Colouring the dinosaurs
Almost all of us have had the experience of walking through a museum where, invariably, there comes an exhibit on dinosaurs. Usually there are life-sized replicas of dinosaur skeletons (if not the real skeletons themselves) and often recreations/models of what it’s thought the dinosaurs looked like, such as the reconstruction (below) of the feathered dinosaur Archaeopteryx displayed in the Melbourne Museum.
And whilst these models are a welcome, colourful addition to the exhibit, I often wonder just how accurate these models are. Did Archaeopteryx really have a yellow head and red patterning on its wings, back and tail? How would we know whether these coloured models are accurate or not? It’s just an artist’s interpretation, right?
Wrong! Scientists do now have a way to predict, in a fairly accurate manner, the colouring of feathered dinosaurs from analysing fossilised feathers.
Melanin is a chemical derived from the amino acid tyrosine that provides colour to skin, hair and feathers as well as being a light-absorbing polymer that protects skin from light and having a role in temperature regulation. Two types of melanin have been identified in fossils: eumelanin, which provides black, grey, brown and yellow colours to hair and feathers; and phaeomelanin, which produces reddish colours.
Melanin is produced in cells called melanocytes, located in the base of the epidermis of the skin, and granules of it are packaged into vesicles called melanosomes inside these melanocytes.
Image source: http://palaeo.gly.bris.ac.uk/melanosomes/melanin.html
Melanosomes are then incorporated into keratinocytes, being the cells that produce keratin – the protein that serves as a major structural component of hairs and feathers. From the keratinocytes, the melanosomes make their way into the hairs or feathers to give them colour (depending on how they’re packed in the hairs/feathers, melanosomes can also present iridescence that shows as bright blues).
Depending on what type of melanin they contain, melanosomes have different shapes. Melanosomes containing eumelanin typically have an elongated shape and are typically between 800 and 100 nanometres in length, whereas those containing phaeomelanin are more oval or semi-spherical in shape and are typically between 500 and 700 nanometres long and are generally wider than eumelanin-containing melanosomes.
It is these shape differences in melanosomes that allow scientists to determine what colour fossilised feathers once were. This was first achieved in 2008 when researchers found melanosomes in a 100-million year old bird feather.
In 2010, a team based in Liaoning Province in China, an area rich in well-preserved fossils, made the first discovery of melanosomes in dinosaurs.
Image and caption source: http://palaeo.gly.bris.ac.uk/melanosomes/Jeholdinos.html
One of these specimens was a dinosaur that, when alive, was covered in ‘fuzz’. These hair-like structures, dubbed ‘protofeathers’, are thought to be the evolutionary precursor to the true feather seen in modern birds today. This specimen was a carnivorous dinosaur named Sinosauropteryx that was about the size of a turkey when it was alive. From the shapes and stacking of the fossilised melanosomes, the researchers were able to reveal what coloration this dinosaur had when it was alive.
Reconstruction of Sinosauropteryx based on fossilised melanosomes
Image source: http://news.nationalgeographic.com/news/2010/01/100127-dinosaur-feathers-colors-nature/
While the inferences of colour of these fossil feathers must be regarded as not being absolute since feather coloration is often influenced by dietary factors and because the pigment chemicals degrade after death, the findings open the door for reasonably accurate determination of what kinds of colours animals likely exhibited in prehistoric times. This could give further insights into the evolutionary development of camouflage and the use of colour for courting behaviours.
For the first time, reconstructions of the colours of fossilised species can be based on scientific evidence and not speculation by scientists and artists. So next time you see a reconstruction of a dinosaur when you’re walking through a museum, it’s quite possible that what you’re looking at is pretty close to the real thing.
For more information:
2010 paper: Nature, doi:10.1038/nature08740
National Geographic: http://news.nationalgeographic.com/news/2010/01/100127-dinosaur-feathers-colors-nature/
University of Bristol Palaeobiology: http://palaeo.gly.bris.ac.uk/melanosomes/Index.html