This dinosaur femur is so large that Chinese fossil hunters in the Gobi desert first mistook it for the leg bone of a huge plant-munching sauropod. It wasn't until further fossils were uncovered that the previously unknown Gigantoraptor was revealed.

The new species is all the more surprising for belonging to a group of small, birdlike dinos called oviraptorosaurs. Gigantoraptor was about 300 times heavier than its earlier feathered relatives but likely carried similar plumes on its tail and arms, researchers say.

The study team suggests these feathers could have been used for display or for incubating its eggs.

The fossil showed other body features previously found only in birds.

Most theories suggest birds evolved from two-legged dinosaurs that became progressively smaller over time, but Gigantoraptor's huge size appears to contradict this evolutionary trend.

"Now after the discovery of Gigantoraptor, things get more complicated," lead researcher Xing Xu said.

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