Colouring in the Past
One of the coolest discoveries of recent times in palaeontology has been the finding that we are able to distinguish which colours some extinct animals were. This has been done for several species of theropod dinosaur (e.g. Sinosauropteryx), early birds (Microraptor, Archaeopteryx) and even fossil penguins (Inkayacu). We are able to determine the colour of these extinct beasties by looking at fossilised microscopic structures that are preserved in their fossil feathers called melanosomes. These are organelles that contain the light absorbing pigment known as melanin, the shape of the melanosome determining what colour the feather will be. But did these melanosomes evolve as soon as feathers evolved and they were still simple filamentous structures or did it not happen until later in their evolution when they were being used for flight? A new study, published last week in the journal Nature has attempted to answer this question.
The team, consisting of researchers from institutions from the USA and China sampled melanosomes from 181 extant animals including mammals, reptiles, turtles, crocodiles and birds. They also sampled 13 fossil taxa which included reptiles, turtles, ornithischian dinosaurs, pterosaurs, theropod dinosaurs and early birds. What they found was that only theropod dinosaurs, birds and living mammals possess the diversity in structure of their melanosomes that can allow scientists to decipher their original colour. The other groups sampled showed a much more limited diversity in their melanosome shape making it difficult to infer what colour they were based on their melanosomes.
But when did this increase in melanosome structure diversity occur? The researchers discovered that the increase happened suddenly just before the evolution of feathers used in flight (known as pinnate). "This points to a profound change at a pretty discrete point," says author Julia Clarke of The University of Texas at Austin's Jackson School of Geosciences. "We're seeing an explosion of melanosome diversity right before the origin of flight associated with the origin of feathers."
The group were also surprised by the division between high and low melanosome structure diversity in the living groups. "Only in living, warm-blooded vertebrates that independently evolved higher metabolic rates do we see the melanosome diversity we also see in feathered dinosaurs," said co-author Matthew Shawkey of The University of Akron. What this is potentially hinting at then is that there was a change in the physiology of these feathered dinosaurs that may have allowed these animals to become energetic enough for flight. "We are far from understanding the exact nature of the shift that may have occurred," says Clarke. "But if changes in genes involved in both coloration and other aspects of physiology explain the pattern we see, these precede flight and arise close to the origin of feathers."
This area is at the cutting edge of palaeontological research. There will no doubt be many more amazing discoveries to come as we learn more about these little structures that appear to not only show us what colour these magnificent animals were, but can also potentially tell us about their physiology too. Fossils really are awesome.
Quotes were taken from the EurekAlert! press release on this article.
Quanguo Li, Julia A. Clarke, Ke-Qin Gao, Chang-Fu Zhou, Qingjin Meng, Daliang Li, Liliana D’Alba &Matthew D. Shawkey, 2014. Melanosome evolution indicates a key physiological shift within feathered dinosaurs. Nature. Doi:10.1038/nature12973.