A shiny dinosaur –four-winged Microraptor gets colour and gloss

For the longest time, artists could only speculate about what dinosaurs looked like. Sure, we could reconstruct their silhouettes from their bones, but the colour of their skin was a mystery. That’s not quite true anymore. Thanks to some well preserved fossils and some ingenuous detective work, scientists have started to assign the right palettes to these prehistoric reptiles.

The latest species to get this treatment is Microraptor. This Chinese dinosaur was about the size of a crow. Its body was covered in feathers. Long plumes on both its arms and legs gave it a distinctive four-winged, baggy-trousered look, and may have allowed it to glide or fly. And thanks to a new study by Quanguo Li form the Beijing Museum of Natural History, we know that Microraptor was probably black and certainly shiny.

It was a iridescent dinosaur, with the same metallic sheen that you see on today’s hummingbirds, peacocks, and swallows. If you travelled back in time and stumbled across Microraptor, you might think that you’d found a Cretaceous starling.

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Li’s study was led by Jakob Vinther, who pioneered the study of dino-colours. Several years ago, Vinther realised that he could infer the hues of prehistoric feathers by studying microscopic structures within them, known as melanosomes. These structures are sacs of pigments called melanins, and their shape corresponds to their colour. Spherical melanosomes tend to be reddish-brown, while rod-like ones are black or grey.

By studying melansomes, Vinther worked out that a fossilised Cretaceous feather probably had black and white stripes. Later, he concluded that another feather had an iridescent sheen. A few years later, these analyses went from single plumes to entire animals.

A rival group headed by Fucheng Zhang found that Sinosauropteryxfound that Sinosauropteryx – a small predator related to Microraptor and Velociraptor – was probably a rusty red colour, with “chestnut to reddish-brown” stripes running down its tail. A week later, Li and Vinther announced that another small predator called Anchiornisanother small predator called Anchiornis had a dark grey body, black-tipped white feathers on its arms, red freckles on its black face, and a reddish-orange crest. They later painted a prehistoric giant penguin.

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The team turned to Microraptor when they came across a new specimen of the tiny dinosaur, with a thorough coat of well-preserved feathers. “It’s one of the most beautiful Microraptor specimens out there,” says Vinther. The feathers were full of melanosomes, but the team realised that they were longer than usual, and arranged end-to-end in regular sheets. There are similar melanosomes around today, and they’re found in the feathers of glossy black birds.

Most feathers get their colours because bundles of pigment, like melanosomes, selectively absorb and reflect different colours of light. But in iridescent feathers, it’s the way the melanosomes arranged that matters. They’re either found in one smooth layer or in stacks, and they’re surrounded by keratin, the substance your nails and hair are made of. As light travels between the different materials, some passes through and some is reflected. The reflected beams of light reinforce each other. Depending on the size of the gaps and the thickness of the keratin layer, you might get a subtle gloss or a vivid burst of colour.

Working with Matthew Shawkey from the University of Akron, the team sampled melanosomes from a wide range of modern birds, from rockhopper penguins to palm cockatoos. “I spent two days in a Copenhagen collection plucking feathers from birds of paradise,” recalls Vinther. “We found out that melanosomes have a characteristic shape when they’re in iridescent feathers.” They’re longer and narrower – widescreen versions of the ones found in matte-coloured birds.

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The team created a model that can identify iridescent feathers from the shape and arrangement of their melanosomes. When they eventually looked at feathers from their Microraptor specimen, they found that the majority were iridescent, and the rest were black.

It’s possible that the dinosaur could have had more splendid colours, but the team can’t tell from looking at their fossil. Even though it is beautifully preserved, the gaps between the melanosomes aren’t clear enough to measure, and the keratin layer surrounding them is lost. Without these elements, it’s impossible to say if more vivid colours reflected from the feathers. “Black is the conservative guess,” says Vinther. “That’s the simplest kind of iridescence we see.”

The study shows that iridescence isn’t an exclusive bird innovation – it was around in their dinosaur predecessors too.  In fact, it seems to be an easy trait to evolve, independently turning up many times in bird evolution. It just takes subtle tweaks to the shape of the melanosomes and the way they are stacked to fashion shiny feathers from matte ones.

Once again, we see that feathered dinosaurs were much like their modern descendants. “Many of these dinosaurs had colours like the birds you see outside your window,” says Vinther. “When we looked at Anchiornis, we realised that it looked more or less like a woodpecker. Here in Texas, we have grackles, and it’s cool how much they resemble Microraptor.”

But the research isn’t just an aide to artists. Knowing that Microraptor was iridescent also tells us about its life. Some scientists have suggested that it was nocturnal, based on the large size of the bony ring around its eye. But iridescent birds are active during the day. If Microraptor was glossy, it probably wasn’t nocturnal.

We also know that modern birds mostly use iridescence during courtship displays – think about the seductive dance of a peacock for an obvious example. Maybe Microraptor did something similar. Its tail ends in several long feathers.  People have suggested that these fanned out in a teardrop, and might have helped the animal to fly. But Li’s specimen, along with a few others, suggests that the tail feathers were spread too narrowly to have generated much lift. Instead, they might have been useful for signalling, much like the elaborate streamer tails of modern birds like widowbirds or the paradise flycatcher.

Reference: Li, Gao, Meng, Clarke, Shawkey, D’Alba, Pei, Ellison, Norell & Vinther. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science http://dx.doi.org/10.1126/science.1213780

Images: reconstruction by Jason Brougham; fossil photos by Mick Ellison

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