The fossil of an ancient turtle hatchling has been found to contain pigment, meaning that the survival trait of a dark shell used by today's baby turtles originally evolved at least 54 million years ago.
The turtle in question is from the species Tasbacka Danica, which floated through the oceans of the Eocene period, around the time that primates first appeared.
The fossil was discovered in 2008, preserved in limestone, in the well-known Fur formation in Jutland, Denmark. It is tiny at just 7.4 centimeters (3 inches) in length.
In 2013, Johan Lindgren, a palaeontologist from Lund University, found soft tissue residues close to the turtle’s “shoulder”. So, he collected some samples to analyze.
Today’s baby sea turtles have dark pigmentation in their shells. This is a clever tactic that helps them avoid aerial predators like seagulls, as they blend in well with the dark murky color of the sea. The coloration also allows the turtles – which are cold-blooded – to absorb a lot of sunlight, which warms up their bodies and helps them to survive. Being able to warm up quickly also means that the tiny turtles grow faster, minimising the amount of time they spend being easy prey for a whole host of larger animals.
Pigmentation like this is caused by structures called melanosomes, and the researchers noticed that the fossil had some similar-looking features. They analyzed the tissue using various techniques, including transmission electron microscopy (TEM), infrared microspectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS).
Using these techniques, along with a number of others, the scientists were able to confirm that the components of blood, pigment, and protein were all present in the tissue. They identified beta-keratin (a protein that adds rigidity to a reptile’s skin), hemoglobin (a component of blood), tropomyosin (a muscle protein), and eumelanin (a pigment that can give humans a dark skin color).
The molecules were all original to the fossil, meaning that ancient turtles shared their dark pigmentation with the turtles of today. The findings are published in the journal Scientific Reports.
“[W]e know that these hatchlings had the dark coloration common to modern sea turtles,” said co-author Mary Schweitzer, professor of biological sciences at North Carolina State University, in a statement.
"The data not only supports the preservation of multiple proteins but also suggest that coloration was used for physiology as far back as the Eocene, in the same manner as it is today," she added.
“We have previously identified eumelanin in fossilized skin, but the fact that the organic matter from this baby turtle contained such a wide range of identifiable biomolecular remains came as a big surprise,” Lindgren told Sci News.