The skull of R. kriegsteini is dwarfed by the skull of a full-sized descendant
A 3m-long dinosaur fossil from China which predates T. rex by 60 million years is a blueprint for the mighty carnivore, say researchers.
They tell Science magazine that the fossil displays the same features as T. rex but in miniature.
The new species, Raptorex kriegsteini, would have weighed around 65kg; its descendants were 90 times as massive.
Scientists believe it could be the "missing link" between earlier species of dinosaur and T. rex.
The 125-million-year-old specimen suggests that T. rex's characteristic big head with enhanced jaw, relatively small forearms and huge back legs were inherited from this much smaller dinosaur, and that the body type changed little over millions of years except in size.
In the past 10 years, fossils from earlier and more primitive species have been discovered in rocks between 100 and 176 million years old (Middle Jurassic to Early Cretaceous Periods) in Europe, North America and China.
In short, much of what we thought we know about tyrannosaur evolution turns out to be either simplistic or out-and-out wrong
Stephen Brusatte, American Museum of Natural History
These previous finds have confirmed that the Tyrannosaur family of dinosaurs is descended from small-bodied, long-armed predecessors.
But there has been little evidence of a direct link between these relatively small dinosaurs and the massive fossils of T. rex.
The team believes that Raptorex kriegsteini is that link. It looks identical to a big T. rex, even down to the shape of the skull with its enlarged olfactory bulbs, which would have given it the same acute sense of smell as T. rex.
Paul Sereno describes the tiny dinosaur (Video: Chicago Media Initiatives Group)
Lead researcher Dr Paul Sereno, from the University of Chicago, said: "It's as close to the proverbial missing link on a lineage as we might ever get for T. rex.
"From the teeth to the enlarged jaw muscles, the enlarged head, the small forelimbs, the lanky, running, long hind-limbs with the compressed foot for hunting prey: we see this all, to our great surprise, in an animal that is basically the body weight of a human or 1/90th the size that ultimately this lineage would reach in T. rex at the end of the Cretaceous."
The team believes that the new fossil completely overturns accepted opinion on the evolution of tyrannosaurs.
Until now it had been thought that their strange body shape evolved as a consequence of their large size.
The fossil record bears out the previous theory that as tyrannosaurs developed truly giant size over time, they needed to modify their entire skeletons so they could continue to function as a predators as they grew.
According to co-author Stephen Brusatte of the American Museum of Natural History in New York: "Raptorex, the new species, really throws a wrench into this observed pattern.
The animal was entombed at the edge of a lake in north east China
"Here we have an animal that's 1/90th or 1/100th of the size of T. rex, about my size, but with all the signature features - the big head, the strong muscles, the tiny little arms - that were thought to be necessary adaptations for a large body predator.
"So really we can say that these features did not evolve as a consequence of large body size but rather that they evolved as an efficient set of predatory weapons in an animal that was just 1/100th of the size of T. rex and that lived 60 million years before T. rex."
The fossil record tells us that tyrannosaurs only grew to huge sizes during the final 20 million years of the Cretaceous.
Dr Brusatte said: "So that means that for most of their evolutionary history, about 80% of the time that they were on Earth, tyrannosaurs were small animals that lived in the shadow of other types of very large dinosaur predators.
"In short, much of what we thought we know about tyrannosaur evolution turns out to be either simplistic or out-and-out wrong."
Dr Sereno estimates that this specimen of Raptorex kriegsteini was a young adult when it died.
The team made casts of the bones and reconstructed the animal as they believe it would have appeared. They think it would have been 1m high and that it would have had similar feathers to an ostrich.
It would have preyed on small birds and other smaller scampering animals that lived around the lake beds where it was discovered.
The forearm of raptorex is a blueprint for that of an adult T Rex
The researchers say that fragments of sand and sediment on the skeleton indicate that it came from an area of northeastern China rich in fossils. It was dug up illicitly and spirited out of the country and ultimately sold.
It is named after Henry Kriegstein, a private collector, who bought it from a dealer. He contacted Dr Sereno three years ago, asked him to analyse the specimen, and agreed to give it back to science and ultimately to China. It is now in a museum in Inner Mongolia.
Mr Kriegstein is a co-author on the paper.
Dr Sereno said: "I hope this is a pathway that other important specimens will follow.
"This is more than just a Chinese specimen. It has given us some real insights into our own tyrannosaurs here in North America. So it really belongs to science. It belongs to the world."
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