Fossils reveal what the fur of early mammals looked like

While many dinosaurs and pterosaurs flaunted flamboyant feathers, early mammals were a dull lot. A study of the fossilised fur of six mammals that lived during the Jurassic and Cretaceous periods has found they all had greyish-brown fur.

“They were dinosaur food,” says Matthew Shawkey at Ghent University in Belgium. “You didn’t want to be conspicuous.”

Working out what animals that lived in the distant past looked like used to be thought impossible. But since the 1990s, thousands of fossils with feathers and fur have been discovered.

In some cases, traces of melanosomes – cell organelles that contain the pigment melanin – can be seen when these fossils are examined under a microscope.

Melanin comes in two variants – black-brown and yellow-red – and the shape of melanosomes varies according to their composition. So, knowing the shape of melanosomes in fur or feathers gives you a good idea of their colour.

Shawkey’s team started by looking at the melanosomes in the fur of a diverse range of 116 living mammals. From this, the researchers developed a model that predicts fur colour based on melanosome shape and applied this to six fossils of different early mammals.

All six fossils came from the same deposits in China, but the species lived at different times ranging from the Middle Jurassic to the Early Cretaceous, around 165 million to 120 million years ago. One of them was a newly described gliding mammal named Arboroharamiya fuscus that lived around 159 million years ago.

Given that these mammals are all thought to have been nocturnal, it is no surprise that they were all rather plain.

“We expected them to have pretty subdued colours,” says Shawkey. “The one thing I was surprised by is how invariant they were. The colours were even more similar than I would have predicted.”

The team plans to expand its study by looking at additional early mammal fossils from elsewhere in the world, but Shawkey doesn’t expect the results to be much different. It was only after the extinction of the dinosaurs 66 million years ago that many mammals became active in the daytime and that is probably when their colours became more diverse, he says.

Some fossils of dinosaurs and marine reptiles include preserved skin, but there have been few attempts to work out their skin colour from fossils.