A team of paleontologists from the United States and the United Kingdom has discovered the reddish brown color of extinct bats from fossils dating back about 50 million years, marking the first time the colors of extinct mammals have been described through fossil analysis.

The team, headed by Dr Jakob Vinther of the University of Bristol, UK, determined the color of two species of bat – Palaeochiropteryx sp. and Hassianycteris sp., which lived in Eocene, 56-33.9 million years ago.

By studying microscopic spherical and oblong-shaped structures in the fossils, the scientists found that the bats were reddish-brown in color.

“Microscopic structures traditionally believed to be fossilized bacteria are in fact melanosomes – organelles within cells that contain melanin, the pigment that gives colors to hair, feathers, skin, and eyes,” Dr Vinther and co-authors explained.

Fossil melanosomes were first described in a fossil feather in 2008 by Dr Vinther. Since then, the shapes of melanosomes have been used to look at how marine reptiles are related and identify colors in dinosaurs and, now, mammals.

“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,” Dr Vinther said.

“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. In addition to shape, melanosomes are chemically distinct.”

In order to identify the origin of these structures, the team replicated the conditions under which the fossils formed using high pressure, high temperature autoclave experiments.

They showed that the fossils contain fossilized melanin by using Time-of-flight secondary ion mass spectroscopy, which had changed in chemical composition over time.

“By incorporating these experiments, we were able to see how melanin chemically changes over millions of years, establishing a really exciting new way of unlocking information previously inaccessible in fossils,” said team member Caitlin Colleary, a PhD student at Virginia Polytechnic Institute and State University.

“This is a great leap forward in our understanding of how fossils are preserved. We now know how melanin is preserved and we have the methods to confidently detect it,” Dr Vinther added.

The study was published online September 28 in the Proceedings of the National Academy of Sciences.

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Caitlin Colleary et al. Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils. PNAS, published online September 28, 2015; doi: 10.1073/pnas.1509831112