How do scientists know what colors dinosaurs were?

For a handful of dinosaurs, mostly feathered ones, researchers have found traces of melanin, the pigment behind blacks, browns, grays and reddish tones, or the tiny packages that hold it, called melanosomes. Because the shape of melanosomes in living animals tends to match feather color, scientists can make informed estimates of color in fossils. The method is strongest for patterns and melanin-based colors, and weaker for the full true appearance of the animal. [1] [2]

Melanosomes are durable, and they survive in some fossil feathers even when little actual melanin remains. According to BBC Science Focus, reddish phaeomelanin is typically found in round melanosomes about 400 to 500 nanometres wide, while black or brown eumelanin sits in sausage-shaped ones about 300 nanometres long. Long, skinny, flat or hollow melanosomes point to iridescence, and big, fat ones indicate gray or blue pigment, according to Live Science. [1] [3]

For the feathered dinosaur Anchiornis, a team measured melanosomes from 12 black, 12 brown and 12 gray modern bird feathers. Using length, width, aspect ratio and variation in shape, a statistical method predicted feather color with 90 percent accuracy. Applied to the fossil, it suggested a mostly gray body, white wing and leg feathers with black tips, and a ruddy crest. [4]

A second approach looks for the pigment chemistry itself. Johan Lindgren used time-of-flight secondary ion mass spectrometry to find eumelanin in fossils, including an Anchiornis specimen, and concluded that at least some of the microbodies in fossils are melanosomes. Derek Briggs called the matter closed. Other researchers, including Mary Schweitzer, argue that sampling a few chips could mislead, since the method covers only a limited number of spots. Edwards and colleagues use X-ray analysis, which does no damage, to map pigments across a whole fossil. [5]

Researchers agree that melanin colors can be reconstructed in some fossils, and the results include a black-and-white Archaeopteryx, a blue-black glossy Microraptor and an iridescent Caihong. They disagree about how far those reconstructions can be trusted as true colors. [1]

Maria McNamara of University College Cork cautions that many bright bird colors come from carotenoid pigments, which rarely fossilize. She says current reconstructions are a fair picture of melanin-based pigmentation but may have no relationship to the animals' actual color. A Smithsonian interview makes a similar point: a red cardinal feather would leave only melanosomes after fossilization, so melanin may show patterning but not actual hue. Most feather fossils are compressions that lose non-melanin color structures, and the interviewee suggested fossils preserved in calcium phosphate would be better targets. Dinosaur skin is mostly preserved as impressions, and none of the thin films studied so far has yielded pigment details. [1] [6]

Other pigments can survive in rare cases, such as porphyrin pigments in theropod dinosaur eggshells. [2]