A pair of 2.4-metre-long forelimbs belonging to the Deinocheirus mirificus was discovered in Mongolia in 1965. The size of the limbs led some to speculate the dinosaur was some kind of gargantuan theropod — much larger than Tyrannosaurus rex.
But the rest of the skeleton and location of the quarry were lost until paleontologists located them in 2009 using a hand-drawn map — the only clue they had.
"We looked for years to find the quarry where the Deinocheirus came from," said Phil Currie, professor and Canada research chair in dinosaur paleobiology at the University of Alberta.
"We had a map, but it was a hand-drawn map, so as you can imagine it was very difficult to find."
Evidence in the quarry included isolated bones, broken blocks and some money tucked under a rock as an offering to the gods, which all suggested that the area had been poached.
"The poachers knew what they had, but they only took certain more-saleable parts," said Currie.
"The damaged specimen was missing its skull, hands and feet, but included an easily recognizable hefty left forearm, making it clearly identifiable as Deinocheirus."
Although it is rich in prehistoric assets, Mongolia suffers from widespread poverty, making it vulnerable to fossil poaching. Poachers often take the bones with the highest value, such as skulls, or those that are the easiest to transport, such as claws and teeth.
Word of the find made its way to those who deal in the sale of dinosaur artifacts in the underground fossil market. Currie was contacted about a fossil dealer in Europe, who had what was clearly a Deinocheirus arm, but also feet and even more incredibly a skull.
"What gave us a clue that it was the poached Deinocheirus specimen that we collected was the fact that the hand was divided between the specimen in Europe and the poached specimen we collected in Mongolia."
The skull and other pieces found in Europe fit perfectly with the rest of the specimen that was unearthed in Mongolia, nearly completing the skeleton.
"I was just blown away," said Currie.
The results of the find were released in a paper Wednesday in Nature magazine.
Currie said the Deinocheirus was 11 metres long with an estimated body weight of 6.4 tonnes. But it was not the behemoth that its huge arms suggested.
"The apparently disproportionately large forearms were more likely used for digging and gathering plants in freshwater habitats, or for fishing."
Among its unusual attributes were tall dorsal spines, truncated hoof-like claws on the feet to prevent sinking into muddy ground and bulky hind legs that indicate it was a slow mover.
The now near-complete Deinocheirus specimen has been returned for further study to the Mongolia Centre for Paleontology.
— By Bill Graveland in Calgary
Follow @BillGraveland on Twitter
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