A relatively small, slender cousin of the T. rex shared its environment with larger tyrannosaurs, paleontologists say.

This artist's impression shows A. altai as it might have appeared. (Jason Brougham)The new dinosaur species, Alioramus altai, was a thin, graceful relative of Tyrannosaurus rex, about four metres long from its long snout to the tip of its tail.

The researchers, describing A. altai in this week's Proceedings of the National Academy of Sciences, estimate that it weighed 369 kilograms, compared to a T. rex's weight of up to 6,800 kilograms.

"Compared to tyrannosaurus, this new animal is like a ballerina," said Stephen Brusatte, a graduate student affiliated with the American Museum of Natural History in Manhattan, in a statement.

A. altai was found on a 2001 expedition to the Gobi Desert in Mongolia. The fossil of another tyrannosaur, called tarbosaurus, was found at the same site, meaning the two dinosaurs lived in the same place at the same time — between 85 million and 65 million years ago.

Tarbosaurus looked very much like its more famous North American cousin T. rex, with massive jaws and teeth for crushing through bone. A. altai had a small skull, slender teeth and weak muscle attachments, so it couldn't crush bone and likely fed on smaller prey.

"We now have evidence of two very different tyrannosaurs that lived in Asia at the same time and place — just like today, where lions and cheetahs live in the same area but look dissimilar and exploit their environment differently," said Mark Norell, chair of the division of paleontology at the museum and a member of the Gobi expedition.

The skull of A. altai has a long snout with eight horns that were about 13 centimetres in length. No other tyrannosaur species had these features, but CT scans of the skull show that the interior looks very much like a tyrannosaur, the researchers say.

A microscopic analysis of the fossilized bone suggests the animal died at nine years old, making it an adolescent, about 85 per cent of its full size.

The research is the second recent description of a small T. rex relative. While A. altai lived at the same time as other tyrannosaurs, Raptorex kriegsteini, described two weeks ago in the journal Science, lived tens of millions of years earlier.