BioEd Online

A painstaking search through thousands of chunks of amber has unearthed 11 prehistoric feathers. They promise an unprecedented look at the history of these peculiar structures in both birds and non-avian dinosaurs.

The amber samples are between 70 and 85 million years old, and come from a site called Grassy Lake in western Canada that was once home to a conifer forest. The site is well known for the wide range of insects found preserved in its amber.

Palaeontologist Ryan McKellar and his colleagues at the University of Alberta in Edmonton were keen to find out whether any feathers had been preserved, so they sifted through more than 4,000 of the amber inclusions held in museum collections. Once feathers were found, the samples were mounted on slides and polished, before being analysed using microscopy. The team reports the 11 feathers identified, as well their range of structures, today in Science¹.

From fuzz to feather

During the past 15 years, researchers have unearthed the remains of numerous dinosaurs sporting remnants of fossilized feathers. Feathers' evolutionary origin remains murky, but palaeontologists propose that they started off as simple, flexible filaments similar to those in the coat of 'dino fuzz' that covered the small predatory dinosaur Sinosauropteryx. From there, feathers adapted to become complex branching structures, eventually culminating in the asymmetrical flight feathers of the early bird Archaeopteryx and its living relatives.

The collection of amber-preserved feathers provides palaeontologists with the most diverse and best preserved set of samples yet, says McKellar. Fossilized feathers are normally flattened and carbonized, but the amber-preserved specimens retain their three-dimensional shape, along with minute structural details.

Because the amber-encased feathers are not associated with body fossils, the researchers cannot be sure whether they come from birds or from non-avian dinosaurs. Both were present in prehistoric Canada 70-85 million years ago. But the authors do make some guesses.

The more complex specimens include asymmetrical feathers made up of interlocking elements, and highly specialized feathers similar to those used by modern sandgrouses and grebes to absorb water. McKellar says that these feathers probably came from birds. However, the group also found simple, wispy feathers similar to those that have been found on non-avian dinosaurs such as Sinosauropteryx and Sinornithosaurus.

Thomas Holtz Jr, a palaeontologist at the University of Maryland in College Park, agrees that the find adds a new dimension to our understanding of prehistoric plumage. "These are the closest we have to pristine non-avian feathers in the fossil record," he says, adding that they may prove useful for future studies of the structure and composition of dinosaur feathers. They may even contain signs of prehistoric parasites and fungi, Holtz suggests, "giving us a better look at the biota that bothered the dinosaurs".

Colour codes

The microscopy also revealed how dark or light the feathers were. McKellar says that the more complex feathers display a range of shades similar to that of modern birds, whereas the simpler protofeathers were mostly dark colours.

Last year, palaeontologists reconstructed the partial colouring of Sinosauropteryx² and the full plumage palette of Anchiornis³ by analysing the shape, size and density of microscopic structures called melanosomes present inside their fossilized feathers. In the Grassy Lake feathers, such details are locked behind their amber coating.

The specimens are too rare to be broken open but Julia Clarke, an expert in fossil feathers at the University of Texas at Austin, suggests that it might in future be possible to study their melanosomes non-destructively using high-resolution X-ray imaging.