Ancient mammals had mostly dark brown coats during the dinosaur era, new study reveals

CBC

16-03-2025

Ancient mammals that lived in the time of dinosaurs were mostly the same dark-brown colour, according to a new study providing clues about how those mammals evolved as they faced giant predators.
The study, published in the journal Science, used scientific techniques that have similarly shown the colouring of various dinosaurs and ancient birds from their fossils. Advances in dinosaur knowledge have trickled into museums and popular depictions of the animals over the past years, something the new study's authors hope can now happen for ancient mammals.
"Just the way the first colour map in an extinct dinosaur opened the door to a whole new area of inquiry, this paper also does that. And that's pretty exciting," said co-author Julia Clarke, a vertebrate paleontologist at the University of Texas at Austin.
Using six well-preserved fossil specimens and powerful microscopy, the researchers were able to detect the shape of pigment-producing parts of cells known as melanosomes. Previous work has shown that shape corresponds to the colour of the animal's fur.
Apart from building a clearer picture of those early mammals, knowing the colour is also a huge breakthrough, according to Clarke, for understanding other aspects of the animals' lives and how they evolved over the millennia. Present-day mammals, while not as colourful as birds, do have more variety than those early mammals.
"We know how important colours are for animals," said co-author Lilian D'Alba, an evolutionary biologist at the Naturalis Biodiversity Center in Leiden, Netherlands.
"It's a way for animals to interact with their environment. Colours can also tell us much about how these organisms interact with temperature, humidity, with other organisms. And so we we can get a lot of information from looking at colours."
Why colour matters
Coat colours in mammals are important for many things, according to D'Alba, like controlling their body temperature, hiding from predators using camouflage, and being a marker of aggressiveness or strength to other animals.
D'Alba said that future research could look at how mammals got more colourful coats — after a major extinction event, about 66 million years ago, killed off dinosaurs, their major predators.
"The mammals that survived suddenly found themselves in a place where there was plenty of space. They had new places to colonize," she said. This was in contrast to the lives of mammals during the time of dinosaurs, when they are believed to have been active mostly at night, in the darkness, and relied more heavily on senses of smell and touch rather than vision.
"So a lot of new different types of lifestyles evolved. And with these lifestyles, new environments, you see … an explosion of new species in mammals."
The paper suggests that these new species came with new colours, for all the expanded activities these new mammals could now do.
"It's long been the assumption that Mesozoic mammaliaforms were nocturnal, hiding in the dark to avoid being devoured by the many predatory dinosaurs and other animals would have snacked on these little animals. But, this fur colouration study is the first to find general support for this across all the fossils sampled," said Hans Larsson, associate professor and curator of vertebrate palaeontology at McGill University, who was not involved with the study.
Mesozoic mammaliaforms are early mammals that lived during the time of the dinosaurs.
He said the findings are interesting in understanding how being nocturnal may have led to the early evolution of many other things mammals have, including our large brains, eyesight, hearing, reproductive biology and parental care.
"Ecologically, it's interesting because it suggests mammaliaforms were able to take advantage of a nocturnal lifestyle for millions of years and perfect it to the point that mammals are the dominant nocturnal predators of many of today's ecosystems," Larsson said.
Fossils found in China
Doing research on the colours of ancient animals can be a game of waiting and luck, according to D'Alba. The paper relied on six fossil specimens found in China, which has a few areas of the perfect geology to find these kinds of fossils.
That's enough to draw an inference for most mammals during that era, according to Caleb Brown, curator of dinosaur systematics and evolution at the Royal Tyrrel Museum in Drumheller, Alta.
"Most dinosaurs are known from less than one skeleton, but yet we we make inferences about their taxonomy," he said.
"We have to live with that, but we also have to keep working on collecting more fossils because as you get more and more fossils, your statistical power increases."
At the moment, China is where these fossils, with preserved skin or hair or feathers are being found, according to D'Alba and Brown. A lot of it depends on luck.
"There's a suite of deposits in China that preserve basically lake deposits or volcanic ash deposits, and these are very fine grained sediments and very rapid burial," Brown said.
"And in those cases it's quite common to find dinosaurs with feathers, but also mammals with with hair."
D'Alba hopes research will continue on the mammals to learn more about those colours … and see that ending up depicted in popular culture.
"The last [Jurassic Park movie] was actually quite exciting for me because they showed feather, feathered dinosaurs, and some of them had the patterns that we predicted," D'Alba said.
"It's always good to see that some of the accuracy of our studies is is reaching the public."