The team speculates that its main function was to make the animal more noticeable or attractive to others of its species.
The estimated ages of the two dinosaurs seemingly support this idea.
Fossil bone-growth rings suggest one animal died at age 12, while the other was only 6. This younger specimen, yet to reach sexual maturity, had a much smaller crest.
"It looks very much like a display structure," agreed dinosaur researcher Paul Barrett of the Natural History Museum in London.
"There's no other good, obvious function for it."
Barrett, who wasn't part of the study team, says the feature could have played a role in species recognition, mating displays, or in establishing territory.
"This weird crest is something that's completely unprecedented among big predatory dinosaurs," he added.
"There's a very interesting combination of features in this animal."
Barrett says the new find represents an important missing link in the evolutionary line leading to later, much larger tyrannosaurs, including T. rex.
"It gives us the first real solid evidence of an early member of this group and shows us what they would have looked like," he said.
Teeth and Skull
The team identified Guanlong as a tyrannosaur based on a number of physical characteristics, including U-shaped front teeth, the makeup of its pelvis, and the shape of its skull.
Unlike T. rex, however, the animal had three finger-claws instead of two, a shallow snout, and long arms.
"Guanlong represents a specialized lineage very early in the evolution of tyrannosauroids, so it has only a few features of this group," study co-author Clark said.
Researchers say the species, which they suspect was covered in simple feathers like another primitive tyrannosaur described in 2004, Dilong paradoxus, may also help scientists understand how birds evolved.
Tyrannosaurs grew out of a group of lightweight, carnivorous, bipedal dinosaurs that also gave rise to birds.
The first known bird is Archaeopteryx, which emerged soon after Guanlong.
"Tyrannosaurs are fairly closely related to the origins of birds," Barrett, of the Natural History Museum, said. "Guanlong ties tyrannosaurs in with that other group of birdlike [dinosaurs]."
Xing Xu said, "We have to go back deep back in time to reconstruct this important evolutionary transition. In this regard, Guanlong looks like just what paleontologists have been expecting for a primitive tyrannosaur."
For example, he says, Guanlong has a characteristic type of wrist bone that links dinosaurs and birds.
"Even after so many great discoveries, we have to say there is still a lot we don't know about dinosaurs," Xu added. "They really are a diverse group of animals."
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