SYDNEY: Microscopic details in fossilised plumage could reveal the colour of extinct birds, or even feathered dinosaurs, according to a new report.

Researchers at Yale University in Connecticut, U.S., have found that dark stripes on a fossilised feather were preserved 'melanosomes', microscopic structures that contain the pigment melanin and give feathers their colour. As they report in the journal Biology Letters the striped fossil was discovered in Brazil and is thought to be the remains of a 100-million-year-old bird.

"Melanin is very resistant to decay," said Derek Briggs, palaeontologist and lead author. "It preserves the shape of the melanosomes that contain[ed] it."

Curious stripes

That's important, he said, because different colours of melanin are contained in melanosomes of different shapes. Black and brown melanosomes are sausage-shaped; while red and yellow ones are more circular. Some arrangements of melanosomes can even produce iridescent colours, said Briggs.

While occasional fossils of shells or beetles retain their original colours, feathers and skin do not. Most reconstructions of the colour of extinct animals rely on comparisons with living species.

However, the fossils themselves can have light or dark patches, like this Cretaceous period feather. Earlier research had suggested that the dark banding found in some feather fossils were the remains of feather-degrading bacteria, but Briggs thought this was unlikely, so his team set out to test the idea.

"We knew that some fossil feathers preserved stripes but we didn't know how," said Briggs. Co-author Jakob Vinther, who has studied melanin in fossilised squid ink, suggested they look for melanosomes in the fossil feathers.

The team examined the fossil with a scanning electron microscope, which shows extremely fine structural details, and energy dispersive X-rays, which reveal the chemical composition of the fossil. Though scientists routinely use energy dispersive X-rays to examine fossils, Briggs said this may be the first time this technique has been used to investigate colour.

The researchers found that the dark parts of the fossil were covered in carbon-rich, microscopic structures that appeared to be the remains of melanosomes. A comparison with the structure of melanosomoses in a present-day blackbird feather suggested the fossil stripes had been black, and the feather was therefore likely to have been black and white during life.

Behavioural clues

Melanosomes are found in fur and skin as well as feathers, so – if they can find them – scientists may be able to use them predict the colour of fossilised mammals or even dinosaurs, said Briggs.

Andrew Parker, who studies fossilised visual systems at the University of Sydney, Australia, said the findings could be especially useful for predicting whether fossilised birds or dinosaurs used camouflage.

"Certain patterns are associated with certain behavioural strategies," he said, meaning that an understanding of the colour of a species can tell us about its behaviour too. "Disruptive colouration - to break up the outline of an animal against its background - carries the most potential here."

While this is the first time melanosomes have been found in fossil feathers, they have been found in other kinds of fossils, said Parker, who is not one of the study's authors. For example, frog fossils have been found with melanosome structures that suggest yellow and red skin.

Briggs said the next step is to compare the melanosomes of fossil birds with those of colourful living relatives.