Paleontologists have discovered the fossilized remains of a previously unknown pachycephalosaur species in Mongolia’s Gobi Desert. The fossil is both the most skeletally complete and geologically oldest pachycephalosaurian discovered globally.
Young Zavacephale rinpoche duel for territory along a lakeshore 108 million years ago. Image credit: Masaya Hattori.
“The dome-headed pachycephalosaurians are among the most enigmatic dinosaurs,” said Dr. Tsogtbaatar Chinzorig, a researcher with the Institute of Paleontology at Mongolian Academy of Sciences and North Carolina State University.
“Bearing a hypertrophied skull roof and elaborate cranial ornamentation, members of the group are considered to have evolved complex socio-sexual systems.”
“Despite their importance in understanding behavioral ecology in Dinosauria, the absence of uncontested early diverging species has hindered our ability to reconstruct the origin and early evolution of the group.”
Named Zavacephale rinpoche, the new pachycephalosaur species lived in what is now Mongolia around 108 million years ago (Early Cretaceous epoch).
At the time, the area was a valley dotted with lakes and surrounded by cliffs or escarpments.
“Zavacephale rinpoche predates all known pachycephalosaur fossils to date by about 15 million years,” Dr. Chinzorig said.
“It was a small animal — about less than 1 m (3 feet) long — and the most skeletally complete specimen yet found.”
The fossilized bones of Zavacephale rinpoche were found in the Khuren Dukh Formation in the Eastern Gobi Basin.
“Zavacephale rinpoche is an important specimen for understanding the cranial dome development of pachycephalosaurs, which has been debated for a long time due to the absence of early diverging or pre-Late Cretaceous species and the fragmentary nature of nearly all pachycephalosaurian fossils,” Dr. Chinzorig said.
“The consensus is that these dinosaurs used the dome for socio-sexual behaviors,” said Dr. Lindsay Zanno, a researcher at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences.
“The domes wouldn’t have helped against predators or for temperature regulation, so they were most likely for showing off and competing for mates.
“If you need to headbutt yourself into a relationship, it’s a good idea to start rehearsing early,” she says.
Zavacephale rinpoche fills in huge gaps in the pachycephalosaur timeline — both in terms of when they lived and how they grew.
“This specimen is an once-in-a-lifetime discovery,” Dr. Zanno said.
“It is remarkable for being the oldest definitive pachycephalosaur, pushing back the fossil record of this group by at least 15 million years, but also because of how complete and well-preserved it is.”
“Zavacephale rinpoche gives us an unprecedented glimpse into the anatomy and biology of pachycephalosaurs, including what their hands looked like and that they used stomach stones to grind food.”
“The newly-recovered materials of Zavacephale rinpoche, such as the hand elements, the stomach stones (gastroliths), and an articulated tail with covered tendons, reshape our understanding of the paleobiology, locomotion, and body plan of these ‘mysterious’ dinosaurs,” Dr. Chinzorig said.
The discovery is described in a paper published today in the journal Nature.
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T. Chinzorig et al. A domed pachycephalosaur from the early Cretaceous of Mongolia. Nature, published online September 17, 2025; doi: 10.1038/s41586-025-09213-6
