The remains of a giant, birdlike dinosaur as tall as the formidable tyrannosaur have been found in China, a surprising discovery that indicates a more complicated evolutionary process for birds than originally thought, scientists said Wednesday.

Fossilized bones uncovered in the Erlian Basin of northern China's Inner Mongolia region show that the specimen was abouteight metres long, five metres tall and weighed 1,360 kilograms, said Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology & Paleoanthropology in Beijing.

The height is comparable to the meat-eating tyrannosaurs. But the dinosaur, called Gigantoraptor elrianensis, also had a beak and slender legs and likely had feathers. It was 35 times larger than its likely close relation, the Caudipteryx, a small, feathered dinosaur species, Xu said.

That puts the Gigantoraptor's existence at odds with prevailing theories that dinosaurs became smaller as they evolved into birds, and that bigger dinosaurs had less birdlike characteristics, he said.

"This is like having a mouse that is the size of a horse or cow," said Xu, who co-authored a paper on the finding published Thursday in the journal Nature. "It is very important information for us in our efforts to trace the evolution process of dinosaurs to birds. It's more complicated than we imagined."

Creature was an oviraptor

The Caudipteryx and the Gigantoraptor belong to a group of dinosaurs called oviraptors, which tend to be human-sized or smaller. In recent years paleontologists have found turkey-sized, feathered representatives of the group, but they never found anything close to the scale of Gigantoraptor.

"It's one of the last groups of dinosaurs that we would expect to be that big," said Mark Norell, curator of paleontology at the American Museum of Natural History in New York City.

'It's one of the last groups of dinosaurs that we would expect to be that big.' — Mark Norell, American Museum of Natural History

But Philip Currie, a paleontologist at the University of Alberta, said the size of the Gigantoraptor would be a natural step in the evolutionary process of the oviraptors. "Almost every group that has evolved has tended to evolve giant forms."

Animals tend to become bigger with evolution because larger creatures have an easier time getting food, impressing potential mates and avoiding predators.

But size has disadvantages, too.

Bigger animals need more food and territory, have fewer offspring and reproduce less frequently than smaller animals do. That means they are particularly vulnerable when environmental conditions change, as they abruptly did about 65 million years ago. Just a few million years after Gigantoraptor evolved, it and every other dinosaur species on Earth became extinct.

It has not been determined whether the Gigantoraptor was a herbivore, which have small heads and long necks, or a carnivore, which have sharp claws. The dinosaur has both, Xu said.