Pelage coloration, which serves numerous functions, is crucial to the evolution of behavior, physiology, and habitat preferences of mammals. However, little is known about the coloration of Mesozoic mammals that coevolved with dinosaurs. In a new study, scientists from China, Belgium, Netherlands, the United Kingdom and the United States used a dataset of melanosome (melanin-containing organelle) morphology and quantitatively measured hair colors from 116 living mammals to reconstruct the coloration of six Mesozoic mammals, including a previously undescribed species of euharamiyidan mammal.
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Life reconstruction of Shenshou lui, an extinct squirrel-like euharamiyidan species from the Middle Jurassic of China. Image credit: Nobu Tamura, http://spinops.blogspot.com.
From communication to camouflage, animal coloration plays an important role in numerous behavioral ecological functions.
While some animals, like birds, exhibit a striking and vivid array of plumage, mammal furs are generally limited to muted tones due to their reliance on the single pigment melanin.
Although lacking in palette, mammals have evolved diverse and distinctive coat patterns.
However, due to the scarcity of data on the pigmentation of extinct mammals, the evolutionary history of pelage coloration remains poorly understood.
Recent studies have shown that melanosomes — the organelles responsible for pigmentation — can be preserved in fossilized specimens.
Similar techniques have successfully reconstructed dinosaur coloration but have not been widely applied to fossil mammals, despite well-preserved fur specimens.
In new research, Dr. Ruoshuang Li from the China University of Geosciences and colleagues analyzed melanosomes from 116 living mammals to create a predictive model for reconstructing pelage color based on melanosome morphology.
The authors applied the model to fossilized melanosomes of 6 Mesozoic mammaliaforms, including a newly-described euharamiyidan species that lived 158.5 million years ago (Late Jurassic epoch).
The authors discovered that these early mammals’ fur was predominantly and uniformly darkly colored, without any patterns like the stripes and spots that adorn many modern mammals.
This suggests that, despite evolutionary divergence in their phylogeny and ecology, early mammals’ melanin color system remained largely unchanged.
This stands in stark contrast to the varied melanosome structures found in feathered dinosaurs, early birds, and pterosaurs, indicating a distinct evolutionary pattern for mammalian coloration.
“The dark, uniformly dull fur found in these species — typical of modern nocturnal mammals like moles, mice, rats, and nocturnal bats — supports previous hypotheses that early mammals were also largely nocturnal and colored for camouflage,” the researchers said.
“Additionally, the high melanin content in their fur could have been beneficial for thermoregulation and providing mechanical strength for protection.”
“Following the Cretaceous-Paleogene extinction event, mammals rapidly diversified into niches previously occupied by dinosaurs, leading to more diverse melanosome structures and new pelage color strategies better suited to a wider variety of environments.”
The findings were published today in the journal Science.
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Ruoshuang Li et al. 2025. Mesozoic mammaliaforms illuminate the origins of pelage coloration. Science 387 (6739): 1193-1198; doi: 10.1126/science.ads9734
