Rather than tearing prey limb from limb with talons that gave birth to its monicker 'horrible hand,' Deinocheirus mirificus was a peaceable giant that fed on plants and small fish, palaeontologists said.
It probably used its magnificent 2.4-metre (eight-foot) arms for digging and gathering vegetation, they reported in the journal Nature.
And the reptile likely supplemented its diet by scooping up little fish with a duck-like bill as it roamed a lush river habitat 70 million years ago in what is now the Gobi desert in Mongolia.
"For 50 years Deinocheirus has remained one of the most mysterious dinosaurs," the study's authors wrote.
Its incomplete profile was long based solely on a pair of massive arms with fearsome claws, a handful of ribs and some shoulder bones discovered in 1965.
But from new fossil finds, including a stolen skull recovered from a private collector this year, a near-complete picture now exists of this largest known member of a group of ostrich-like dinosaurs called ornithomimosaurs.
"Deinocheirus turned out to be a gentle giant that mainly ate herbaceous plants and small fishes, not a horrible, humongous meat-eating dinosaur," Yuong-Nam Lee of the Korea Institute of Geoscience and Mineral Resources in Daejeon said.
At 11 metres (36 feet) from nose to tail, and weighing in at more than six tonnes, Deinocheirus was a slow-moving beast with no teeth, and needed to swallow stones to grind up its dinner.
It was heavy-built with relatively short hind legs, a long snout and a humped back with a tall sail on it. Most other ornithomimosaurs were built for running, with slender bodies and long hind limbs.
Deinocheirus' spoon-like bill was suited to rummaging for soft plants on land and in shallow water, and scales and bones found with the fossil suggest it also ate fish, the scientists said.
“The shape of the mouth suggests the presence of a massive tongue that when manipulated would create suction for ingesting the organic material cropped and disturbed by the broad bill as it foraged on the bottom of streams, lakes and ponds," they wrote.
Wide, flattened foot bones would have prevented the dinosaur sinking into mud. And wide hips with upper femur bones longer than the lower tibia indicated it was a slow mover.

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