By Doreen Walton
Science reporter, BBC News
The dinosaur had short powerful arms and a large claw on each hand
A newly discovered fossil has shed light on why a group of dinosaurs looks like birds, say scientists.
Haplocheirus sollers may not be as charismatic as T. rex or as agile as a pterodactyl but it's thought to solve a long standing puzzle.
Researchers believe its short arms and large claw show how bird-like dinosaurs evolved independently of birds.
The 3m-long skeleton, found on an expedition to China's Gobi desert, is described in the journal Science.
The fossil is a member of the Alvarezsauridae family, a group of bird-like dinosaurs. The group shares features with birds, including fused wrist elements and a loosely structured skull.
But the researchers say the new fossil shows the Alvarezsauridae group split from birds much earlier on the evolutionary tree than was thought.
"Haplocheirus is a transitional fossil," Jonah Choiniere from George Washington University told the BBC.
"Previously we thought the Alvarezsauridae were primitive, flightless birds. This discovery shows they're not and that the similarities between them evolved in parallel."
The fossil is of a nearly complete adolescent dinosaur skeleton and was found in orange mudstone beds in the Junggar Basin, Xinjiang, China.
It was spotted when a member of the team noticed the pelvis at the ground's surface. The rest of the skeleton was found only inches down.
The new dinosaur shows an early evolutionary step in the development of the short, powerful arm typical to the Alvarezsauridae group.
"The rest of the members of this group have really short forelimbs with huge muscle attachments, like body-builder arms. The fossil shows the first step in the evolution of this weird arm and claw," said Mr Choiniere.
The researchers believe the fossil shows development of the two diverged in the Late Jurassic period, about 160 million years ago. Until now there was no evidence of this type of dinosaur living at that time.
"It's like finding a great, great grandfather in your family which doubles the age of your family tree," said Mr Choiniere.
Scientists believe that birds descended from theropods or bird footed dinosaurs in the Late Jurassic. Theropods include alvarezsaurs, other bird-like dinosaurs including the well known Velociraptor, meat eaters like T. rex and modern birds.
Haplocheirus sollers means simple, skillful hand. The fossil shows the dinosaur had small teeth and researchers believe the claw may have been used for digging termites.
"It may have had a very general diet, tackling smaller animals like lizards, very small mammals and very small crocodile relatives," explained Mr Choiniere. "It was a lightly built animal and could run very quickly."