About 17 million years ago in the grassy plains of north China, an odd-looking creature met one of its own.
But it was no friendly encounter.
The giraffe-like animal didn't have massive teeth or claws to rip its opponents to shreds, but its fossils show that it was perfectly built for another combat style: head-butting.
So, it went head to head with its opponent.
Known as Discokeryx xiezhi, the hoofed herbivore sported a rock-solid skull, helmet-like headgear, and chunky neck joints that could absorb the violent impact of fighting better than today's head-butting heavyweights of the animal world.
The findings were published today in Science.
Jin Meng, a vertebrate palaeontologist at the American Museum of Natural History in New York, said the new species was a long-lost member of Giraffoidea, a superfamily that includes modern-day giraffes, okapis and pronghorns.
Its distinct head and neck features are an example of just how varied ancient members of this group were, said Dr Meng, who co-authored the study.
"It shows how diverse life could be when living under different conditions, and that they will evolve in a totally different way, even though they are related."
A 'strange beast'
Dr Meng and his team stumbled across the fossils during excavations at a site in the Junggar Basin in Xinjiang, north-west China almost three decades ago.
Embedded in the 16.9-million-year-old sediment layer was a set of cervical vertebrae, a jawbone with teeth, and a thick skull fragment crowned with a flattened piece of bone.
"It was just like a big pancake on the skull," Dr Meng said.
The researchers spent years comparing the oddball remains to museum specimens around the world to identify them, but they couldn't find a match.
"We didn't know what it was," Dr Meng said.
In 2015, the mysterious fossil bones caught the eye of Shi-Qui Wang, a palaeontologist at the Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
"It was a puzzle for me," said Dr Wang, who led the study.
Dr Wang used a CT scanner to take a deeper look at the structure of the skull fragments and compared them to 3D scans of modern-day ruminants, including the giraffe (Giraffa camelopardalis), goitered gazelle (Gazella subgutturosa) and Reeves's muntjac (Muntiacus reevesi).
The bony structure of the animal's inner ear was most similar to a giraffe's, indicating that it was likely from the same broad group.
Its distinctive headgear also showed similarities to horn-like, bony structures seen in giraffes and their extinct relatives called ossicones.
The team named the ancient giraffoid Discokeryx xiezhi after "Xiezhi", the one-horned mythical creature from Chinese legends.
The anatomy of a headbutter
The haul of fossils also offers clues about D. xiezhi's battle style.
The pancake-like headgear had a rough texture, which suggested that it was once covered in tissue that formed a dome-shaped helmet.
Its thick skull also hinted that it could take a few knocks to the head.
But it was the massive joints connecting the creature's skull with its neck and upper spine that held the key to its head-butting capabilities.
The sheer size of these joints suggests they were able to absorb the impact generated by high-speed head-slamming, says Rex Mitchell, who studies skull morphology and biomechanics at Flinders University, and who was not involved in the study.
"What you tend to find with large parts of the bone anatomy is that they're large to accommodate higher forces," Dr Mitchell said.
Put through virtual crash-test paces
Taking things a step further, Dr Wang and his team decided to give D. xiezhi's head-butting chops a crash test.
The researchers digitally reconstructed the head and neck joints and used a technique called finite element analysis to test how much force they could withstand.
This type of simulation is handy for exploring how well bones and other objects disperse impact forces under different conditions, including intense head-butting duals, Dr Mitchell said.
"It's an excellent choice of methodology here, because it allows researchers to digitally crash-test 3D models."
When the team simulated head-butting, there was barely any strain on D. xiezhi's head and neck joints.
For comparison, the team created another head-neck joint model that was thinner than the original.
When they applied the same force to this flimsier model, the neck bent wildly out of shape, rendering it useless for head-to-head fighting.
Not just another sheep
While these crash tests confirmed that D. xiezhi's joints were nicely adapted to head-butting, the team wanted to see how they stacked up against today's head-to-head fighters.
The team ran the simulation on models on three classic head-butters: the muskox (Ovibos moschatus), argali sheep (Ovis ammon) and blue sheep (Pseudois nayaur).
The strain these animals' joints underwent was up to five times higher than D. xiezhi's, indicating that the prehistoric beast could handle heavier blows to the head than today's top head-butters, Dr Mitchell said.
The team also analysed the chemical signatures in D. xiezhi's tooth enamel to gain a glimpse of its eating habits.
By looking at these telltale imprints — which are picked up in food and stored in teeth for millions of years — the team found that the ancient head-butter was an open-land grazer.
D. xiezhi's chemical signatures were distinct from other herbivores in the region at the time, suggesting it accessed food that competitors couldn't access.
To put things in the bigger evolutionary picture, the researchers compared the horn shapes of several animal groups, including giraffes, cattle, sheep, deer and pronghorns.
Even though giraffe species today all have similar headgear, the team found that their ancestors sported the widest variety of horn shapes of all the groups millions of years ago.
Dr Meng suspected that this explosion in headgear diversity among giraffe ancestors and their relatives was shaped by their environment and fighting styles, with D. xiezhi's single "horn" adding another extreme adaptation to the list.
"Although phylogenetically related to the giraffe, it shows evolution in a totally different direction."
