Fur colour of ancient mammal relatives revealed for the first time

Dark brown was in fashion for mammals and their relatives over 150 million years ago.

While it’s rare for hair, feathers and skin to become fossilised, it’s not completely unheard of. A group of well-preserved fossils from a variety of species unearthed in China have offered scientists the chance to look at the fur of small mammals from the Jurassic and Cretaceous.

By examining the remains of pigment-containing structures called melanosomes, the researchers have been able to uncover what colour the fur of these animals might have been. Despite coming from different groups, and having a range of different lifestyles, these ancient species all seem to have had uniformly dark hair.

Dr Neil Adams, who curates our fossil mammal collection, says that the methods developed in the paper could help to reveal the appearance of more extinct species.

“While scientists have looked at dinosaur and bird melanosomes before, not as many have looked at mammal melanosomes from this time,” says Neil. “As far as I’m aware, this is the first study which uses them to work out the colour and pattern of Mesozoic mammals.”

“It would be exciting to apply these new methods to other exceptionally preserved fossil mammals, like those from the 47-million-year-old site of Messel in Germany. This would allow us to test how the diversity of colour changed at different times across different mammal groups and in other parts of the world.”

The findings of the study were published in the journal Science.

How do we know the colours of ancient animals?

Animals have adapted their skin in many different ways for many different functions. Some have developed thick fur, others armoured scales, yet more bristly feathers.

In addition to the structure, the colour of these features often plays a role in what they’re used for. Dark fur, for instance, can help an animal to absorb heat; greens and browns are useful to blend into woodland and bright reds and blues can signal to potential mates.

But while some groups of animals can adorn themselves in a rainbow of colours, mammals are generally limited to shades of blacks, browns, greys, yellows and reds. Only one mammal, the golden mole, is able to go beyond this with its iridescent hairs.

This is because modern mammals rely on two pigments for their hair colour. Eumelanin provides the blacks and browns, while phaeomelanin is used for reds and yellows. They are both contained within tiny cell structures called melanosomes, whose size and shape are linked to the colour they contain.

While these pigments are thought to have evolved tens of millions of years ago, it was thought to be almost impossible to know what colour ancient mammals were. This is because the melanosomes either weren’t preserved or were mistaken for fossilised bacteria.

This changed in the late 2000s, when researchers studying ancient feathers of fossil birds and dinosaurs such as Anchiornis announced they had discovered their colourations. The shape of the melanosomes were preserved in the fossils, so they could be compared with those of living birds to reveal the colour of the long extinct animals.

While similar techniques have revealed the colouration of other dinosaurs, there’s not been as much focus on ancient mammals. The scientists behind the current study set out to change that.

China’s ancient mammals

The team examined six fossils of extinct mammal relatives discovered in the Yanliao and Jehol biotas in China. These deposits are renowned for preserving fossils in great detail, offering important insights into the evolution not only of mammals, but also pterosaurs, dinosaurs and flowering plants.

One of the mammal fossils the palaeontologists looked at was so distinct they named it as a new species called Arboroharamiya fuscus. It would have looked somewhat similar to a modern flying squirrel, with folds of skin between its body and limbs that allowed it to glide.

Other mammals from the sites included burrowing, climbing and ground-living animals from various parts of the mammal family tree. But when the researchers analysed their colouration, they found a surprising amount of similarities.

“The authors scanned hairs from over 100 modern mammals to find out which colours were associated with which melanosome shapes,” Neil explains. “They then compared the results to the fossilised melanosomes to predict the original colour of the extinct mammals, and found they were all very similar shades of dark brown.”

“It’s an interesting result, given their range of different lifestyles.”

The study speculates that the wider range of colours and colour patterns seen in living mammals evolved after the dinosaurs died out, when the diversity of our mammal ancestors exploded. But Neil isn’t yet convinced.

“This is mostly a Jurassic sample with only one specimen from the Eary Cretaceous,” adds Neil. “The idea that diversity in mammal colouration only really got going after the Cretaceous mass extinction isn’t impossible, but it overlooks growing evidence that suggests more modern mammal groups were diversifying during the Late Cretaceous.”

“We might therefore predict diversification in colour and patterning during this time too. Unfortunately, exceptionally preserved fossils with hairs and melanosomes from diverse Late Cretaceous mammals haven’t been found so far, but if they are it would be interesting to see whether the researchers’ theory is borne out.”