The fossilised remains of a giant bird-like dinosaur have been uncovered in the region of Inner Mongolia, China.
While some have theorised that meat-eating dinosaurs got smaller as they evolved to be more bird-like, this beast weighed about 1,400kg (3,080lbs).
That is about 35 times heavier than other dinosaurs of its ilk.
Nature journal reports that the beaked animal was 8m (26ft) long and twice as tall as a man at the shoulder; yet it was only a young adult when it died.
The authors suggest the dinosaur's enormous size was due to a fast growth rate, faster even than the precocious Tyrannosaurus rex.
However, precisely what it ate is a mystery. Gigantoraptor erlianensis had some features associated with meat-eating dinosaurs, such as sharp claws for tearing flesh; but it also had some features associated with plant-eaters, such as a small head and long neck.
Chinese researchers uncovered the fossilised remains of the flightless giant in the Erlian basin in Inner Mongolia.
The researchers had originally thought they had found the bones of a tyrannosaur - the group of dinosaurs to which T. rex belongs - due to their large size.
The team has established that the creature lived about 70 million years ago, in the late Cretaceous Period.
The bones were so big, they were thought to be from a tyrannosaur
According to lines of arrested growth detected on its bones, it died in its 11th year of life.
The team says it is very probable G. erlianensis had feathers - although none were identified from the recovered fossils.
"It was a very surprising discovery, not at all what we expected," said Xu Ling, a palaeontologist at the Chinese Academy of Sciences in Beijing and co-author on the Nature paper.
"We think it's the largest feathered animal ever to have been discovered."
Gigantoraptor could probably run relatively quickly on its long, powerful legs.
"It belongs to a very unusual group of theropod dinosaurs, which are normally meat-eaters. But this one doesn't have any teeth, so what it ate is a mystery," commented Dr Paul Barrett, a dinosaur researcher from the Natural History Museum in London, UK.
"They show that it had a very fast growth rate so it probably got big by growing very rapidly, rather than growing for a very long period of time."
Dr Barrett added that the animal was not on the direct evolutionary line leading to today's birds. This supported the notion, he said, that the features we associate with modern birds probably arose more than once in their close relatives.