In a study published Monday in the Proceedings of the National Academy of Sciences, researchers say they've provided even more evidence that these structures do indeed contain melanin.
The scientists examined the molecular structure of the mystery objects, determining that they were most likely not bacterial fossils, and that they compared closely to modern melanosomes. They solidified the comparison by subjecting the new melanosomes to stresses that would mimic some of the process of fossilization and deterioration, allowing them to determine how different kinds of melanin would look after millions of years in the ground.
"It was important to bring microchemistry into the debate, because discussion has been going on for years over whether these structures were just fossilized bacteria or specific bodies where melanin is concentrated," MIT's Roger Summons, who worked on the cuttlefish study but was not involved in the most recent research, said in a statement. "These two things have very different chemical compositions."
By using the battered modern melanosomes as a guide for what to expect from the fossilized versions, the researchers were able to make informed guesses about color.
"Very importantly, we see that the different melanins are found in organelles of different shapes: Reddish melanosomes are shaped like little meatballs, while black melanosomes are shaped like little sausages, and we can see that this trend is also present in the fossils," senior author Jakob Vinther, a molecular paleobiologist at the University of Bristol, said in a statement. "This means that this correlation of melanin color to shape is an ancient invention, which we can use to easily tell color from fossils by simply looking at the melanosomes shape."
The bats described in the study -- which were reddish brown, by the way -- represent the first extinct mammals to have their color determined by this method, though many researchers have already used melanosomes to determine colors in dinosaur feathers.