Pelage coloration, serving numerous functions such as thermoregulation, crypsis, aposematism, sexual display and communication, has been playing a crucial role in the evolution of behavior, physiology, and habitat preferences of mammals (Caro, 2005). However, the scientific discussion on the evolution history of integumentary coloration in amniotes has primarily focused on dinosaurs and early birds. Limited data exist on the color of extinct mammals known only from fossils, and no such attempt has been made for Mesozoic mammaliaforms.
Colors of mammalian hairs are produced by a mixture of two morphologically different types of melanosomes containing either eumelanin (producing blacks and browns) or pheomelanin (producing yellows and reds), therefore, the colors of hair have been associated with the morphology of melanosomes. Here, we reconstructed the pelage coloration in six Mesozoic mammaliaforms of different phylogeny and ecomorphotypes of the Yanliao and Jehol Biotas from northeastern China, using melanosome morphology and trace metal distribution, and provided novel insights into the evolutionary history of mammalian pelage coloration. We developed a predictive model by using MCMCglmm to analyze melanosome geometry (via scanning electron microscopy) and microspectrophotometric color data from 116 extant mammalian species (covering Prototheria, Eutheria and Metatheria), revealing that melanosome length and width correlate with color expression: elongate melanosomes correspond to darker, unsaturated hues (e.g., blacks and browns), while spherical melanosomes associate with brighter tones (e.g., yellows and reds). Melanosome impressions preserved in the six fossils were analyzed across taxa from Haramiyida (including the newly described Euharamiyidan Arboroharamiya fuscus), Docodonta, and Eutheria, representing arboreal gliders, fossorial, terrestrial, and scansorial ecomorphotypes, spanning from the Middle Jurassic to the Early Cretaceous.
Results indicate uniformly dark-brown pelage in all fossils specimens, with melanosome geometries constrained to low aspect ratios (length/width < 2) and sizes overlapping the upper range of extant mammals. X-ray fluorescence detected elevated copper (linked to eumelanin) and absence of zinc (associated with phaeomelanin), corroborating dominantly eumelanin-based, non-patterned coloration. The absence of striping, countershading, or sexual dichromatism suggests limited roles for visual communication or intraspecific signaling, contrasting with extant mammals. Instead, dark integument likely conferred crypsis, thermoregulatory efficiency, and abrasion resistance in these small-bodied, nocturnal Mesozoic mammaliaforms. This conserved coloration aligns with inferred nocturnality in early mammaliaforms (Gerkema et al., 2013).
Notably, Mesozoic mammaliaforms exhibit melanosome uniformity in stark contrast with the diverse melanosome morphotypes of coeval feathered dinosaurs, early birds and pterosaurs (Li et al., 2014; Cincotta et al., 2022), suggesting a different and unique evolutionary pattern of mammaliaform melanin color system after the sauropsid-synapsid split. After the Cretaceous-Paleogene event, mammals rapidly diversified into vacant niches previously occupied by dinosaurs (Maor et al., 2017), which could have simultaneously propelled the rapid radiation and diversification of pelage color strategies in new and diverse environments

mammaliaforms,melanosome,paleocolor,nocturnality