The newly discovered miniature tyrannosaur pre-dates T-rex by millions of years but has almost identical features

Fossil hunters have unearthed the remains of a man-sized forerunner to the colossal Tyrannosaurus rex from an ancient lake bed in northeastern China.

The remarkable discovery has allowed dinosaur experts to piece together a picture of a diminutive but formidable predator that was so finely tuned to killing they describe it as "Jaws on legs".

The beast, named Raptorex kriegsteini, roamed the Earth 130m years ago, tens of millions of years before the giant T-rex became the most fearsome predator in history.

The finding has stunned palaeontologists because the skeleton resembles the larger tyrannosaurs in every respect except its size. Measurements of bones recovered from the site reveal that the new species was one hundredth the size of T-rex.

Analyses of the remains by researchers at the University of Chicago and the American Museum of Natural History in New York revealed the dinosaur to be a juvenile of five or six years old, measuring nearly 3m from nose to tail and weighing only 60kg (nine stone). A similar aged T-rex could weigh several tonnes.

Though smaller than its more celebrated descendant, Raptorex was the largest meat-eater of its time. It would have enjoyed a varied diet of parrot-beaked psittacosaurs, turtles, primitive birds and a host of small, scampering dinosaurs that would have watered at the ancient lakes it lived near.

The exquisite and almost complete remains only came to light when an American eye surgeon, Henry Kriegstein, telephoned the researchers to say he had bought the fossil from a trader. Paul Sereno at the University of Chicago agreed to document the fossil – and name it after the surgeon's father – on condition that the remains were returned to China afterwards.

The Chicago team has spent the past three years preparing and studying the fossil, which was lodged in a block of sediment removed from the Lujiatun lake beds in northeast China.

Writing in the US journal Science, the researchers describe the delicate operation to clean and prepare the skeleton. The skull was sent through an X-ray scanner at a Chicago hospital before moulds and casts of the bones were made. The X-rays revealed enlarged brain regions that suggest the creature had a highly evolved sense of smell.

The discovery overturns scientists' thinking about how Tyrannosaurus rex evolved. Many of the most striking features of the beast, such as its puny forearms, were thought to be a trade-off during the evolution of its enormous size, but Raptorex shows these features had already evolved more than 60m years earlier.

"So much of what we thought we knew about Tyrannosaur evolution turns out to be simplistic or out-and-out wrong," said Stephen Brusatte, a member of the team.

"The thinking has been that as tyrannosaurs developed to a truly giant size, they needed to modify their entire skeleton so they could function as predators.

"Raptorex, the new species, really throws a wrench into this observed pattern. Here we have an animal that's one 90th or one 100th the size of T. Rex, but with all the signature features, the big head, the strong muscles and the tiny little arms.

"We can now say these features didn't evolve as a consequence of body size, but rather they just evolved as a set of efficient predatory weapons," Brusatte added.

Raptorex had powerful legs to run down its prey and a huge muscular jaw with which to dispatch them. "This is a blueprint for a predator: Jaws on legs," Sereno said.

Researchers now believe that tyrannosaurs spent almost all of their time on Earth as small, flighty predators like Raptorex. As other large dinosaurs became extinct, this left the path clear for Raptorex to expand in body size and ultimately become the giant Tyrannosaurus rex. "When it did, there was no turning back until the asteroid hit," said Brusatte.

Sereno said Kriegstein agreed to donate the fossil remains to science if the species was named in honour of his father, Roman.

"The specimen was found perhaps in the dark of night and spirited out of China and ultimately sold. [Mr Kriegman] contacted me and wondered if I would describe it. I said I would if it could be returned 100%, lock, stock and barrel to science and ultimately back to China.

"I think he saw that there was a sense of immortality in having a dinosaur named after your family," Sereno said.