Oldest, most complete “dome-head” dino ever found rewrites the pachycephalosaur playbook

Step out of your tent in the Gobi Desert, spot a rounded knob sticking out of a hillside, and you might just change dinosaur history. That’s essentially what happened to paleontologist Tsogtbaatar Chinzorig, whose team has uncovered in Mongolia the oldest and most complete pachycephalosaur on record—a juvenile from a previously unknown species now named Zavacephale rinpoche. The find, described in Nature, dates to roughly 108–115 million years ago, pushing the group’s fossil record back by about 15 million years and, crucially, offering far more than the fragmentary skull bits scientists usually get.

Pachycephalosaurs—two-legged plant-eaters famous for their thick, domed skulls—are among the most mysterious dinosaurs because almost everything we’ve ever found of them is headgear. Researchers joke you could fit the world’s known fossils into a bathtub. Zavacephale breaks that drought. More than half the skeleton is there: long legs, short arms with comically small hands, hips, shoulders, a tail stiffened by rodlike tendons, and even a cluster of gastroliths—stomach stones swallowed to grind tough vegetation. Although the animal was still young (about two years old and roughly three feet long, the size of a large housecat), the skull already sported a proper dome, not the flat or mildly thickened roof some had predicted for early relatives.

That single detail lands like a sledgehammer on a lingering debate. The completeness and age of Zavacephale show the signature dome evolved early in pachycephalosaur history and stuck around. It also grew early in an individual’s life, which hints the dome mattered for social signaling or combat long before full adult bulk arrived. Whether these dinosaurs actually butted heads like bighorn sheep is still contested, but the teenage bling was clearly important.

Because this skeleton preserves parts almost never seen in the group, it fills in basic anatomy. Those tiny hands, painstakingly reconstructed bone by bone, clear up a long-standing question about the forelimb. The gastroliths are the first definitively tied to a pachycephalosaur, confirming they used a gizzard-like system to process plants. And the long, runner’s hind legs suggest early members may have moved—and maybe behaved—differently from their later, heavier cousins.

Equally important is the timeline. Finding a true dome-head this early forces a rethink of the family tree and of specimens once thought to represent “primitive,” flat-topped stages. It strengthens the idea that some spiky, small-domed animals named as separate species were actually juveniles of bigger, later forms. It also nudges the origin story firmly into Asia, with later dispersal into North America.

Back in the lab, the “shockingly beautiful” skull and its partner bones are already reshaping what researchers thought they knew about these enigmatic dinosaurs. The fossil has been returned to Mongolia, but high-resolution scans and samples will keep scientists busy for years. For a group long understood through scattered bowling-ball domes and little else, Zavacephale finally gives pachycephalosaurs a body to go with the head—and a much earlier start than anyone expected.

NBC News, the New York Times, and National Geographic contributed to this report.