The juvenile pachycephalosaurs, which predates the previous oldest dome-headed dinosaur by 15 million years, reveals more about how and when this unusual feature developed.
Paleontologists working in Mongolia’s Gobi Desert have unearthed the oldest and most complete dome-headed dinosaur ever found, pushing back the evolutionary timeline of these odd creatures by at least 15 million years.
The fossils, dating between 108 and 115 million years old, belong to a newly described species named Zavacephalae rinpoche. The find not only rewrites the history of pachycephalosaurs — dinosaurs famous for their thick, rounded skulls — but also provides the clearest look yet at how their unusual domes developed.
“Pachycephalosaurs are iconic but remain poorly understood,” said study co-author Lindsay Zanno, a paleontologist at North Carolina State University. “Most fossils are incomplete, usually just pieces of skulls, which makes it hard to track how they grew.”
The new fossil, however, is remarkably well-preserved: about 54% of the skeleton, including a full skull, tail, limbs, and even stomach stones used for grinding food. The animal was relatively small, about 1 meter (3.3 feet) long and weighing around 6 kilograms (13 pounds).
Its skull dome was already fully formed, but analysis of growth rings in the leg bones revealed the dinosaur was still a juvenile when it died. That means pachycephalosaurs may have reached sexual maturity before they were finished growing — a clue that their domes played a key role in courtship or competition.
“Z. rinpoche is a spectacular find,” Zanno said. “It’s the first time we’ve been able to directly link growth stage to dome development.”
The species name reflects its significance: Zavacephalae combines Tibetan and Latin roots meaning “origin head,” while rinpoche — Tibetan for “precious one” — honors the way its skull gleamed like a jewel as it emerged from a cliffside.
Whether pachycephalosaurs actually butted heads like rams or engaged in stranger rituals like kangaroo-style kickboxing remains debated. But this fossil provides unprecedented insight into their biology.
“This specimen is truly a once-in-a-lifetime discovery,” Zanno said. “It opens a new window into the anatomy, growth, and behavior of one of the strangest dinosaur groups to ever walk the Earth.”