Now, scientists have finally uncovered the rest of the dinosaur those armed belonged to — and found an astonishing animal.
"This dinosaur is even more bizarre than we could even possibly have imagined," said Canadian dinosaur expert Philip Currie, who was part of the international research team that made the new discovery.
The team, led by Yuong-Nam Lee at the Korea Institute of Geoscience and Mineral Resources, described their findings in this week's issue of Nature.
Deinocheirus mirificus was the name that scientists gave to the mysterious dinosaur nearly 50 years ago, after the discovery of its 2.4-metre long arms.
"The size was just mind-boggling," said Currie, a paleontology professor at the University of Alberta who has been fascinated by the discovery for decades.
"People started speculating, 'Well if Tyrannosarus rex has arms as short as ours and this thing has arms that are six feet long, this dinosaur must be like 300 feet (91 metres) long' — which was ridiculous, of course."
The international research team recently discovered two nearly complete Deinocheirus skeletons that revealed it wasn't a tyrannosaur. Instead, it belonged to a family of ostrich-like dinosaurs called Ornithomimids – most of which were ostrich-sized or human-sized and had longer arms relative to their bodies than tyrannosaurs did.
Deinocheirus was the largest known dinosaur in the group – it was nearly as long as a bus (11 metres) with an estimated mass of six tonnes, making it similar in size to T. rex.
It also looked very different from its ostrich-like cousins, which had pointed bills and slender legs that ended in sharp-clawed feet.
Deinocheirus had a duck-like bill and stubby legs that ended in hoof-like claws, which may have stopped it from sinking in muddy ground.
Currie said Deinocheirus lived about 70 million years ago (about five million years before dinosaurs went extinct) along a river running through the desert, similar to the Nile. The area was home to about 40 other species of dinosaurs ranging from chicken-sized creatures to massive, long-necked plant-eaters.
Like a suspension bridge
One of Deinocheirus's stranger features was a series of extra-long bones in its back that formed a hump or sail. Researchers think that may have helped it balance and support its huge mass on its hind legs.
"It looks very much like suspension bridges where you have those big pillars in the middle the bridge and those cables coming down on either side," Currie said.
The last few bones of its backbone were fused together like they are in birds such as turkeys.
"This thing probably had a fan of feathers at the end of the tail," Currie said.
"It's the first time we've ever seen that in one of these dinosaurs…. So you know, you can look at anywhere in the skeleton — and it's just weird."
Along with the bones, the researchers also found the remains of the dinosaurs' stomach contents. Those included rocks, suggesting that Deinocherus swallowed stones to grind up plants and seeds the way birds do. But among the rocks were also fish bones and scales, suggesting that Deinocheirus was an omnivore.
Currie said the discovery is a "good warning that even though we think we know a lot about dinosaurs, in fact we don't."
Currie travels to the fossil beds in the Gobi Desert of Mongolia at least once a year to dig for dinosaurs, and searched for years for signs of Deinocheirus.
A shop in France
In 2006 and 2009, he and his collaborators found the remains of two dinosaur skeletons that had been damaged by fossil poachers.
The poachers had removed key parts such as the skull, making them difficult to identify. In one case, Currie's team managed to re-assemble a smashed-up arm bone, revealing that it belonged to Deinocheirus.
Some time later, Currie got a call from Pascal Godefroit, a paleontologist at the Royal Belgian Institute of Natural Sciences, who thought he had found the skull and feet of Deinocheirus in a fossil dealer's shop in France.
It turned out to be parts of the specimen that Currie found in 2009, presumably taken by poachers and then passed along from dealer to dealer before arriving in the shop of François Escuillie.
Escuillie, upon learning that the fossil had been illegally excavated, donated it back to the Mongolian government so it could be officially studied. Paleontologists avoid illegally obtained skeletons to discourage poaching, which causes serious damage to fossils and fossil sites.
While Currie has studied lots of species of dinosaurs over his career and discovered many new ones, he says none of them compares to Deinocheirus.
"It's probably one of the coolest things I've ever done in my life."