How can anyone know what colour a dinosaur was?

By reading tiny pigment capsules called melanosomes. Skin pigmentation is nearly always lost during fossilization, so colour is one of the unknowns of paleontology. But in some fossil skin and feathers, melanosomes, the organelles that make, store and transport melanin, are preserved, and different shapes of melanosome determine different colours. Rod-like melanosomes correspond to dark shades, while spherical ones make red pigment.

Researchers measure the fossil capsules and compare them with those of modern birds. In one Anchiornis, a feathered dinosaur, this revealed black, white and grey feathers all over its body and a crest of dark red or ochre feathers on its head. It was the first Mesozoic dinosaur for which almost the entire life coloration was known. Microraptor, studied in 2012, had iridescent black feathers, stacked melanosomes reminiscent of a blackbird. And Borealopelta, an armoured dinosaur, was reddish brown and counter-shaded, which may indicate that even this large armoured animal was under threat of predation.

There was a plot twist. Dark carbon smears in fossil feathers were once thought to be traces of bacteria that had decomposed them, but they are in fact microscopic imprints of fossilized melanosomes.

The method has limits, and scientists argue over it. A second Anchiornis specimen studied in 2015 showed only gray-black melanosomes, with none of the rufous ones, and the researchers suggested explanations such as different preservation, the colour changing with age, or even different species. Without knowing the structure of a feather's keratin layer, which does not fossilize well, no hue can be assigned for certain.