Fossil Pigments Reveal Skin Color of Ichthyosaurs and Mosasaurs

Fossil pigments reveal skin color of ancient sea creatures

Fossil pigments reveal skin color of ancient sea creatures

A recent study conducted by researchers at SP Technical Research Institute of Sweden and the MAX IV Laboratory at Lund University, in Sweden, has revealed the skin color of ancient, dinosaur-era sea creatures. Soft tissue remains were acquired from a series of sea creatures, including a 55-million-year-old leatherback turtle, a 86-million-year-old mosasaur and a 196-million-year-old ichthyosaur.

Fossil pigments reveal the colors of ancient sea monsters
Soft tissue samples, including the skin from a leatherback turtle (left), the scales of a mosasaur (center) and a tail fin from an ichthyosaur (right). Image Credit: Bo Pagh Schultz, Johan Lindgren and Johan A. Gren.

John Lindgren, a Lund University scientist who led the international research team’s investigation into the latest fossil specimens, discussed his enthusiasm for the undertaking:

“This is fantastic! When I started studying at Lund University in 1993, the film Jurassic Park had just been released, and that was one of the main reasons why I got interested in biology and palaeontology. Then, 20 years ago, it was unthinkable that we would ever find biological remains from animals that have been extinct for many millions of years, but now we are there and I am proud to be a part of it.”

The Ancient Sea Creatures

When dinosaurs roamed the Earth, the seas were occupied by enormous reptiles, including mosasaurs and ichthyosaurs. The first fossilized remains of mosasaurs were unearthed in a Maastricht limestone quarry during the 1700s. The carnivorous marine reptiles – inhabiting the Earth during the Late Cretaceous – spanned lengths of up to 15 meters and weighed in at approximately 20 tons.

Artists illustration of the leatherback turtle, ichthyosaur and mosasaur
Artist’s illustration of the leatherback turtle, ichthyosaur (center) and mosasaur (bottom). Image credit: Stefan Sølberg.

Meanwhile, ichthyosaurs – meaning great “fish lizards” – swam alongside mosasaurs. The earliest ichthyosaurs were dolphin-like in appearance and possessed long, flexible bodies. Swimming in an undulating fashion, they were highly streamlined and built for speed.

Before the latest study, scientists could only conjecture over the possible colors that these majestic sea creatures had. However, Lindgren – along with researchers from the United States, England and Denmark – have established that the ancient marine beasts had dark-colored skin. They believe this color may have contributed towards more efficient thermoregulation, while serving to camouflage and shield them from the damaging effects of ultraviolet radiation.

The fossilized remains comprised of skeletal samples that were scantily coated in “dark skin patches,” that were – at one time – believed to have been home to clusters of micrometer-sized bacteria that aided in decomposition of the deceased sea creatures’ remains. Lindgren and colleagues determined that these microbodies were not bacteria, however, based upon analyses performed on the soft tissue. Instead, the team established that the micrometer-sized bodies were, in actual fact, fossilized melanosomes – organelles responsible for synthesis, storage and transportation of melanin in modern animals.

Per Uvdal, co-author on the study, and an employee at the MAX IV Laboratory, describes the team’s results as “amazing.” Uvdal explains that melanin is exceptionally unstable. However, Lindgren states the black pigment eumelanin to remain extremely persistent within the environment, and the melanosomes could be the principal reason for the survival of the skin patches.

Now, advancement in molecular and imaging capabilities have allowed the team to use the creatures’ own biomolecules to learn more about their appearance, indicates Uvdal. The researchers employed a technique called energy-dispersive X-ray microanalysis, which beams X-rays directly onto the specimen. A reaction is then triggered, based upon the chemical constituents of the biological sample, which the researchers can use to determine its structure. The results of the analysis, indeed, confirmed the presence of melanosomes affixed to the preserved skin.

Leatherback Turtles Explain Advantage of Dark Skin Color?

In a bid to understand how the dark skin could have benefited the ancient sea creatures, the team turned to the leatherback turtle. While both mosasaurs and ichthyosaurs became extinct during the Cretaceous Period, leatherback turtles still endure to this day. The Dermochelys leatherback turtle (Dermochelys coriacea) has a black back into adulthood and survives in a diverse range of habitats. For example, the creatures inhabit the harsh environment of the Arctic circle. The authors suggest that their dark shells help with thermoregulation and retain heat from the sunlight. Lindgren reflects on this intriguing hypothesis:

“The fossil leatherback turtle probably had a similar colour scheme and lifestyle as does Dermochelys. Similarly, mosasaurs and ichthyosaurs, which also had worldwide distributions, may have used their darkly coloured skin to heat up quickly between dives.”

Looking towards the modern sperm whale – one of the few current marine animals to possess dark-colored skin – the authors suggest that they were, perhaps, afforded an advantage. It seems the deep-diving sperm whale’s dark skin could confer it camouflage in the dark depths of the oceans, where light is scarce. Since ichthyosaurs, too, have been posited to operate as deep-sea divers, Lindgren suggests they may have had lifestyles similar to sperm whales.

The research was published in the Jan. 8 issue of the journal Nature.

By James Fenner

Top image credit: Stefan Sølberg


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