It's now accepted that many dinosaurs -- if not almost all -- were feathered. But evidence clearly and directly associating well-preserved feathers with dinosaur remains has been in scant supply. Now, for the first time, it exists.
The smoking gun is a small piece of amber, fossilised tree resin, dating back to the mid-Cretaceous Period, some 99 million years ago. It's about the size and shape of a dried apricot, and was set to be sold in a market in Myanmar for jewellery before paleotologist Lida Xing of China University of Geosciences found it and realised what it was.
The team's research has been published in the Current Biology journal.
Inside the amber is an ant, plant debris and a small section of feathered tail, now known to have belonged to a juvenile coelurosaur, a type of theropod dinosaur (the family of dinosaurs that includes tyrannosaurs and the ancestors of modern birds) about the size of a sparrow.
The researchers were able to conclusively determine that the specimen did not belong to a bird because the vertebrae are articulated. Birds and their ancestors have fused tail vertebrae.
The 3.5-centimetre (1.4-inch) section of tail contains eight vertebrae, remnants of soft tissue and muscle, and is furred with tiny feathers. It's chestnut brown on top, with a pale underside. It's the most detailed sample of dinosaur feathers discovered to date.
"It's visually stunning and the level of detail on the specimen is not something I was expecting at all," co-author Ryan McKellar of the Royal Saskatchewan Museum in Canada told the ABC.
"I've done a lot of work on amber from dinosaur bone digs in places like Alberta and Saskatchewan, and there's always the hope that maybe you'll find a fragment of a feather. This actually has part of the animal in it in terms of the skeletal remains too. It's a totally different ball game."
The sample could also provide clues about the evolution of feathers. They're an intermediate form of feather, without a well-developed central shaft, indicating that the bars and barbules -- the parts of feathers that branch from the shaft -- evolved before the shaft. This structure is closer to ornamental feathers than flight feathers, which indicates that the dinosaur probably wasn't winging its way through the air.
And this isn't because flight feathers were yet to evolve. Research published last month detailed the fossil of a bird from 130 million years ago with flight feathers.
The sample also contained ferrous iron, remnants of blood haemoglobin from the dinosaur's soft tissue. This could open up new avenues for future study, gathering chemical information about this dinosaur and others if more researchers get as lucky as Xing.
"It's one of those things where if there hadn't been the right person on the ground at the time, I think it would have disappeared into a private collection or gone entirely unnoticed," McKellar said.