Earliest feathers fan controversy

The features look like feathers

A small, lizard-like creature that lived 220 million years ago has re-ignited the debate about the evolution of birds by seriously questioning whether they evolved from dinosaurs.

Researchers studying the fossil remains say the animal, Longisquama insignis, had elongated structures on its back and arms that look very much like the feathers of modern birds. This suggests an evolutionary link between the two.

But Longisquama, the scientists say, was not a dinosaur, and in any case was around when the great reptiles had only just begun to walk the Earth.

And they argue that it is unlikely that features as complex and specialised as feathers evolved more than once.

Entrenched view

"These are some amazing fossils, and at the very least they prove that feathers did not evolve in dinosaurs," said Professor John Ruben, of Oregon State University and one of the scientists investigating Longisquama.

"The supposed link between dinosaurs and birds is pretty entrenched in palaeontology, but it's not as solid as the public has been led to believe."

Longisquama insignis appeared 75 million years before Archeopteryx

He added: "Feathers are a very complicated structure. The odds of them evolving first in Longisquama and then separately at some later point in dinosaurs or any other group of animals would have been astronomically small."

The OSU researchers, and colleagues, report their analysis of the fossils in the journal Science.

Old specimen

The Longisquama specimen was actually discovered three decades ago in central Asia by a Russian palaeontologist who specialised in insects.

When the scientist published the first report of the fossil in 1970, he described a row of long narrow appendages down the animal's back, interpreting them as a frill of extremely long scales.

The feathers would have evolved for flight rather than insulation

The Science authors, who have pored over every detail of the fossils, which include most of the skeleton except for the hind part, have now challenged this view.

They think the appendages show some of the most recognisable features of a modern-day feather.

Developing feather

They have identified a long, thin tube called a "shaft" running down the centre of each appendage.

A short distance from the base, a dense row of fine strands called "pinnae" project from either side. Neither the shaft nor the pinnae are typically thought to be features of reptilian scales.

The shaft also comes to a point at the base and appears to poke into a follicle in the skin. These and other clues point feathers as the only logical explanation for the features, the scientists say.

The pinnae of modern feathers first develop inside a tube called a feather sheath and then unfurl as the feather grows. The Longisquama fossil shows a new feather that seems to be developing in just the same manner.

Flight not warmth

"We can identify certain structures in these fossils that you only find in feathers and just don't see anywhere else," said Terry Jones, also an OSU palaeontologist and a co-author of the study.

"So we're quite sure we're looking at the earliest feather. But beyond that, this animal looks like an ancestral bird even if you ignore the feathers. The teeth, pectoral structure, neck, and skull are just like those of birds."

The researchers think the feathers evolved for flight rather than insulation. Providing warmth is the more likely function of the downy feathers sported by some much later dinosaurs.

Longisquama probably glided, rather than flew, using its long aerodynamic forelimbs for steering.

Dr Alan Feduccia, a co-author from the University of North Carolina at Chapel Hill, who in 1979 proved for the first time that Archeopteryx, the earliest recognisable bird, could fly, said: "These are the earliest structures in the fossil record that can be called feathers.

"They pre-date the so-called 'fuzzy dinosaurs' from China by at least 100 million years. Here we show unequivocally that the earliest known feathers evolved in the context of flight and not thermo-regulation."