Microscopic features found in fossils of dinosaurs and ancient birds are the remains of structures that contained pigment, new work reveals. The structures provide new clues about what these creatures looked like, including the possibility of patches or stripes with a sporty yellow or russet color scheme.
The fossils, some of them described for the first time, were found in northeastern China and contain well-preserved, sub-micrometer-sized structures called melanosomes, says Michael Benton, a paleontologist at the University of Bristol in England. In modern-day creatures, these pigment-bearing structures come in two forms: the cigar-shaped eumelanosomes that hold black melanin, and the egg-shaped or near-spherical phaeomelanosomes that contain pigments ranging from reddish brown through yellow. The new analyses by Benton and his colleagues, appearing online January 27 and in an upcoming Nature, are the first to report structures that appear to be phaeomelanosomes in the fossil record.
Several lines of evidence indicate that the tiny structures are melanosomes, rather than the fossilized remains of bacteria, which are about the same size and shape, says Benton. For one thing, in these fossils the structures appear within the remains of what are believed to be fossilized feathers and featherlike filaments, not on outer surfaces, where bacteria would be expected. Also, the structures are densely packed in some parts of a feather or filament but absent in similarly preserved portions nearby, whereas bacteria would be expected to indiscriminately colonize a feather on a carcass, the researchers note. Finally, the layered arrangement of the melanosomes in some of the fossils matches that seen in the feathers of some modern birds but doesn’t resemble any arrangement that bacteria might produce, says Benton.
Further analyses also ruled out the possibility that that the structures are tiny bits of iron pyrite, which can form during the fossilization process. Even the smallest pyrite grains are typically much larger than melanosomes and have angular, faceted shapes. But the structures seen in these fossils are iron-free, carbon-rich and rounded, says Benton.
Previously, paleontologists discovered signs of black pigment in fossilized feathers (SN: 8/2/2008, p. 10). The new findings provide tantalizing clues that dinosaurs and early birds weren’t limited to a drab color scheme. For instance, the pattern of melanosomes in the fossils of Confuciusornis — a bird that lived about 125 million years ago in what is now China — show the creature likely had patches of white, black and orange-brown feathers.
Sinosauropteryx, a dinosaur that lived in the same area a million or so years after Confuciusornis, probably sported alternating rings of white and orange-brown bristle-like filaments on its tail, the researchers propose.
The presence of pigment in those filaments indicates that the structures were outside of the dinosaur’s skin, says Benton. Some paleontologists have argued that such filaments, found on many dinosaurs, are merely the frayed remnants of collagen fibers that in life were part of the creatures’ skins. Richard O. Prum, a paleontologist at Yale University, agrees with Benton: “The presence of melanin confirms that these filaments are integumentary [or external] structures,” he says.
Alan Feduccia, a paleontologist at the University of North Carolina at Chapel Hill, isn’t so sure, however. The new research is an interesting attempt to devise a microscopic assay for pigment in fossilized feathers, he says, but adds that the implications for the filament-sporting Sinosauropteryx aren’t clear. He contends that those filaments were under the skin. When the dinosaur’s carcass was compressed during fossilization, he says, melanosomes from the skin could have been embedded in the filaments.