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NEWS FEATURE NEWS FEATURE Prehistoric animals, in living color Paleontologists are looking beyond bones to reveal the hues of prehistoric animals that vanished millions of years ago. But the young field has its share of disagreements. Amber Dance, Science Writer Michael Benton used to tell his paleontology students that Most of these colorful revelations have emerged from they would never know the true color of a dinosaur. After fossils that contain evidence of melanin pigments— all, even fossils that sport light or dark patches may not responsible for earth tones, such as red, black, brown, ’ indicate the creature s original hue. But in recent years, the and buff—or the tiny cellular bags, called melano- vertebrate paleontologist at the University of Bristol in the somes, which produce and store melanins. But some United Kingdom has had to revise those lectures. scientists are already identifying the brighter hues of In 2010, Benton and colleagues found evidence ancient snakes and insects. “Probably all of the colors that the feathered dinosaur Sinosauropteryx prima can eventually be identified,” predicts Benton. had reddish-brown stripes on its tail (1). The same year, The work is not only helping to repaint the colorful another group claimed that birdlike Anchiornis huxleyi pictures of dinosaurs that charm schoolchildren and bore a red crest on its head (2). Since then, others have discovered that a marine ichthyosaur was dark-colored, museum-goers. Color can provide clues about how and that the early bird Confuciusornis sanctus possessed animals lived, mated, or died. Bright feathers suggest dark feathers with light wing tips (3, 4) (Table 1). Sud- that animals used those hues to select a desirable denly, the color of prehistoric animals has become an mate, for example, whereas green coloration would active topic for research rather than speculation, turning offer camouflage amid foliage, indicating there must Benton into an optimist: “If you’re a scientist, never say have been predators about. “Color has roles in lots of anything is impossible,” he now tells students. different parts of animal behavior and social interactions,” Researchers reconstructed the plumage color of a Jurassic birdlike dinosaur called A. huxleyi using color-imparting melanosomes, the morphology of which had been preserved in fossil feathers. Image courtesy of Michael DiGiorgio (artist). 8552–8556 | PNAS | August 2, 2016 | vol. 113 | no. 31 www.pnas.org/cgi/doi/10.1073/pnas.1610795113 Downloaded by guest on September 27, 2021 Illustrations courtesy of Lucy Reading (graphic artist). says Nick Edwards, a paleontologist at the University be responsible for helping to preserve fossil feathers, of Manchester in the United Kingdom. says Benton. But the fledgling field of paleo-color has also gen- Nineteenth century fossil hunters certainly knew that erated controversy. Some scientists dispute whether melanins could stand the test of time. Some even wrote melanosomes could be preserved in fossils, and argue letters with the melanin pigment ink that they found that researchers are being fooled by what are in fact preserved in the ink sacs of fossilized squids. And it was fossilized bacteria. Others question the use of certain during his own studies of squid fossils in 2006 that Jakob metals as proxies for pigments. “I think what’s happening Vinther, then a graduate student at Yale University in is what often happens when people come up with new New Haven, Connecticut, began to wonder whether ideas,” says Luis Chiappe, Vice President for Research and melanins could persist in other fossils. A handful of sci- Collections at the Natural History Museum of Los Angeles. entists, including his supervisor Derek Briggs, had pre- “We got super excited because we had figured out ways viously seen minuscule, sausage-shaped formations in a of potentially devising the colors of these extinct ani- variety of fossils, and labeled them fossil bacteria (5, 6). mals... now we’re starting to realize it’snotassimple.” Vinther suggested these formations could be mela- nosomes instead. Melanosome shape, he reasoned, The Race to Color a Dinosaur could even give clues to color: sausage-shaped mela- In retrospect, it’s not so surprising that pigment traces nosomes contain the black pigment eumelanin, whereas can survive for so long. Melanins, derived from the amino round melanosomes hold reddish-brown pheomelanin. acid tyrosine, are remarkably tough polymers, and they Briggs, a paleontologist, was skeptical, but willing to tightly cross-link with proteins, such as the keratin in test the idea. He suggested that Vinther take a closer look feathers. This strengthens tissues, and could even at a fossil feather that had dark and light stripes. If the blobs Dance PNAS | August 2, 2016 | vol. 113 | no. 31 | 8553 Downloaded by guest on September 27, 2021 were melanosomes, they should be found only in the dark A. huxleyi specimen (8, 9). Lindgren concludes that sections. If they were bacteria, they ought to be all over the at least some of