The Mystery of Body Coloration in Mesozoic Mammaliaforms

Body coloration plays an important role in the behavior, physiology, and habitats of animals, and mammals are no exception. Modern mammals display a wide variety of colors and patterns. Yet the coloration of early mammaliaforms that lived during the Mesozoic Era has long remained largely unknown.

A recent study addressed this question by analyzing the coloration of six Mesozoic mammaliaforms discovered in northeastern China. These fossils span a long time interval, from the Middle Jurassic to the Early Cretaceous, covering more than forty million years. This temporal range provides valuable insight into the early evolutionary history of mammalian coloration.

Research Methods

As in previous studies reconstructing the colors of fossil organisms, the key material used in this research was melanosomes, the microscopic organelles that contain melanin pigments.

The research team first examined 116 species of living mammals with diverse appearances. By analyzing the shapes of their melanosomes and measuring the colors of their hair, the scientists established a database linking melanosome morphology with hair coloration. This relationship makes it possible to infer the color of fossil animals from the preserved shapes of their melanosomes. In general, elongated melanosomes are associated with darker colors, while rounder melanosomes tend to correspond to yellow or reddish hues.

Once this relationship was established, the researchers compared it with melanosomes preserved in the fossils of six Mesozoic mammaliaforms. These taxa include:

1. Arboroharamiya fuscus

2. Megaconus mammaliaformis

3. Vilevolodon diplomylos

Vilevolodon diplomylos specimen（Image source：Jonathan Chen， CC BY-SA 4.0 ）

4. Docodontan specimen SDUST-V0006

5. Docodontan specimen SDUST-V0007

6. Eutherian specimen SDUST-V0008

Research Results

The analyses revealed a surprising result. All six animals were reconstructed as having dark brown coloration. None showed evidence of stripes, spots, gradients, or other patterns. Their fur appears to have been uniformly colored.

For the researchers, this result was both expected and unexpected.

It was expected because many Mesozoic mammaliaforms lived under strong predation pressure from dinosaurs. As a result, these small mammals are widely believed to have been primarily nocturnal. Animals that are active at night typically do not rely heavily on conspicuous coloration. Modern bats provide a familiar example, as most species possess relatively plain body colors.

However, what surprised the researchers was the remarkable consistency of the coloration. Despite differences in evolutionary lineage, ecology, and lifestyle, all six species appear to share essentially the same dark brown color.

Interpretations

Because these animals lived across a very long time span and occupied a variety of ecological niches—including terrestrial, burrowing, arboreal, and gliding lifestyles—the researchers proposed several possible explanations for their uniform coloration.

One explanation involves camouflage. For nocturnal mammals living alongside dinosaur predators, darker body coloration would allow them to blend more effectively into the nighttime environment, reducing the risk of detection.

Another possibility relates to thermoregulation. Nighttime temperatures are generally cooler than daytime conditions, and animals active at night must cope with heat loss. Dark coloration absorbs heat more efficiently, which may help small animals retain warmth.

The researchers also plan to extend their investigation to fossils from other parts of the world. Nevertheless, they suspect the results may be similar. They propose that mammalian coloration remained relatively uniform throughout much of the Mesozoic. Only after the end-Cretaceous extinction, when non-avian dinosaurs disappeared and mammals rapidly diversified into newly available ecological niches, did mammalian coloration begin to diversify.

A Newly Described Species

Among the six animals examined in this study, Arboroharamiya fuscus represents a newly described species. It belongs to the mammaliaform group Haramiyida and lived about 160 million years ago in what is now Inner Mongolia, China.

The fossil preserves clear impressions of a patagium, indicating that this animal possessed a gliding membrane. Combined with other anatomical features, this suggests that Arboroharamiya fuscus was an arboreal gliding mammal. The preserved patagium also contains abundant melanosomes, providing excellent material for studying its body coloration.

Conclusion

During the past decade, researchers have discovered melanosomes preserved in the feathers and skin of certain dinosaurs, allowing scientists to reconstruct a wide range of complex and colorful dinosaur patterns. In contrast, relatively little research has focused on the coloration of early mammaliaforms.

This study helps fill part of that gap by revealing the general trend of body coloration in Mesozoic mammaliaforms. At the same time, the discovery of the new species Arboroharamiya fuscus adds further diversity to the known mammaliaform fauna of the region, improving our understanding of the composition of these ancient ecosystems.

Future research expanding this approach to additional fossil sites around the world may reveal even more about the evolution of mammalian coloration and help clarify many remaining mysteries.

Author: Bai Leng

Reference:

Li, R., D’ALBA, L., Debruyn, G., Dobson, J. L., Zhou, C. F., Clarke, J. A., Vinther, J., Li, Q., Shawkey, M. D. (2025). Mesozoic mammaliaforms illuminate the origins of pelage coloration. Science.

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