"It is an absolutely new dinosaur that was not known before," said Ursula Gohlich, a palaeontologist at the University of Munich in Germany.

Remains of small dinosaurs from the Late Jurassic period are rare finds. The new fossil is nearly complete, apart from a missing part of its long tail, and shows soft tissue and an imprint of the skin but no feathers.

"Scientists had thought that all representatives of the group coelurosaurs should have feathers," Gohlich told Reuters.

"Now we have a little dinosaur that belongs to coelurosaurs that does not show feathers. This is a problem."

COMPLEX EVOLUTION

Feathers were thought to have evolved very early within coelurosaurs. All members of the group were thought to be feathered.

But Gohlich and Luis Chiappe, of the Natural History Museum of Los Angeles County in California, believe the evolution of feathers may be more complex than previously thought.

Feathers may have evolved early but then were replaced by scales in some creatures because they were not needed.

"Another possibility perhaps is that some representatives of coelurosaurs were not entirely covered with feathers, only certain areas," said Gohlich.

The newly discovered Juravenator was very young so may not have lived long enough to develop feathers. But Gohlich said that despite its age, she would have expected it to have had feathers.

"We think that feathers evolved. We have several fossils that support this theory. But our fossil asks some questions," she added.

The oldest known bird, Archaeopteryx, was also found in southern Germany. It too lived about 150 million years ago and had feathers but it is uncertain whether they were used to fly or to keep warm.

Xing Xu, of the Chinese Academy of Sciences in Beijing, said whatever the explanation, the discovery of Juravenator has enriched knowledge of early feather evolution. It could also indicate where future research could be concentrated.