the microbodies in fossils are indeed feather. When Vinther examined the light sections with a melanosomes. scanning electron microscope, he saw nothing more than Vinther, now a paleontologist at the University of the imprint of the feather itself. But in the dark sections he Bristol, also uses ToF-SIMS; he and other scientists say found the oblong bodies, along with their impressions in this direct evidence of the chemical signature of the rock (7). His hypothesis appeared to be correct. melanin pigments clinches their argument. “As far as Once Vinther’s idea went public in 2008, the race I’m concerned, case closed,” says Briggs. was on to color in a dinosaur. It ended in a dead heat: However, some critics point out that these analysis Benton’s and Vinther’s teams unveiled the colors of methods may offer a misleading picture of pigmentation. stripe-tailed S. prima and red-headed A. huxleyi within Essentially, they can only look at a limited number of days of each other in early 2010 (1, 2). spots from a given specimen. Scientists have to chip off a Other discoveries followed fast, and Benton says small bit of the fossil to subject it to mass spectrometry; that the majority of paleontologists have come to accept to minimize damage they typically only look at a handful that the preserved blobs are melanosomes. Even Michael of chips. And Vinther and colleagues studied only Wuttke, the paleontologist recently retired from the one isolated feather to decide that Archaeopteryx lith- General Department of Cultural Heritage Rhineland- ographica was probably black (10). This kind of extrapo- Palatinate in Germany, who originally thought they were lation could be wrong, say Schweitzer and others. Imagine bacteria (5), says he is now “absolutely convinced” that trying to determine the coloration of a modern-day pea- melanosomes are preserved in hairs and feathers. cock from pigments at just a few dozen spots, she cau- However, evolutionary biologist Mary Higby Schweit- tions: you might not come up with the right pattern. zer, of North Carolina State University in Raleigh, calls To avoid this problem, Edwards and colleagues use the conclusions “overstated,” a position she admits has X-ray analysis (which causes no damage) to probe pig- made her unpopular among most paleo-color aficio- ments across an entire fossil. Along with collaborators nados. “I’m not against melanosomes preserving,” says Schweitzer. But, she adds, it’s “more parsimonious to at the Stanford Synchrotron Radiation Lightsource (SSRL) assume a microbial origin until disproven with data in Menlo Park, California, these researchers use a tech- other than pictures.” nique called synchrotron rapid-scanning X-ray fluo- rescence that causes different elements to emit a Rainbow of Possibilities characteristic burst of light. In fossil organisms such as Researchers have started to collect such data. Johan C. sanctus, the scientists see copper, which is one of Lindgren, a paleontologist at Lund University in Swe- several metal ions that can bind to melanins, and is also den, uses time-of-flight secondary ion mass spec- found in tyrosinase, an enzyme that controls the produc- trometry (ToF-SIMS) and other methods to analyze tion of melanins (4). Another technique, X-ray absorption the chemical composition of fossils. By exciting the spectroscopy, revealed that the copper was bound to or- fossil surface with an ion beam, Lindgren can mea- ganic molecules, which were “most probably derived from sure the masses of the atoms and molecules that precursor pigment molecules,” says Roy Wogelius, a Uni- fly off, and he has discovered black eumelanin pig- versity of Manchester geologist on the team. Based on ment itself in fossils, such as a fish eye and an these techniques, the researchers concluded that the tips Elucidating Structural Color Pigments are not the only source of animal colors. The bright hues of a butterfly’s wing and the metallic sheen of a beetle are structural colors, produced when light bounces off multilayer, nanoscale structures that scatter the rays. Maria McNamara, of the University College Cork in Ireland, has shown that these nano- structures can occasionally survive the fossilization process (15), allowing researchers to decipher the color of insects that lived long ago. McNamara and her team examined the greenish-blue fossils of 47-million-year-old forester moths by using electron microscopy and spectrophotometry to analyze how they scatter light (16). The researchers concluded that the moths’ wings contained multilayer structures that would have produced color in life, but the wings probably weren’t originally greenish-blue. That’s because the fossilization process can change the architecture of the nanostructures, and therefore the color they produce. To simulate this process, McNa- mara exposed modern-day beetles to high temperatures and pressure in an autoclave. She found that the highest temperatures turned their wings black, whereas lower temperatures shifted the hue toward the blue end of the spectrum (17).

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