Ichthyosaur is the New Black
Just yesterday, a group of 2nd graders asked me what color dinosaurs were. I was pretty excited to tell them we actually do know this through looking at tiny structure on feathers called melanosomes. Melanosomes are sub-millimeter sized round or cigar shaped organelles inside of animal cells that contain melanin, the substance that causes pigmentation. When we examine melanosomes in modern organisms, their chemical profiles (known as a spectra) tells us what type of melanin is in that melanosome. Eumelanin is responsible for black and brown pigment and pheomelanin is responsible for red or pink coloration. Carotenoids and porphyrins are other non-melanin pigments, and can produce other colors in birds like bright yellow and green. Interestingly, a blue coloration is produced not by pigment but the scattering of light from unique keratin ‘air pockets’ in certain type of bird feathers. This means blue is a structural, not a pigmented color.
Different types of melanin are really the easiest for us to distinguish in fossil specimens due to the fact their chemical profiles are preserved in melanosomes. From this type of research on dinosaurs, we have found out that Microraptor was shiny and black. A feather from the 1861 Archaeopteryx specimen indicates it too was black. This research, up until this point, has mostly focused on examining the melanosomes in birds, but this week, in Nature, this same idea was used to determine the colors of ancient marine reptiles.
This time, Johan Lindgren and coauthors were interested in the colors of extinct marine giants like ichthyosaurs, the mosasaur Tylosaurus, and an extinct leatherback turtle. Scrapings of “skin” were taken from these specimens and put into some special microscopes. The skin of these specimens is a blackish sort of film that, to the naked eye, doesn’t look like anything special. But up close in the scanning electron microscope, it is immediately apparent that the telltale cigar-shaped melanosomes are present. It is important to note fossil bacteria can look very similar to fossilized melanosomes, but distinct chemical signatures only on the body areas of the fossils and not on the sediment outside that zone seem to indicate they were present in life and did not form after death.
The chemical profiles of the melanosomes in all three of these marine reptiles indicated they were black in color because eumelanin was detected. Interestingly, it seems the ichthyosaur was black on the entire body- not just on the dorsal side. The mosasaur and turtle possibly had a dark top and a lighter underside. You may be wondering why any of this matters- what difference does it make if an ichthyosaur was black all over? In the paper, the authors aptly mention coloration is a trait that is subject to natural selection. Coloration in animals influences sexual displays, thermoregulation, and camouflage. Certain animals can be better adapted to colder environments if their dorsal sides are darker in color, which will allow them to absorb more heat. Cordylid lizards in South Africa that had lower skin reflectance (more melanin) had a higher fitness than species that had lighter, more reflective skin.
Ichthyosaurs may have had eumelanin on their entire body due to a benefit in thermoregulation. If they spent time close to the surface, they could absorb more sunlight and stay warmer in cold environments. Extant leatherback turtles are dark on the top of their shells, possibly for this reason. This pigmentation pattern is called thermal melanism. On the other hand, most turtles and cetaceans are countershaded–dark on top and light on the bottom. This shading in cetaceans seems to obscure their own shadows while they are diving, making it easier to sneak up on prey. Species of whales that possess uniform dark coloration could be better adapted for diving to extreme depths where there is no light, but the evidence for this is mostly anecdotal at this time.
This study indicates for the first time there is convergent evolution of melanism in secondarily aquatic tetrapods, but more importantly expands fossil melanosome analysis beyond just feathers. Not that long ago, determining the color of extinct animals seemed impossible, but now it is very clearly possible for more organisms than we ever imagined!
References:
Carney, R. et al. 2012. New evidence on the colour and nature of the isolated Archaeopteryx feather. Nature Communications. 3 (637). doi:10.1038/ncomms1642
Clusella-Trullas, S., Hvan Wyk, J., Spotila, J.R. 2009. Thermal benefits of melanism in cordylid lizards: a theoretical and field test. Ecology 90:2297–2312. http://dx.doi.org/10.1890/08-1502.1
Li et al. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science 335 (6073) 1215-1219. DOI: 10.1126/science.1213780
Lindgren, J. et al. 2014. Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature. doi:10.1038/nature12899