If you were to travel more than 120 million years back in time to the Early Cretaceous or Jurassic Period, you might see dinosaurs with striped tails and ruddy crests, primitive birds with iridescent feathers, and forests dominated by giant tree-like ferns. What you’d be less likely to notice are your closest relatives. Early mammals were small, strictly nocturnal, and altogether less showy–verging on downright drab, as supported by a study published March 13 in the journal Science.
The new research is the first to look back at early mammals in full color. Using advanced fossil imaging methods and a thorough examination of the pigment-producing cells present in living mammals, the team have uncovered what the fur of our long-lost kin probably looked like. Across six different specimens representing five distinct near-mammalian (mammaliaform) or mammalian groups, the researchers found striking uniformity. All of the early mammal fossils examined indicated the extinct animals had unpatterned, dark-brown coats.
Though perhaps not the most immediately exciting news, the finding is notable for illustrating what science has only previously been able to guess at. These new findings have implications for our understanding of mammalian evolution and hints at a future where we can reconstruct every era of the distant past in technicolor.
“When I was growing up, all of the books on fossils said that we would never know the color of extinct species,” Steve Brusatte, a paleontologist and evolutionary biologist at the University of Edinburgh who was uninvolved in the research, tells Popular Science. “So I’m always flabbergasted with studies like these that seem to do the impossible. This is fantastic work,” he adds.
The first mammal-like animals emerged alongside dinosaurs in the Mesozoic Era, aptly known as “the Age of Reptiles.” Based on fossilized bone and skeletal impressions, paleontologists have long theorized that these early pre-mammals (and the later true mammals) were fairly small. Most were no larger than rodents, and they were predominantly active at night to avoid predation. For decades, measures of eye sockets have been some of the only concrete evidence of that nocturnal lifestyle. Now, this glimpse at some of the first mammals’ true colors adds proof.
Though we mammals may not be quite as colorful as our avian counterparts, modern species still display hues from purple to orange, along with varied patterns. Fur color isn’t just aesthetic. It’s a multi-functional adaptation that can serve purposes as varied as thermoregulation, camouflage, mate attraction, communication, and defense. Understanding how early mammals looked helps us understand how they lived and when all the present diversity evolved, says Matthew Shawkey, co-senior study author and an evolutionary biologist at the University of Ghent in Belgium.
The findings “confirm what we [thought] about early mammals–that they were living in the shadows of dinosaurs. Not only was their size and diversity constrained by the presence of dinosaurs, but also their coloration,” Shawkey tells Popular Science. “They’re basically small, dinosaur food.”
To reach that conclusion, Shawkey and his colleagues built a database of living mammals. They used spectrophotometry to quantify the color of 116 modern species, and an electron microscope to closely examine the melanosomes (melanin-production and storage sacs inside melanocyte cells) of each of those animals. The team found correlations between hair color and melanosome shape and size, and used those trends to build a predictive model.
Then the scientists took the same extreme, close-up view of six very well-preserved ~165 to 120-million-year-old fossils unearthed from northeastern China. The fossils were selected because they’re detailed enough to include intricate impressions of fur and individual hairs, and also represent a wide swathe of early mammals and mammaliaforms. The fossils included two flying squirrel-esque gliders, a mole-like burrower, a tree-climber, and two different terrestrial creatures. One of the extinct taxa, a tiny tree-dweller with prehensile claws and wide furry membrane for sailing between branches, Arboroharamiya fuscus, is described and named for the first time in this new study.
They found that the fossilized melanosomes were very similar to each other: oval shaped and mid-sized, generally lacking in the extremes present in some living mammals. Overall, the extinct species showed far less diversity than the modern ones. Using their mammalian model, the scientists predict that these melanosome measurements would have corresponded to a mousy grey-brown shade. There was some small variation between and within the specimens–“it’s biology after all,” says Shawkey–but not enough to call them different colors. Likely, their universal dull, dark coloring helped these early mammals to better camouflage at night and stay warm. “We came from such humble beginnings,” he says.
Luke Weaver, a paleontologist studying mammal evolution at the University of Michigan who wasn’t part of this study, is excited about the new research. “This gives a glimpse of mammal ecology that is otherwise hidden from view,” he says.
[ Related: This extinct tree-dwelling mammal may be among humans’ closer relatives.]
However, Weaver notes the study comes with limitations. The sample size of six fossils is small, encapsulating only a particular time period, region, and subset of all early mammals, he says– though adds that’s not abnormal for paleontology. He also cautions that this dataset doesn’t necessarily allow for broad inferences about mammals later on in the Cretaceous, as the fossils are mostly representative of the earlier part of the Period.
The study authors suggest early mammals were stuck in a narrow ecological and evolutionary lane until the extinction of the dinosaurs. But Weaver notes that recent studies have found evidence for earlier diversification events, as much as 30 million years before the dinosaurs died out. Some early mammals displayed advanced social behaviors, as documented in his own research, and some were even dino predators–not just prey.
“There’s often the tendency to think of mesozoic mammals as these kind of meager critters that were just running around hiding,” says Weaver. Though that may be true for many, particularly the earliest groups, “I would be hesitant to draw that conclusion about all early mammals,” he adds. “I think there’s a lot of interesting things that are happening, especially in the late Cretaceous.”
Additional research, examining a broader range of specimens, would be needed to assess how far-reaching the trend of brown-ish, boring mammals was, and when exactly mammals began to gain stripes, spots, patches, and brighter colors. A graduate student in Shawkey’s lab has already started a project to reveal just that, in an attempt to build a detailed timeline of fur evolution.
In addition to illuminating our own long evolutionary journey, Weaver and others hope this and similar work underscore the value of museums and preserving fossil remains. When these fossils were initially collected, “we didn’t have the technology to interrogate these sorts of questions,” he says. “You never know what sort of new insights you can get from old fossils.”
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