By Caitlin Colleary, Department of Geoscience, Virginia Tech
The past is often portrayed in black and white. The old films and photos of our great grandparents don’t represent a colorless world and neither do the extraordinarily preserved fossils that retain feathers, hair, and skin – which often appear black because they are preserved as carbonaceous films.
Biologists study the colors of living animals because they are extremely informative. Color is camouflage, it attracts mates, and can even actively protect an animal (e.g., squid ink). But until recently, this important behavioral information was inaccessible in the fossil record.
Melanin is the chemical pigment that colors hair, feathers, and skin. The two types of melanin: eumelanin and pheomelanin are contained in organelles called melanosomes. The two different types of melanin produce different colors and the shape of the melanosomes correlates to that color. Eumelanin is black and eumelanosomes are oblong in shape. Pheomelanin is red and pheomelanosomes are spherical. This distinction is important because we now know that melanosomes are preserved in fossils.
In 2008, Dr. Jakob Vinther, now a lecturer at the University of Bristol, first described what he thought were melanosomes preserved in a Cretaceous black-and-white banded feather from Brazil. The organelles that he found, which were about a micron in length, were only preserved in the black bands and absent in the white bands, which is exactly how melanosomes are arranged in a modern black-and-white feather.
This discovery led to the identification of the original colors of a number of feathered dinosaurs and it became clear very quickly that these structures are preserved a lot more than anticipated.
The goal of our study was to identify the chemical remains of the pigment melanin in association with these structures, to see if the pigment itself was also preserved and to test that the shape of the melanosome would correlate to the type of melanin, like it does in modern animals.
To do this, we used scanning electron microscopy to identify melanosomes in the fossils and mass spectroscopy to study the chemical structure associated with these structures. We identified melanosomes in fossil fish, birds, squids, frogs, tadpoles, and mammals. And we found the chemical traces of melanin associated with each one. However, when we compared the chemistry to modern melanin, it looked a bit different. To understand why this may be the case, we used pyrolysis experiments to test pressure and temperature variables to see if the chemistry of the melanin had changed over the millions of years it had been buried.
When we examined the variance in the types of melanin, we found that the experimentally matured melanin plotted in between the fresh and fossil samples. And we could also distinguish between eumelanin and pheomelanin, which allowed us to correlate the chemistry with the shapes. We determined that the shape of the melanosome does indeed match the type of melanin in fossils.
For the first time, we were able to determine the color of a mammal from the fossil record. The two species of bats were about 49 million years old and were preserved in the Messel formation in Germany. Microscopically, they both had small spherical melanosomes preserved in their fur and the chemical signature associated with pheomelanin. Therefore, we inferred that they were a reddish brown color.
The ability to make these interpretations from the fossil record is very exciting. We can learn so much about animal behavior and the types of environments they lived in. Now the dinosaurs drawn in children’s books can be colored in with confidence instead of artistic license.
C. Colleary, A. Dolocan, J. Gardner, S. Singh, M. Wuttke, R. Rabenstein, J. Habersetzer, S. Schaal, M. Feseha, M. Clements, B. Jacobs, L. Jacobs, R.L. Sylverstersen, S. Gabbot, J. Vinther. Chemical, experimental and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils. Proceedings of the National Academy of Sciences. 2015. 12592-12597.
To learn more:
Listen to an interview with Caitlin about the research.
You can follow Caitlin on twitter at @hypothecait