Photo: Courtesy of Chinese Academy of Sciences

A joint research team from China and abroad has for the first time found that mammaliaforms from the Jurassic and Cretaceous periods exhibited highly similar, uniform dark coloration, the Chinese Academy of Sciences reported on Tuesday. The discovery has been published in Science magazine. 

"This phenomenon of mammaliaforms that lived during the Jurassic and Cretaceous periods exhibiting uniform dark coloration may be linked to their ecological niches and nocturnal habits," Li Ruoshuang, the study's first author from the China University of Geosciences, told the Global Times on Tuesday. "This dark color may have helped them hide from dinosaurs."

According to Li Quanguo, Li Ruoshuang's advisor, these extinct early mammals had diverse lifestyles, ranging from ground-dwelling, and burrowing to gliding through treetops like modern flying squirrels. The exceptional preservation of these fossils and their fur impressions allowed researchers to reconstruct both their ecology and coloration.

Most modern mammals are covered in fur and exhibit a wide variety of colors and patterns that play important roles in temperature regulation, camouflage, species recognition, and sexual display. However, the evolution of mammalian fur color and patterns has long remained a mystery, according to the China University of Geosciences. The research team's goal is to unravel this mystery.

The team focused on six exceptionally well-preserved early mammal fossils from the Jurassic Yanliao Biota and the Cretaceous Jehol Biota in China. These fossils included Megaconus mammaliaformis (one of the earliest known hairy mammals), the gliding Arboroharamiya fuscus (a newly identified euharamiyidan species), and other tree-climbing and burrowing mammals, according to the research team. 

Using advanced techniques such as microspectrophotometry, high-powered electron microscopy, and new statistical models, the team analyzed melanosome morphology in the fossilized fur and compared it to data from 116 modern mammals. The result shows that in modern mammals, darker hairs contained longer melanosomes, while brighter hairs contained rounder melanosomes, according to the China University of Geosciences. 

Modern mammal melanosomes display a wide range of morphological diversity. In contrast, the early mammals exhibited strikingly invariant melanosome shapes which were predicted to produce dark colors. 

According to the research team, despite the varied taxonomy and lifestyles of early mammals, all fossil specimens exhibited uniformly dark colored fur, devoid of patterns, countershading, or sexual dichromatism - a stark contrast to the vibrant colors seen in contemporaneous dinosaurs and modern mammals.

"Modern mammals exhibit a greater diversity in coloration and melanosome morphology, a variation that may not have emerged until after the Cretaceous mass extinction," Li Ruoshuang noted.  

According to the research team, following the end-Cretaceous extinction of dinosaurs, mammals rapidly radiated and occupied more diurnal ecological niches. This shift likely drove the evolution of more diverse body colors and striking patterns to accommodate needs such as species recognition, sexual selection, and camouflage.  

Zhou Changfu, a paleontologist at the Shandong University of Science and Technology, added that after the extinction of non-avian dinosaurs, mammals were free to diversify into the wide array of colors we see today, from the fiery orange of tamarins to the soft gray of koalas and the bold black-and-white patterns of zebras.

"We plan to expand their study by analyzing early mammal fossils from other regions around the world, though we do not expect significant differences in the results," said Matthew Shawkey, a member of the research team from Ghent University in Belgium.

This research was led by the China University of Geosciences (Beijing) and Ghent University in Belgium. Other participants include researchers from the Shandong University of Science and Technology in China, the Naturalis Biodiversity Center in the Netherlands, the University of Texas at Austin, US, and the University of Bristol in England.