Oldest-known dome-headed dinosaur discovered sticking out of a cliff in Mongolia's Gobi Desert

Paleontologists have uncovered the oldest and most complete dome-headed dinosaur fossils to date in Mongolia's Gobi Desert.

The fossils, which are between 108 million and 115 million years old, push back the timeline for the emergence of dome-head dinosaurs, or pachycephalosaurs, by about 15 million years. The new fossils could also reveal details about the evolution and development of these dinosaurs' bizarre rounded skulls.

"Pachycephalosaurs are iconic dinosaurs, but they're also rare and mysterious," study co-author Lindsay Zanno, a paleontologist at North Carolina State University, said in a statement. These dinos thrived during the Late Cretaceous period (between 86 million and 66 million years ago).

Some scientists think the thick skulls of pachycephalosaurs helped them attract mates and fend off competition, while others suggest that these weirdos didn't butt heads, but instead kick-boxed like kangaroos. But how their rounded skulls developed is a mystery. Most known pachycephalosaur fossils are incomplete, and scientists haven't found many fossils from early in their development.

Now, in a new study, published Wednesday (Sept. 17) in the journal Nature, a group of paleontologists report a fossil that might hold some answers. In eastern Mongolia, the team found the fossilized remains of a dome-headed dinosaur from the Early Cretaceous period (145 million to 100 million years ago). They named the newly discovered species Zavacephalae rinpoche.

Zavacephalae comes from the Tibetan word "zava," meaning root or origin, and the Latin word "cephal," meaning head. "Rinpoche" is a Tibetan word meaning "precious one," so named because the team found the skull sticking out of a cliff like a polished jewel.

The fossil included about 54% of the dinosaur's bones, including the skull and entire tail, as well as several hand and leg bones and stomach stones that helped Z. rinpoche grind its food. In life, the dinosaur likely measured about 3.3 feet (1 meter) long and weighed about 12.9 pounds (5.85 kilograms).

"We age dinosaurs by looking at growth rings in bones, but most pachycephalosaur skeletons are just isolated, fragmentary skulls," Zanno said. "Z. rinpoche is a spectacular find because it has limbs and a complete skull, allowing us to couple growth stage and dome development for the first time."

Z. rinpoche's dome was fully developed, but growth rings in its lower leg bone suggest that the animal was a juvenile and was still growing when it died. Because pachycephalosaurs probably used their domes to attract mates, this suggests that the dinosaur reached sexual maturity before it was fully grown.

"If you need to headbutt yourself into a relationship, it's a good idea to start rehearsing early," Zanno said.

"This specimen is a once-in-a-lifetime discovery," Zanno added. "Z. rinpoche gives us an unprecedented glimpse into the anatomy and biology of pachycephalosaurs."