initially mistook them for something the dinosaur had swallowed. The limb bones also revealed that the animal was at least two years old—a juvenile, yet already sporting a fully developed dome. As Zanno put it, "Whatever they were doing with those domes, they started practicing at a very young age."
The fossil’s journey was equally dramatic. After its excavation in 2019, it remained in Mongolia during the pandemic before being shipped to North Carolina in 2022 for further study. Today, it’s back home at the Institute of Paleontology of the Mongolian Academy of Sciences, a testament to Mongolia’s commitment to protecting its natural and cultural heritage.
Experts outside the study were awestruck. Cary Woodruff, a curator at the Frost Museum of Science in Miami, told NPR, "How he even found the specimen is just bonkers. 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." David Evans, of the Royal Ontario Museum in Toronto, echoed the sentiment: "I was just stunned by the beauty and completeness of this particular fossil. This is a specimen that we’ll be learning from for many, many years to come."
Back in Montana, the discovery of Brontotholus harmoni adds another piece to the puzzle of pachycephalosaur evolution. According to the Zoological Journal of the Linnean Society, this herbivore was about 10 feet long and, based on the size of its dome, is the third-largest pachycephalosaurid ever found in North America. The region where it was discovered, the Two Medicine Formation, was shaped by cycles of sea level changes during the Late Cretaceous, which influenced the evolution of many dinosaur clades—including tyrannosaurids, ceratopsids, hadrosaurids, and, of course, pachycephalosaurids.
What sets B. harmoni apart are its distinct morphological features: a bipedal gait, thickened domes, and ornamented skulls. The discovery not only increases our understanding of pachycephalosaurid diversity but also demonstrates that relatively large-bodied members of this group existed as early as the Middle Campanian stage of the Late Cretaceous.
These two discoveries, separated by continents and millions of years, are reshaping what we know about dome-headed dinosaurs. Zavacephale rinpoche fills a crucial gap in the early evolution of the group, while Brontotholus harmoni expands the known diversity and size range of pachycephalosaurids in North America. As Evans told NPR, "This dinosaur fills in a critical gap in the early evolution of this famous group of dome-headed dinosaurs."
With each new fossil, paleontologists are piecing together a more complete picture of these mysterious, head-butting herbivores. And with finds as stunning as these, who knows what other secrets lie buried, waiting to dazzle the next lucky scientist?
