Dinosaur books have become more colourful affairs of late, with the dull greens, browns and greys of yesteryear replaced by vivid hues, stripes and patterns. This has largely been a question of artistic licence. While fossils may constrain an artist’s hand in terms of size and shape, they haven’t provided any information about colour. But that is starting to change.
The fossils of some small meat-eating dinosaurs were covered in filaments that are widely thought to be the precursors of feathers. And among these filaments, a team of Chinese and British scientists have found the distinctive signs of melanosomes, small structures that are partly responsible for the colours of modern bird feathers.
Melanosomes are packed with melanins, pigments that range from drab blacks and greys to reddish-brown and yellow hues. Their presence in dinosaur filaments has allowed Fucheng Zhang to start piecing together the colours of these animals, millions of years after their extinction. For example, Zhang thinks that the small predator Sinosauropteryx had “chestnut to reddish-brown” stripes running down its tail and probably a similarly coloured crest down its back. Meanwhile, the early bird Confuciusornis had a variety of black, grey, red and brown hues, even within a single feather.
Zhang’s discovery also launches another salvo into a debate over the very nature of “feathered” dinosaurs. Beautiful fossils, mainly from China, show that several species of dinosaur had feathers akin to the flight-capable plumes of modern birds. Species like Caudipteryx and the four-winged Microraptor had true feathers with asymmetric vanes arranged around a central shaft.
The feather is an extraordinary biological invention and the key to the success of modern birds. It has to be light and flexible to give birds fine control over their airborne movements, but tough and strong enough to withstand the massive forces generated by high-speed flight. It achieves this through a complicated internal structure that we are only just beginning to fully understand, with the aid of unlikely research assistants – fungi.
At a microscopic level, feathers are made of a protein called beta-keratin. The same protein also forms the beaks and claws of birds, and the scales and shells of reptiles. It’s close (but less rigid) relative, alpha-keratin, makes up the nails, claws and hairs of mammals. Zoom out, and we see that feathers have a central shaft called the rachis with two vanes on either side. Each vane is composed of barbs that branch off the rachis. Even thinner barbules branch off from the barbs, and are held together by small hooks that give the feather its shape.
What’s much less clear is how the keratin fibres and filaments are organised into the rachis, barbs and barbules. To work that out, scientists would typically slice the rachis in cross-sections and look at it under an electron microscope. But feathers don’t give up their secrets so easily. Their fibres are stuck together with a chemical glue that makes them virtually impossible to separate. Imagine gluing a bundle of matches together and cutting them cross-ways. You could see the fibres that make up the component matches, but if they were glued together tightly enough, you wouldn’t be able to tell where one match started and another began. So it is with feathers and their keratin.
Theagarten Lingham-Soliar from the University of Kwazulu-Natal solved the problem by recruiting fungi as research assistants. He used four species, which like to grow on keratin, to digest the complex molecules that glue individual filaments together. The process was very slow. Even after a year, the feathers seemed in pretty good shape and it was only after 18 months that they had broken down enough to be studied under the microscope.