How Do We Know What Colour Dinosaurs Were? Fossil Pigment Under a Microscope
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For a century and a half every dinosaur in every book was painted grey, green or brown, on the reasoning that large reptiles are drab and that nobody could ever know. Since 2010 a few dozen species have had their colours read directly from the fossils, because the structures that make colour in a feather or a hair are small bodies of pigment that can survive in rock for a hundred million years and can be told apart under an electron microscope. The answers so far include a ginger dinosaur with a striped tail, a black-and-white one with an iridescent throat, and a camouflaged armoured giant.
What makes colour in an animal
Animal colour comes from two sources. Pigments are molecules that absorb some wavelengths and reflect others, and the commonest in vertebrates is melanin, which comes in two kinds, eumelanin for black and grey and phaeomelanin for reddish brown and ginger. Structural colour comes from the physical arrangement of layers at the scale of a wavelength of light, which produces iridescence, the blue of a jay's wing and the shifting green of a starling. Both leave traces. Melanin is packed into small capsules called melanosomes, a micrometre or so long, and the shape of the capsule differs by pigment, eumelanin in sausage-shaped rods and phaeomelanin in shorter spheres; structural colour needs melanosomes stacked in ordered layers, and the stacking can be seen.
The discovery
Microscopic bodies had been seen in fossil feathers since the early 1980s and were taken for fossilised bacteria that had eaten the feather. In 2008 Jakob Vinther, then a graduate student, looked at a 100-million-year-old feather from Brazil and noticed that the bodies were arranged in bands that followed the feather's own stripes, which bacteria would not do, and concluded that they were melanosomes. Within two years two teams had applied the method to feathered dinosaurs: one reported that Sinosauropteryx, a small Chinese predator, had a tail ringed in ginger and white, and the other mapped the whole plumage of Anchiornis, a four-winged dinosaur, as grey and black with white-spangled wings and a rusty crest, like a woodpecker's.
What has been read so far
The technique works only where soft tissue is preserved in fine detail, so the sample is small and biased toward small feathered species from a few exceptional sites:
- •Sinosauropteryx: a ginger and white striped tail, a ginger crest along the back, and a bandit mask across the eyes, with countershading that suggests open habitat
- •Anchiornis: grey body, black and white wings, red crest
- •Microraptor: glossy black all over with a blue-green iridescence, read from melanosomes stacked in the ordered layers that make structural colour, the same arrangement as a modern crow's
- •Borealopelta, an armoured dinosaur from Alberta: reddish brown above and pale beneath, countershading that in living animals indicates a hunted species in open ground, which is a surprise in a 1.3-tonne animal covered in armour
- •Psittacosaurus: countershaded in a pattern that a 2016 study matched to the light of a forest floor
- •Fossil eggs: a 2018 study found the pigments that colour birds' eggs in the shells of several dinosaurs, so that some laid blue-green or speckled eggs
What it cannot do
Melanin is only part of the palette. The carotenoids that make a flamingo pink and a goldfinch yellow are chemically fragile and have never been recovered from a fossil, and the porphyrins and pteridines that colour some birds and reptiles are lost too, so a dinosaur that was yellow, red or pink in the way a modern bird is would read as blank. Scaly skin is worse than feathers, since it preserves melanosomes rarely. There is also a live dispute about whether some of the structures reported are melanosomes at all, and about whether heat and pressure in the rock distort their shapes enough to change the reading, which laboratory experiments that cook modern feathers in a press have been used to test. The conclusion most workers accept is that the method is sound where the preservation is exceptional and silent everywhere else.
Why it matters
Colour is behaviour. Countershading, dark above and light below, is the pattern of an animal that is hunted and that lives in a particular light, so measuring it on Psittacosaurus and Borealopelta says something about the habitat and the predators of each; iridescence and a crest on Microraptor and Anchiornis say that these animals were displaying to each other, which implies good colour vision and some kind of courtship. The paintings in books have accordingly changed, from grey lizards to animals that look like birds, striped, spangled and, in at least one case, ginger.
The takeaway
Dinosaur colours are read from melanosomes, the microscopic capsules of melanin preserved in exceptionally fossilised feathers and skin, whose shape distinguishes black from ginger and whose ordered stacking indicates iridescence; the method was recognised in 2008 and has given Sinosauropteryx a ginger striped tail, Microraptor a glossy blue-black sheen and the armoured Borealopelta reddish countershading. Carotenoid colours such as yellow and pink do not survive, so the palette recovered so far is partial.