Mesozoic mammaliaforms illuminate the origins of pelage coloration.

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 mammaliaforms that coevolved with dinosaurs. In this study, we used a dataset of melanosome (melanin-containing organelle) morphology and quantitatively measured hair colors from 116 extant mammals to reliably reconstruct the coloration of six Mesozoic mammaliaforms, including a previously undescribed euharamiyidan. Unlike the highly diverse melanosomes discovered in feathered dinosaurs, hairs in six mammaliaforms of different lineages and diverse ecomorphotypes showed uniform melanosome geometry, corresponding to dark-brown coloration consistent with crypsis and nocturnality. Our results suggest that the melanosome variation and color expansion seen in extant mammals may have occurred during their rapid radiation and diversification after the Cretaceous-Paleogene extinction. Editor's summary: In the past decade or so, the identification of melanosomes in some dinosaur feathers and skin has led to the suggestion that many bore bright and complex coloration. Less work has been done to explore the coloration of early mammals. Li et al. looked at melanosome patterns in multiple species of Mesozoic mammals and compared these with those seen in more than 100 species of extant mammals. There was little color variation, and the animals were entirely dark in color. The authors argue that this was likely due to the animals' nocturnal nature, and that brighter and more varied colors in mammals may have arisen after the extinction of dinosaurs at the end of the Cretaceous period. —Sacha Vignieri [ABSTRACT FROM AUTHOR]