The fossilized skull of the Cretaceous Period dome-headed dinosaur.Alessandro Chiarenza / via Reuters

Scientists have unearthed in Mongolia the oldest and most complete fossil of a pachycephalosaur, a group of dinosaurs known for their dome-shaped skulls, according to a new study published Wednesday in the journal Nature.

The fossil, which dates back between 108 and 115 million years, belongs to a juvenile from a previously unknown species that researchers are calling Zavacephale rinpoche.

It was discovered in the Khuren Dukh formation in the Gobi Desert in Mongolia, a landlocked nation between China and Russia, by a team led by paleontologist Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences.

Very little is known about pachycephalosaurs, two-legged herbivores that lived in Asia and North America during the Cretaceous Period and are among the "most enigmatic" dinosaurs, the study said. They have become an object of growing popular fascination thanks to their distinctive, bony skulls as well as appearances in media such as the Jurassic Park films.

Previously discovered pachycephalosaur fossils were mostly limited to skulls.

"You could fit all of the pachycephalosaurs [fossils] in the world within a bathtub, more or less, or maybe two bathtubs -- there are very few fossil pachycephalosaurs," Michael Pittman, a paleobiologist from the Chinese University of Hong Kong, told NBC News.

The Mongolia discovery is the "most skeletally complete," the study said, providing researchers with far more detail about the anatomy of pachycephalosaurs than they had before.

Researchers said the Zavacephale individual they found was at least 2 years old and about 3 feet long. Its skeleton, more than half of which has been found, features long legs, short arms and small hands, as well as stones known as gastroliths that the animal would have swallowed to help grind down the vegetation that it ate.

Though the animal was not fully grown, its skeleton also already had the pachycephalosaur's signature thickened bone on top of the skull.

The completeness of the fossil "makes it an important specimen for understanding how the cranial dome of pachycephalosaur developed," Chinzorig told the National History Museum in London.

The purpose of the dome remains unclear, though scientists have suggested pachycephalosaurs may have used them to butt heads in combat or to impress potential mates.

The fossil is also about 15 million years older than previously discovered pachycephalosaur specimens, helping to shed light on how the dinosaurs evolved.

"It's the oldest pachycephalosaur, so it's giving us information of what earlier species were like and how they changed through time," Pittman said. "So finding an earlier one that still has it shows that actually they've had that dome for a long time."

Zavacephale was smaller than later pachycephalosaurs, which could grow to about 14 feet long.

Oldest and most complete fossil of a dome-headed dinosaur is found in Mongolia