View Images The original “terrible hands”. Credit: Eduard Solà

For 50 years, the dinosaur was just a pair of arms.

But what arms! Each was eight feet (2.4 metres) long, and ended in three eight-inch (20-centimetre) claws. You can understand why the scientists who discovered this beast called it Deinocheirus mirificus, from the Greek for “terrible hand, which is unusual”.

The arms, hands, and shoulder girdle were discovered in 1965 in Mongolia’s Gobi desert, nestled within a 70-million-year-old sandstone formation. But the rest of the skeleton was missing, save for a few uninformative fragments. Palaeontologists have repeatedly ventured into the Gobi to try and find the rest of the animal, but without success.

These failures have turned Deinocheirus into one of palaeontology’s most enduring mysteries. What kind of dinosaur was it? It was big, but how big? What did it eat? How did it live? No one could say. It was a riddle, wrapped in an enigma, hidden behind two gigantic arms.

Now, a team of palaeontologists led by Yuong-Nam Lee from the Korea Institute of Geoscience & Mineral Resources has finally discovered two well-preserved specimens of Deinocheirus, which reveal the complete body behind those terrible hands.

“These new specimens really solve the mystery once and for all,” says Stephen Brusatte from the University of Edinburgh. “And they tell us Deinocheirus was much weirder than anyone could have imagined—a colossal, slow-moving, horse-headed, hump-backed dinosaur that looks like something out of a bad sci-fi movie.“

The fossils confirm what many scientists had suspected: that Deinocheirus was one of the ostrich-like ornithomimosaurs. These dinosaurs are mostly lithe and fast-running, but Deinocheirus was built for size, not speed. At 11 metres long and 6,000 kilograms in weight, it was huge, almost as big as the infamous Tyrannosaurus rex. It wasn’t a ferocious predator, though. It couldn’t move quickly or bite strongly, and most tellingly of all, its long, duck-like snout had no teeth! In fact, bite marks on the original bones suggest that it was meat for the unfriendly neighbourhood tyrannosaur, Tarbosaurus

Lee suspects that Deinocheirus fed on soft plants, especially those growing on the bottom of streams and lakes. It could have rustled these up with its broad bill, and then sucked them up using the huge tongue that undoubtedly sat inside its cavernous lower jaw. It then ground up its food by swallowing stones and using them as an internal mill, like ostriches and many other birds do today. Lee’s team found more than 1,400 of these gizzard stones, or gastroliths, inside the torsos of their specimens.

But they also found fish remains among the stones. This supports the idea that Deinocheirus frequented freshwater, but it suggests that the giant dinosaur ate pretty much anything. “This alien creature was a monstrous omnivore, a garbage-disposal type of dinosaur that fed on fish, small vertebrates, plants, and probably about anything it could get its hands on,” says Brusatte. And if that’s the case, its terrible hands were probably nothing more than extravagant gathering devices, used to dig for food or pull down high branches.

Lee’s team also showed that the bones at the end of Deinocheirus’s tail were fused into a single structure called a pygostyle—a feature that, in modern birds, supports tail feathers. If Deinocheirus had a pygostyle, it most likely wafted a fan of tail feathers too.

But the most surprising parts of its anatomy were the thick bony spines sticking upwards from its backbone, creating a… sail? Lee cautions that this structure was thicker than a real sail, like those of Spinosaurus or Ouranosaurus. Nor was it like a camel’s thick hump. Instead, Lee thinks that ligaments coming off the spines helped to support the creature’s huge abdomen and legs. The closest analogues are man-made structures—cable-stayed bridges that support a long surface with cables branching off from a few central towers.

View Images Deinocheirus reconstruction. Credit: Yuong-Nam Lee

“It is amazing to see what Deinocheirus looked like in its entirety after being known from only two gigantic arms for the past 50 years,” says Darla Zelenitsky from the University of Calgary. “It’s also sad in a way. As a kid, your imagination would run wild about the nature of the beast behind those massive arms. That mystery is now gone.”

Lee’s team made their initial breakthrough in August 16, 2009, in a quarry at Mongolia’s Nemegt Formation. Poachers had already been to work at the site, which was full of isolated bones and several broken fossil-filled blocks. Still, among the remnants, the team found what was clearly the left arm of a Deinocheirus, along with a largely complete skeleton. It was missing parts of the spine, the right arm, and the hands, but the rest of it was there. Jackpot!

After analysing this specimen, the team realised that they already had another Deinocheirus in their collection! It had been collected three years earlier from a different quarry, but since it was missing its front half, no one realised that it belonged to the same animal as terrible hands of 1965. “These specimens remind us of the potential problems with reconstructing a particular dinosaur’s appearance from a few bones or incomplete skeletons,” says Zelenitsky.

Lee now plans to study the bones and brain case of Deinocheirus in more detail, and to really understand the function of the weird back spines. And he wants to find more specimens on his yearly trips to the Gobi Desert. “It will be much easier than before because we know almost all skeletal features of it,” he says.

“Nothing shocks me with dinosaurs anymore,” says Brusatte. “We’ve already seen bizarre new specimens of long-snouted tyrannosaurs, chickens from hell, monstrous dreadnought sauropods, and shark-eating spinosaurs within the last few months, and now this. Who knows what we’ll find next?”

Reference: Lee, Barsbold, Currie, Kobayashi, Lee, Godefroit, Escuillie & Chinzorig. 2014. Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature http://dx.doi.org/10.1038/nature13874