A fossil penguin gets its colours

All Almost all modern penguins wear a suit of black feathers, but prehistoric members of the group didn’t go for the dinner jacket look. A newly discovered penguin, known as Inkayacu, was dressed in grey and reddish-brown hues.

Inkayacu paracasensis is named after the Quechua words for “water king” and the Paracas National Park where it was discovered by Julia Clarke from the University of Texas. Clarke’s team are no strangers to giant fossil penguins. In 2007, they unveiled two extinct species: Perudyptes, about the size of the modern king penguin; and Icadyptes, which was larger than any living species and had an unusually long, spear-like beak. Like Icadyptes, Inkayacu also swam off the coast of ancient Peru, had a long beak, and was one of the largest penguins in history. It weighed around twice as much as the heaviest of today’s penguins – the emperor.

In many ways, Inkayacu is no more significant a find that Icadyptes was three years ago. It is neither the oldest nor the largest penguin fossil, it doesn’t hail from a new part of the world, and it provides few clues about the group’s evolution. However, it does have one stand-out feature that probably secured its unveiling in the pages of Science – its feathers.

These are so well preserved that in some, you can even see the individual filaments (barbs) branching off from the central stem (rachis). Clarke could inspect their structure are a microscopic level and based on that, she could work out what colour they were.

Clarke teamed up with Jakob Vinther, who has become somewhat of a specialist in the palettes of extinct feathered animals. Vinther pioneered a technique that reconstructs the colours of fossil feathers, including those of dinosaurs, by analysing their melanosomes, small structures that are partly responsible for the colours of modern bird feathers. They come in two major types: spherical ‘phaemelanosomes’, which contain a reddish-brown or yellow pigment; and rod-like ‘eumelanosomes’, which have black-grey tints.

To understand Inkayacu’s melanosomes, the team first had to analyse those of modern penguins. They found something completely new: the striking black colours of the living birds come from giant eumelanosomes that are around 50% wider than those of any other bird.

But none of Inkayacu’s melanosomes came close to these dimensions – no dinner jacket look for this bird. Instead, based on the size, shape, distribution and packing of its melanosomes, the team concluded that Inkayacu must have been gray or reddish-brown in colour. These colours are more commonly associated with baby penguins, but Inkayacu’s skeleton suggests that it was an adult.

This raises an obvious question: why did later penguin species evolve giant melanosomes? There may have been several advantages to looking blacker, perhaps including a better ability to regulate temperature. But other bird species manage to look black with far melanosomes. Clarke speculates that penguins did so in a unique way as an adaptation to life underwater.

Inkayacu’s body already had many of the features that make living penguins such graceful swimmers. Its flippers were narrow and streamlined. The central rachises of its feathers were broad and flattened allowing the feathers to pack tightly together. This arrangement would have reduced turbulent flows of water around the flipper, allowing the bird to cut smoothly through the water.

Once those large-scale features were in place, tweaks to the nano-scale structures of the feather might have led to even better performance. Clarke points out that the melanin doesn’t just give feathers their colours; it also makes them more resistant to fractures. For now, this is still speculation. Perhaps Peru will yield well preserved fossils that will provide an even clearer picture of how evolution fine-tuned the penguin’s body.

For more on Inkayacu, you can almost certainly find more detail at March of the Fossil Penguins – a blog devoted entirely to… well, you can guess.

Images: reconstructions by Katie Browne, Clarke photo by Adam Smith, bone/feather images by AAAS.

More on melanosomes:

What colours were dinosaur feathers?

The renaissance of technicolour dinosaurs continues (and the gloves come off…)

If the citation link isn’t working, read why here
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