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Imagine stepping back over 120 million years into the Early Cretaceous or Jurassic Period, where dinosaurs roamed with striped tails and vibrant crests. You would also encounter primitive birds adorned with iridescent feathers, and expansive forests of massive tree-like ferns. However, your closest relatives at that time—early mammals—would likely remain elusive and unremarkable. These small, primarily nocturnal creatures were characterized by a lack of color and flair, a detail emphasized in a recent study published in the journal Science on March 13.
This groundbreaking research marks the first comprehensive exploration of early mammals’ coloration. Utilizing pioneering fossil imaging techniques and extensive analysis of pigment-producing cells in contemporary mammals, the research team has gained insights into the appearance of our ancient relatives. Their examination of six specimens across five distinct mammalian or near-mammalian groups revealed a clear pattern: these early mammals exhibited unpatterned, dark-brown fur.
While this may not seem like thrilling news at first glance, it holds significant implications for our comprehension of mammalian evolution. Such discoveries may pave the way for more vibrant reconstructions of extinct species in the future.
“When I was a child, fossil books stated that we would never ascertain the colors of extinct species,” remarked Steve Brusatte, a paleontologist and evolutionary biologist from the University of Edinburgh who did not participate in the research. “Studies like this continually astonish me as they accomplish what once seemed impossible. This is remarkable work,” he added.
The lineage of mammal-like animals emerged during the Mesozoic Era, often referred to as “the Age of Reptiles.” Paleontologists have theorized for a long time that these early species were relatively small, typically no larger than modern rodents, and predominantly nocturnal to evade predators. For years, physical features like eye socket sizes were the primary indicators supporting this nocturnal behavior. Now, the revelation of these early mammals’ color adds another layer of understanding.
Despite modern mammals boasting a diverse color palette—ranging from rich purples to vibrant oranges—fur coloration serves a crucial role beyond aesthetics. It plays vital functions such as thermoregulation, camouflage, mate attraction, communication, and defense. Insights into the coloration of ancient mammals provide a deeper understanding of their lifestyles and the evolutionary timeline of present-day diversity, according to Matthew Shawkey, co-senior author and evolutionary biologist at the University of Ghent in Belgium.
Shawkey noted, “These findings reaffirm our perception of early mammals as shadows to the dinosaurs. Their size and diversity were limited not only by the dominance of dinosaurs but also by their muted coloration. They essentially served as ‘dinosaur food.’”
To arrive at this insight, Shawkey and his colleagues compiled a database of contemporary mammals. They measured the colors of 116 species through spectrophotometry and employed electron microscopy to scrutinize the melanosomes within their hair follicles. The data revealed correlations between hair color and the morphological characteristics of melanosomes, leading to a predictive model for coloration.
The research team then applied these methods to six exceptionally well-preserved fossils, dating from approximately 165 to 120 million years ago, discovered in northeastern China. These fossils were chosen for their intricate details, including fur impressions, representing various early mammals. Among them was a new species, Arboroharamiya fuscus, a tree-dwelling creature equipped with prehensile claws and a furry membrane, allowing it to glide between branches.
The analysis of these ancient melanosomes revealed they were largely uniform in shape—oval and mid-sized—indicating limited color diversity among the species. Predicted to exhibit a dull grey-brown hue, this muted coloration likely enhanced their ability to blend into the nocturnal environment and retain warmth. “We stemmed from such modest beginnings,” Shawkey concluded.
Luke Weaver, a paleontologist at the University of Michigan who was not involved in the research, expressed enthusiasm about the findings. He stated that this research provides a previously hidden insight into mammalian ecology.
[ Related: This extinct tree-dwelling mammal may be among humans’ closer relatives.]
However, Weaver highlights the limitations of this study, noting that the sample size is small and represents only a specific time frame, geographic area, and subset of early mammals. He cautioned that the dataset does not lend itself to extensive conclusions about later Cretaceous mammals, as it primarily reflects the earlier phase of the period.
The authors propose that early mammals were constrained in their ecological and evolutionary pathways until the dinosaurs’ extinction. Yet, Weaver refers to recent research indicating that diversification may have occurred up to 30 million years before the dinosaur extinction. Evidence shows some early mammals exhibited advanced social behaviors, and some were even predator species rather than mere prey.
“This narrative of Mesozoic mammals as diminutive creatures just hiding is common; however, it doesn’t apply universally,” Weaver stated. “Many fascinating developments occurred, especially during the late Cretaceous.”
To ascertain the extent of the trend toward dull coloration in early mammals, further research will be necessary. Expanding the study to include a broader array of specimens could help determine when mammals began developing a variety of markings and more vibrant colors. A graduate student in Shawkey’s lab is already embarking on such a project, aiming to chart the timeline of fur evolution.
The findings not only shed light on our own evolutionary heritage but also emphasize the importance of preserving fossils and museum collections. As Weaver points out, when these specimens were originally gathered, the technological capabilities to explore these questions were lacking. “The potential insights we can gain from existing fossils are unknown until we apply innovative techniques,” he added.
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