Paris - Palaeontologists have fired a broadside over a fossil which is the cornerstone evidence to back the theory that birds descended from dinosaurs.
The row focuses on Sinosauropteryx, a fossil found in 1994 by a farmer in Liaoning province, northeastern China, a treasure trove of the Early Cretaceous period some 130 million years ago.
About the size of a turkey, the long-tailed meat-eating dino was covered with a down of fibres that, Chinese researchers claimed, were primitive feathers.
That claim had the effect of a thunderclap.
Although the "feathers" were clearly not capable of flight, their existence dramatically supported a theory first aired in the 1970s that birds evolved from dinosaurs. As a result, a once-outlandish notion has become the mainstream concept for the ascent of Aves, as birds are classified.
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But a new study, published by a team led by South African academic Theagarten Lingham-Soliar at the University of KwaZulu-Natal, sweeps away the proto-feathers claim.
The two-branched structures, called rachis with barbs, that were proclaimed as early feathers are quite simply the remains of a frill of collagen fibres that ran down the dinosaur's back from head to tail, they say.
The evidence comes from a recently discovered specimen of Sinoauropteryx, also found in the same Yixian Formation at Liaoning, that Lingham-Soliar put to the scrutiny of a high-powered microscope.
"The fibres show a striking similiarity to the structure and levels of organisation of dermal collagen," the kind of tough elastic strands found on the skin of sharks and reptiles today, the investigators say.
The fibres have an unusual "beaded" structure, but this most likely was caused by a natural twisting of these strands, and a clumping together caused by dehydration, when the dinosaur died and its tissues started to dry.
The tough fibres could have been either a form of armour to protect the small dinosaur from predators, or perhaps had a structural use, by stiffening its tail.
The first known bird is Archaeopteryx, which lived around 150 million years ago.
What is missing are the links between Archaeopteryx and other species that would show how it evolved. But fossil record is frustratingly small and incomplete and this is why debate has been so fierce.
The birds-from-dinos theory is based on the idea that small, specialised theropod dinosaurs - theropods are carnivorous, bipedal dinos with three-toed feet - gained an advantage by developing plant-eating habits, growing feathers to keep warm and taking to the trees for safety.
From there, it was a relatively small step to developing gliding skills and then the ability to fly.
Lingham-Soliar's team do not take issue with the theory itself.
But they are dismayed by what they see as a reckless leap to the conclusion that Sinoauropeteryx had the all-important "protofeathers", even though the this dinosaur was phylogenetically far removed from Archaeopteryx.
The evidence in support of the primitive feathers lacked serious investigation, Lingham-Soliar says.
"There is not a single close-up representation of the integumental structure alleged to be a protofeather," Lingham-Soliar says.
Given that the evolution of the feather is a pivotal moment in the history of life, "scientific rigour is called for".
The study appears on Wednesday in Proceedings of the Royal Society B, a journal of the Royal Society, Britain's de-facto academy of sciences.
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