Fossilized melanosomes reveal colour patterning of a sauropod dinosaur

Abstract Integumentary fossils have improved understanding of dinosaur physiology, appearance and ecological niches. Fossil melanin and fossil melanosome organelles that produced melanin have made it possible to reconstruct dinosaur colour patterns, evidencing fundamental but previously elusive behaviours like camouflage. However, the colouration of several important groups, including sauropods, is still unknown. Here, we propose the first evidence of colouration in a sauropod based on potential melanosome-bearing epidermal scales. The fossil skin originates from juvenile diplodocids from the Mother’s Day Quarry of the Morrison formation in Montana, USA. Scanning electron microscopy reveals two fossilized epidermal layers in the scales that vary in microbody and carbon density. Two distinct microbodies are grouped together and dispersed within the potential outermost epidermal layer. The first are oblong-shaped and interpreted as melanosomes. The nature of the second disc-shaped microbody is unclear, but their flat shape is reminiscent of platelet melanosomes, though they are smaller in size.