The early mammals that lived alongside the dinosaurs upwards of 150 million years ago (mya) were likely covered in dark and dusky greyish-brown fur, according to a quantitative reconstruction of Mesozoic mammal coloration, hinting at their shrouded and nocturnal nature. The findings, drawn from a comparative analysis of fossilized melanosomes, provide insights into the ecology and evolutionary history of early mammals. 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. Using scanning electron microscopy and precise spectrophotometric data, Ruoshuang Li and colleagues analyzed melanosomes from 116 living mammals to create a predictive model for reconstructing pelage color based on melanosome morphology. Li et al. then applied the model to fossilized melanosomes of 6 Mesozoic mammaliaforms – including a newly described euharamiyidan species from the Late Jurassic (~158.5 mya). 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. According to the authors, 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. 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.