Marine fossils show their true colours

12 January 2014
Posted by Chris Smith.

Fossilised remains have revealed that ancient marine-dwelling dinosaurs and turtles used the pigment melanin for camouflage and keeping warm, new Mosasaurresearch has shown.

Lund University scientist Johan Lindgren and his colleagues examined the fossils of a 200 million-year-old ichyosaur (a form of fish lizard), an 86-million-year-old mosasaur and a 55 million-year-old-turtle using a technique called time-of-flight mass spectrometry.

This uses a stream of charged particles to dislodge molecules from the surface of a specimen before feeding them into an analyser that decodes their chemical make-up.

The analysis was focused on regions of the outer layers of the fossils that would have corresponded to the animal's skin. Here, visible with an electron microscope, were small egg-shaped structures less than 1/1000th of a millimetre across.

These structures bear a striking resemblance to bodies called "melanosomes" seen in modern-day animals. They're sites where the dark pigment melanin is generated to produce skin and feather colouration.

The chemical fingerprints from the fossil skin were a strong match for melanin produced by modern-day leatherback turtles, which the team used for comparison. However, the team didn't detect any of the other chemicals sometimes used for skin colouration by modern-day reptiles, like chameleons, suggesting that these ancient animals were the "Model T Fords" of the dinosaur era: they were any colour you like so long as it's black...

These results suggests, say the team in their paper in Nature where the research is published this week, that using melanin as a pigment evolved well before the era of more recent, feathered dinosaurs.

The researchers also speculate that the colouration, which probably would have been uniformly dark, may have played a dual role, providing both camouflage in the deep, low-light feeding habitats frequented by these animals, and a means of keeping warm.

The dark colour would have enabled the creatures to absorb more heat from the Sun whenever they surfaced, or basked, helping them to expand their ranges into colder climes that would otherwise have been uninhabitable.

This so-called "thermal melanisation" strategy is thought to contribute to the ability of today's leatherback turtles to tolerate conditions ranging from tropical seas to the sub-zero conditions in the Arctic.

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