For obvious reasons, when imagining coloration in long-extinct creatures we mostly have to make educated guesses based on modern animals. However, there are exceptions, such as in the case of the early Cretaceous Microraptor, where scans with an electron microscope revealed preserved melanosomes [pigmentation cells] within the feathers, the orientation/stacking of which was consistent with black, iridescent coloration in modern birds such as the starling - this may have served a similar function as in the modern analogue, for sexual display purposes (Li, 2012). Preserved melanosomes have also been found in Sinornithosaurus, though its coloration was not uniform across the body (Zhang et al., 2010). A follow-up study in 2012 showed that the colors were reddish brown, yellow, black, and gray, which were distributed across the body (Naish, 2012). It is important to note that such exceptions are rare, and for the most part we do not know what color dinosaurs were. For this reason, we must look at modern analogues for inspiration. This requires us to understand the uses of coloration in the natural world, so that we may hypothesize the possible roles colors may have played in the lives of dinosaurs.
In order to hypothesize what roles and importance color might have had in the lives of dinosaurs, we first need to understand a bit more about color vision from both an anatomical and evolutionary standpoint. In vertebrates, there are two specialized types of receptors in the back of the eye - rods and cones. The former of the two are responsible for low-light contrast, whereas the latter are responsible for perceiving color. Cone cells contain one of several proteins called opsins, each of which has a different spectral sensitivity - in other words, each type of pigment is best able to detect a particular set of wavelengths in the spectrum. By having many, many cone cells with several different pigment types, the eye can perceive multiple different colors.
Based on this information, it is somewhat intuitive that the number of different types of cone cell pigment an animal has influences its ability to see colors. Humans and other primates have three different types of pigment - in other words, we have trichromatic color vision. However, the earliest vertebrates actually had tetrachromatic color vision, which was then lost in mammals. Thus, reptiles and birds actually both have tetrachromatic vision (Bowmaker, 1998), meaning that they have a much better ability to see color than we do. Taking this into account, along with the many examples of the importance of color in both lineages, it is rather safe to assume that dinosaurs also used color in many different ways.
Another thing we must understand is the mechanisms responsible for coloration in both reptiles and birds. As many people know, color is generated by specialized cells collectively known as chromatophores. The most widely known of these is the melanophores, which produce melanin (the pigment responsible for black and shades of brown). However, reptiles and other poikilothermic vertebrates have two additional types of chromatophore that produce chemical pigments, known as xanthophores (responsible for yellows) and erythrophores (responsible for reds) - these two contain a mixture of different pteridine and carotenoid pigments (Bechtel, 1978). In addition, they also have structural chromatophores called iridophores - rather than containing chemical pigments, these cells reflect different wavelengths of light based on the structure of the cells themselves. Together, different combinations, densities, and distribution of these chromatophores allows for a diversity of colors. However in birds, the only chemical pigment cells are melanophores - the many colors that we see in bird plumage is due to the structure of the feathers themselves. For example, peacock feathers are actually pigmented brown, but their structure interacts with these melanophores to create the vivid colors we perceive (Ball, 2012).
This makes one wonder - when were these specialized chromatophores we find in reptiles lost? My intuition tells me that it likely coincided with the appearance of feathers - since they have an alternate mechanism for generating diverse colors, there is no need for the chromatophores (plus they are found in the skin which is covered by the feathers). However, this is simply speculation and would require further evidence such as molecular data to investigate.
Back to the topic at hand, given the diversity of coloration within reptiles and birds, it is fair to assume that non-avian dinosaurs also came in many different colors. Many modern reptiles and birds use color as a signaling tool, as an indicator of mate quality/health, to intimidate rivals, and obviously to blend in to the environment. Additionally, many reptile and bird species that make use of conspicuous color signals show significant variation in the color of the relevant appendage/body part across their range. A great example of this would be the dewlaps found in the genus Anolis, where a single species may have vastly differing dewlap color and shape between localities. This might have been the case in some dinosaur species if they too used such a signal. For example, perhaps Triceratops would flood its frill with blood to create bright colors to attract mates and intimidate rivals - if it did so, there may have been vastly differing frill colorations between areas of its natural range. Obviously this is purely hypothetical, but it is not beyond the bounds of sound reasoning.
Well, I hope you have enjoyed this week's Sci-Day! Since my current project is related to pigmentation in reptiles, it is helpful for me to explain the mechanisms behind it as well!
Acknowledgements:
Li, Quanguo. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science 335: 1215-1219.
Zhang, Fucheng; Kearns, Stuart L.; Orr, Patrick J.; Benton, Michael J.; Zhou, Zhonghe; Johnson, Diane; Xu, Xing; Wang, Xiaolin. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463 (7284): 1075-1078.
Naish, Darren. 2012. Planet Dinosaur: The Next Generation of Killer Giants. Firefly Books. p. 192.
Bowmaker, J. K. 1998. Evolution of colour vision in vertebrates. Eye 12 (3b): 541-547.
Bechtel,
H. Bernard. 1978. Color and Pattern in Snakes (Reptilia: Serpentes). Journal of
Herpetology 12 (4): 521-532.
Ball, Philip. 2012. Nature's Color Tricks. Scientific American 306 (5): 74-79
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