By Rebecca Morelle
BBC News science reporter
The forefather of the mighty Tyrannosaurus rex has been discovered, scientists report.
The 160-million-year-old fossil is the oldest tyrannosaur ever found.
The researchers were surprised to learn the 3m-long dinosaur sported a spectacular crest on its head which may have been brightly coloured.
The discovery, unveiled in the journal Nature, might reveal how early tyrannosaurs evolved into the T. rex 100 million years later.
The new species was found in the Junggar Basin, an area rich in dinosaur fossils, in the far north-west corner of China.
A local labourer, hired to search for ancient bones, happened upon two dinosaur skeletons: a 12-year-old adult and a six-year-old juvenile. Both were found to be remarkably intact.
The international team have named the dinosaur, which hails from the Late Jurassic Period, Guanlong wucaii (G. wucaii) which is derived from the Chinese for "crowned dragon".
Professor James Clark, an author on the paper and a palaeontologist at George Washington University, US, told the BBC News website of the discovery.
"We found two skeletons of what we call a therapod dinosaur. When we looked at them very closely we found that they are a relative of Tyrannosaurus rex - making them the most primitive tyrannosaur relatives that we have seen," he explained.
Tyrannosaurs were the dominant group of predators during the Late Cretaceous Period. This era, about 65 to 100 million years ago, marked the final chapter before dinosaurs became extinct.
It was during this time the T. rex roamed. The most famed member of the tyrannosaur family; its immense size of 9-13m, huge teeth and tiny but savagely clawed forearms have made it the beast of choice for many Hollywood films.
Professor Clark described how the G. wucaii would have looked: "The most obvious thing was that it had a big crest in the middle of its head. For carnivorous dinosaurs that's pretty unusual.
"We suspect that the crest was highly coloured and probably a display structure of some kind."
He said that it shared some features with the later tyrannosaurs, such as the T. rex. It had sharp teeth, similar muscle scars on its hips and probably ran on two legs.
But the G. wucaii differed markedly in terms of its size: at 3m it was much smaller. In addition, its more primitive skull and pelvic features would suggest that that it was an intermediate animal between tyrannosaurs and the coelurosaurs - an even older, related group of dinosaurs which are thought to be the predecessors to modern birds.
The T. rex was much larger than its relative, the G. wucaii
The researchers hope that the find will reveal more about the primitive phase of tyrannosaur evolution.
"Guanlong shows us how the small coelurosaurian ancestors of tyrannosaurs took the first step that led to the giant T. rex almost 100 million years later," Professor Clark said.
Most of the tyrannosaur fossils that have been found date to the latter years of tyrannosaurs' existence, and there are very few early specimens.
Prior to the discovery of the G. wucaii, the 130 million-year-old feathered Dilong paradoxus (D. paradoxus) reported in 2004, was the oldest tyrannosaur known.
Dr Paul Barrett, a palaeontologist at the Natural History Museum, commented: "The discovery of this new animal pushes the origin of the group containing T. rex further back in time and also shows that early tyrannosaurs had a much wider distribution than previously thought."
The researchers believe the G. wucaii, with its bizarre crest, will begin to fill in some of the gaps of our knowledge of tyrannosaurs.
"This is 160 million years old, there are almost 100 million years of fossil records between it and T. rex and there are only a few tyrannosaurs that we know of between that," said Professor Clark.
"It is telling us that we are just getting into finding the missing records of these early tyrannosaurs."
The Guanlong lies at the base of the lineage of tyrannosaurides
The Eotyrannus was dwarfed by other predators in its environment
The T. rex was a member of the Tyrannosauridae family