Around 150 million years ago, when dinosaurs ruled the Earth, research results suggest that early mammals were likely covered in dark brown fur. This indicates that mammals at the time were primarily nocturnal and may have utilized protective coloration to hide from predators.
An international research team, including members from the China Geological Survey and Ghent University in Belgium, unveiled the results of their reconstruction of Cretaceous mammal colors in the international journal 'Science' on the 14th.
The coloration of animals serves various roles. It can act as a means of communication and perform camouflage functions to evade predators. While birds boast vibrant feathers, mammals primarily rely on a single pigment called 'melanin' and generally exhibit darker tones. Instead, they showcase various appearances through fur patterns such as stripes and spots.
However, there has been little direct evidence of the fur color of extinct mammals. Recent discoveries have revealed that 'melanosomes', the organelles responsible for pigments, can be preserved in fossils. As a result, researchers used this information to analyze and restore the fur colors of Cretaceous mammals.
The research team examined melanosomes from 116 species of modern mammals using scanning electron microscopy and spectroscopic analysis. The analysis showed that the fur color is determined by the size and shape of the melanosomes. Animals with red or yellow fur had rounder melanosomes, while those with dark brown or black fur exhibited elongated melanosomes.
Based on the analysis of today's mammals, the research team developed a model that can predict fur color based on melanosome size and shape. An analysis of six species of mammal fossils that lived during the late Jurassic period, 158.5 million years ago, revealed that early mammals possessed relatively uniform and oval-shaped melanosomes. Applying the newly developed model suggests that the fur color of the time was predominantly a deep and uniform brown or black. Patterns such as stripes or spots commonly seen in modern mammals were not found.
The research team noted that these findings support the existing hypothesis that early mammals were nocturnal. Modern nocturnal mammals, such as mice, moles, and bats, predominantly have dark fur to evade the eyes of predators. In contrast, feathered dinosaurs or pterosaurs from the same era exhibited a variety of melanosome structures. The researchers explained, "Dinosaurs likely required a broader range of colors to emphasize visual signals in courtship behavior or communication, whereas mammals did not need vivid colors while leading a nocturnal lifestyle."
The research team also proposed that melanin may have contributed to temperature regulation and physical protection. Dark fur, rich in melanin, could absorb sunlight more effectively to help maintain body temperature and may have been resilient against friction or damage, thus protecting the skin.
The researchers stated, "It appears that after the Cretaceous-Paleogene extinction event that led to the extinction of dinosaurs, mammals adapted to diverse habitats, evolving a variety of fur colors. Once the dinosaurs disappeared, it is quite likely that mammals occupied ecological niches and began a daytime lifestyle, leading to the emergence of brighter colors and patterns like stripes and spots, similar to today's squirrels and giraffes."
References
Science (2025), DOI: https://doi.org/10.1126/science.ads9734