Archaeopteryx, a winged dinosaur long believed to be the world's first bird, had black feathers, according to a scientific feat reported on Tuesday.
The colour of skin and feathers is one of the big unknowns about dinosaurs, and it is left to the imagination of artists, rather than scientists, to depict how these enigmatic creatures looked.
Researchers in the United States and Europe pored over a remarkably preserved wing feather in an Archaeopteryx fossil unearthed in a German limestone quarry in 1861.
The shape of the feather indicated that it was a "covert," the term for a feather that covers the primary and secondary wing plumage which birds use in flight.
Their next goal was to hunt for fossilised melanosomes, or pigment-producing parts of a cell.
Two attempts to image the tiny, sausage-shaped components -- measuring just a millionth of a metre long and 250 billionths of a metre wide -- failed.
The breakthrough came with a scanning electron microscope at the Carl Zeiss laboratory in Germany, which revealed hundreds of the structures encased in patches in the feather.
"The third time was the charm, and we finally found the keys to unlocking the feather's original colour, hidden in the rock for the past 150 million years," said Ryan Carney, an evolutionary biologist at Brown University, in the northeastern US state of Rhode Island.
Statistically matched against the melanosomes of 87 species of living birds, Archaeopteryx's plumed treasure was estimated to be black, with a 95% certainty, the scientists say.
Black could have been useful as camouflage, for display or to regulate the body temperature.
The alignment of the melanosomes, and tiny overlapping appendages called barbules, are evidence that the wing feather was rigid and durable, rather like the feathers of modern birds.
"If Archaeopteryx was flapping or gliding, the presence of melanosomes would have given the feathers additional structural support," Carney said.
"This would have been advantageous during this early evolutionary stage of dinosaur flight."
The paper is published by the journal Nature Communications.
Archaeopteryx has a hallowed place in palaeontology.
A fossil of the creature, discovered 150 years ago, inspired the belief that this was the forerunner of all birds.
The raven-sized creature had feathered wings and a wishbone as well as the reptilian features of teeth, clawed fingers and a bony tail.
The cherished theory was knocked back last July when Chinese fossil-hunter Xing Xu determined that Archaeopteryx was only one of numerous proto-birds, or feathery dinosaurs, which lived around 150 million years ago.