A trick of chemistry has restored fossil turtles and other marine reptiles to their living color. But the result fell far short of the gaudy transformations wrought by Hollywood in its classic black-and-white movies.
Ancient leatherback turtles, ichthyosaurs and mosasaurs were a rather staid and formal black, maybe with some gray, according to a study published online Wednesday in the journal Nature.
The study offers the first direct chemical evidence of pigmentation in the three species, and illustrates an example of convergent evolution, when animals separately develop the same adaptive features.
Researchers blasted the fossil samples with a beam of ions that enabled them to analyze and image the microbodies embedded in dark film that had the appearance of dark skin. Analysis of those molecules, compounds and fragments showed they were identical to melanosomes, the cellular organelles that produce various types of melanin, which can produce colors from black to yellow.
The samples, however, were dominated by the black- and gray-producing eumelanin, according to the study.
"Every living animal out there has eumelanin, so that by itself is not surprising," said the study's lead author, Johan Lindgren, a vertebrate paleontologist at Lund University in Sweden. "What is surprising is the sheer concentration of these fossilized melanosomes."
Lindgren and his colleagues examined fossils of a leatherback turtle dated to 55 million years ago, a lizard-like mosasaur from 86 million years ago, and an ichthyosaur that was 190-196 million years old. Each is a marine reptile that breathed on the sea surface, and all three once roamed on land, but returned to the sea.
The researchers used the modern leatherback as its frame of reference. That animal has some unique and unusual characteristics, not the least of which is its hard skin that forms its carapace. The largest sea turtle on Earth, the modern leatherback grows very fast, and can live in arctic and subarctic environments.
"In order to do so it has a number of adaptations for these conditions," Lindgren said. "One of them, we presume, is the fact that it has a black skin. The black eumellanic skin enables it to absorb the solar heat more quickly during daylight hours, and these animals are known to bask at the surface during daylight hours in high-latitude areas."
Researchers believe that black skin gave all three ancient marine reptiles a thermal advantage, but also provided protection against ultraviolet radiation and concealment against a dark background, a common dorsal feature of marine life.
A great many marine animals also are counter-shaded: lighter on their underside, to camouflage them better against a light background in shallow water.
Skin samples, however, were not large enough to offer evidence of any lighter shading, Lindgren said. He favors a more monochromatic explanation, particularly for the ichthyosaur, which was deep-diving, where more uniformly dark coloration such as that of the sperm whale is more advantageous.
Lindgren believes the complex techniques used to analyze the fossils, which came from museums in Denmark, Texas and Britain, can be used on many more samples that show imprints of skin material.
"This is a technique that can definitely be used in many, many samples, regardless of what they are made of," he said.