According to a new study published Wednesday in the journal Nature, researchers have discovered the oldest and most complete fossil of a pachycephalosaur—a group of dinosaurs distinguished by their dome-shaped skulls—in Mongolia.
The fossil is from a juvenile of a species that has not yet been identified and is being named Zavacephale rinpoche. It is thought to have existed between 108 and 115 million years ago.
A team led by paleontologist Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences found it in the Gobi Desert's Khuren Dukh formation in Mongolia, a landlocked country between China and Russia.
As per the study, pachycephalosaurs, two-legged herbivores that inhabited Asia and North America during the Cretaceous Period, are among the "most enigmatic" dinosaurs. However, very little is known about them. Their unique, bony skulls and appearances in media like the Jurassic Park movies have made them a subject of increasing public interest.
Skulls were the majority of the pachycephalosaur fossils that had previously been found.
Michael Pittman, a paleobiologist from the Chinese University of Hong Kong, told NBC News, "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."
The study noted that the Mongolian discovery is the "most skeletally complete," giving scientists far more information about the anatomy of pachycephalosaurs than they previously had.
According to researchers, the Zavacephale individual they discovered was roughly three feet long and at least two years old. More than half of its skeleton has been discovered. Its long legs, short arms, and small hands are accompanied by gastroliths, which are stones the animal would have swallowed to aid in breaking down the vegetation it consumed.
The skeleton already had the thickened bone that is characteristic of pachycephalosaurs on top of the skull, even though the animal was not yet fully grown.
Chinzorig told the National History Museum in London that the fossil's completeness "makes it an important specimen for understanding how the cranial dome of pachycephalosaur developed."
Although scientists have hypothesized that pachycephalosaurs may have used the dome to impress possible mates or to butt heads in battle, the exact function of the dome is still unknown.
Additionally, the fossil is roughly 15 million years older than other pachycephalosaur specimens that have been found, which helps to clarify the evolution of the dinosaurs.
"It's the oldest pachycephalosaur, so it's giving us information of what earlier species were like and how they changed through time," said Pittman, adding, "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.
