The Raptorex was only three metres long, but had many of the distinctive characteristics of a T. rex. (Todd Marshall)

A dinosaur with all the traits of a T. rex — big head, little arms, muscular legs — but only three metres long has been found in northeastern China.

The paleontologists who identified the dinosaur, which lived tens of millions of years before Tyrannosaurus rex, have dubbed it Raptorex.

Raptorex kriegsteini had many of the characteristics of its descendant T. rex: a large skull compared with its torso, powerful jaws, tiny forearms and large legs and feet suitable for running. It also had the T. rex's enlarged olfactory bulbs, suggesting a highly developed sense of smell, the researchers say in a study published in Thursday's online edition of the journal Science.

However, Raptorex weighed about 1/100th as much as T. rex, making it "basically our bodyweight," principal author and University of Chicago paleontologist Paul Sereno said in a release.

A Raptorex is shown with its much larger descendent, T. rex., in this drawing. (Todd Marshall)

The researchers said a T. rex body plan on a small dinosaur means that those characteristic physical features didn't evolve as dinosaurs grew in size, but existed in dinosaurs of all sizes throughout the Cretaceous period.

The T. rex-like features of the Raptorex, which the researchers call a "predatory skeletal design," evolved for efficiency in hunting and feeding.

Sereno obtained the nearly complete Raptorex skeleton from a private fossil collector, Henry Kriegstein, who bought it from a vendor. The skeleton was illicitly excavated in Inner Mongolia, an autonomous region in northeastern China.

Sereno and his colleagues examined all parts of the skeleton and compared them with other skeletons in the tyrannosauroid group of dinosaurs. They found that it lived at least 125 million years ago. T. rex lived between 68 million and 65 million years ago.

The researchers found that the skull had enlarged olfactory bulbs by passing it through a CT scanner at the University of Chicago hospital. By examining a thin section of the skeleton's thigh bone under a microscope, Sereno determined that this particular Raptorex lived to be five or six years old.

After a detailed study of the Raporex skeleton, it will be put on display in a museum in Inner Mongolia.