Dark coats may have helped the earliest mammals hide from hungry dinosaurs
The spots and stripes familiar to us today didn't arise until later in mammalian evolution
Small mammals during the Mesozoic Era (some illustrated) may have had uniformly dark coloring, allowing them to blend into nocturnal environments.
Chuang Zhao, Ruoshuang Li
Zebra stripes? Leopard print? Neither were in vogue among the earliest mammals during the Age of Dinosaurs.
Early mammals and their close relatives probably sported dark, drab coats from snout to tail, researchers report in the March 14 Science. The monochrome ensembles may have helped ancient mammals blend into their nighttime surroundings and evade predators.
Many dinosaurs — especially birds — showcase a vibrant array of colorful feathers. But the diversity of fur color among modern mammals is underappreciated, says Matthew Shawkey, an evolutionary biologist at Ghent University in Belgium. “There’s obviously lots of patterns, stripes, spots, blotches, all those types of things,” he says. “But also fairly diverse colors: grays, yellows, oranges.”
Yet very little is known about the evolution of mammals’ colors or their presence in the fossil record, Shawkey says. Though pigment-containing structures called melanosomes have yielded details about the bright feathers of extinct birds, details about fossilized mammals’ coat colors remain scarce, save for recent evidence of reddish fur in a 3-million-year-old mouse fossil.
Shawkey and his colleagues used a scanning electron microscope to analyze melanosomes in the hairs of 116 modern mammals, linking their shapes and sizes to the colors they produce. After organizing the known melanosomes by various physical features, the team used statistics to test their ability to predict which colors others would produce. Melanosomes responsible for brighter colors, such as reds and oranges, are rounder, the researchers found, while blacks and browns generally come from elongated melanosomes.
Working with colleagues in China, the team applied this predictive power to melanosomes in preserved hairs from six early mammals and close relatives that lived in China 120 million to 167 million years ago, mostly during the Jurassic Period — the middle phase of the Mesozoic Era. While modern mammals exhibit an extensive range of these pigment-bearing structures, those from the six protomammals fell within a narrow range associated with dark grays and browns across their bodies, suggesting the ancient beasts had the same dark shade throughout.
The early mammals in this study — like many during the Mesozoic Era — were small creatures much like rodents, shrews or moles. They also appear to have had similarly gray and brown coats. It’s not unexpected given the world they evolved in, Shawkey says.
“They were basically dinosaur food,” he says. “They’re going to be hiding in the shadows. So it makes sense that they were dark.”
Despite being a mix of gliders, burrowers and scamperers, all the mammals in the study had dull, dark fur. This suggests a nocturnal existence was ubiquitous for Mesozoic mammals and their relatives, regardless of their ecological role.
“I think this is the first good evidence that we have of an antipredation strategy among early mammals,” says Luke Weaver, a paleontologist at the University of Michigan in Ann Arbor who was not involved with the research.
Dark, melanin-rich fur may have been useful in other ways. It heats up easily, and potentially helped early mammals stay warm. The dark hairs may have also been particularly tough and wear-resistant, protecting the mammals’ skin.
Shawkey notes that the study was limited to six extinct species, leaving the possibility that some early mammals exhibited patterns or bright colors. The team’s monochrome hypothesis could be upended, he says, if paleontologists “find a [fossil] rat with a giant orange mohawk.”
Determining when those kinds of fashion statements first arose is a natural next step, Shawkey says. “When do we start seeing spots and these patterns? When do we start seeing light browns and oranges and things like that?”
It’s possible the extinction of nonavian dinosaurs some 66 million years ago played a key role. Mammalian diversity exploded in response, and the movement to a wide range of finally-safe daytime habitats may have triggered a broader spectrum of colors, too.
However, Weaver notes, some studies suggest that this diversification might have begun earlier. “There’s emerging evidence to suggest that [mammals] might have been diversifying ecologically — and potentially inhabiting more daylight habitats — prior to the extinction of dinosaurs,” he says. Sampling melanosomes from mammals that lived during the late Cretaceous Period, at the twilight of the Age of the Dinosaurs, would be particularly informative.
Maria McNamara, a paleontologist at University College Cork in Ireland, would like to know if mammals that lived in different biomes or latitudes during the Jurassic Period were similarly dark.
“We need more papers like this to be published,” McNamara says. “It’s really important to demonstrate that modern paleontology is much more than describing dusty old bones. It’s a thriving analytical science.”
