Illustration courtesy Stefan Sølberg
Published January 8, 2014
Black has never been out of fashion, suggests a new study that reveals the color widely worn by marine reptiles for nearly 200 million years. (See also "True-Color Dinosaur Pictures.")
Over the last decade, paleontologists have refined chemical analyses of ancient creatures' hides and feathers, finding evidence of coloration in everything from ancient dinosaurs to penguins. In a new Nature study, Johan Lindgren of Sweden's Lund University and colleagues set out to investigate the hues of ancient sea creatures.
Looking widely across time, the study sampled fossilized skin from three ancient marine reptiles: a 55-million-year-old leatherback turtle, a 86-million-year-old mosasaur, and a 196-million-year-old ichthyosaur. The latter two species are, of course, extinct.
"The most sensational aspect of the study is that it can now be established that the analyzed ancient marine reptiles were, at least partially, dark-colored in life," Lindgren said.
The team that chemically sampled the fossilized hides of these creatures hopes to understand how skin color evolved among ancient sea creatures. The study results, partly supported by a National Geographic Society/Waitt Foundation grant, suggest that darker-colored hides have long offered advantages to ocean dwellers seeking to survive in diverse environments.
"This study offers an exciting window into the past that is just fundamentally fascinating," said evolutionary biologist Rebecca Jo Safran of the University of Colorado in Boulder, who was not part of the study.
"Humans in general are just fascinated by what extinct animals looked like."
In the study, the team used a sophisticated spectrometer and a scanning electron microscope to analyze the individual molecules found in preserved hides of the three museum specimens.
The analysis revealed that dark traces in the soft tissues are marked by melanin, a pigment responsible for black to brown coloration in animal skin, including human skin.
Other color pigments are largely absent, notably ones responsible for red and yellow colors. (Those are harder to detect, however.) In particular, ichthyosaurs, dolphin-like predators also called "sea monsters," appear to have been almost completely dark-colored, while mosasaurs might have been lighter-shaded on their undersides, like the leatherback turtles.
"While the presence of other undetected pigments cannot be ruled out, particularly dark pigmentation in these fossils suggests that they might have been able to live in more [extremely hot] environments or have used pigmentation patterns as camouflage in dark waters," evolutionary behavioral ecologist Ted Stankowich of California State University, Long Beach, said by email.
Pointing to modern-day leatherback turtles, which have black backs, Lindgren suggests that dark pigments evolved separately but uniformly in the ancient species, aiding their internal-temperature regulation, protection against sunburn, and camouflage.
"The ability of leatherback turtles to survive in cold climates has mainly been attributed to their huge size, but it has also been shown that these animals bask at the sea surface during daylight hours," he said. "The black color enables them to heat up faster and to reach higher body temperatures than had they instead been lightly colored."
The study's ancient leatherback turtle probably had a similar color and lifestyle as modern leatherbacks, basking in the midday sun and then diving to great depths to seek prey, even in cold, polar waters. "Similarly, mosasaurs and ichthyosaurs, which also had worldwide distributions, may have used their darkly colored skin to heat up quickly between dives," Lindgren suggests. (Explore National Geographic's sea monster interactive.)
Safran is more cautious, suggesting that more testing needs to take place among living species to see exactly how much of a temperature advantage they truly derive from dark skins.
The study concludes that, most likely, the dark coloration seen on the three species represents an example of "convergent" evolution, which occurs when natural selection favors a similar trait, in this case a dark hide, evolving separately to the same conclusion among diverse species.
Said California State University's Stankowich: "This work is a really cool first step to both understanding the external appearance of extinct marine creatures and using that information to make inferences about their lifestyle and behavior, which, unfortunately for behavioral ecologists, do not fossilize."
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