Monday, 10 June 2013

Colour in Fossils

Dinosaurs are always displayed as colourful, but how do we know what colour they were? The truth is, in most cases, we don't. In the typical depictions of scaly, reptilian dinosaurs, we can make educated guesses based on animals today, and the habitat of the dinosaur in question. For example, herbivorous animals that would need to hide from predators wouldn't be brightly coloured: they would be dull, earthy colours to allow them to fade into the background and hide from predators. Features that are used for some kind of display, like a crest, would likely have been brightly coloured, as it allows an even greater display. 
Caulkicephalus, a pterosaur from England illustrated with a bright blue and yellow crest. Copyright of Mark Witton
More recently, a new way of telling fossil colouration has been discovered. There are several forms of pigment in animals today, including melanin, carotenoids, luciferin, and more. Melanin is found in little packets called melanosomes, and is responsible for brown, black, and red colouration. These packets come in different shapes, for different colours. Eumelanin, responsible for brown-black colouring, is found in elongate sausage-shaped melanosomes, whereas pheomelanin, responsible for red or ginger colouring, is found in spherical melanosomes. They are very small structures, and can be seen using a Scanning Electron Microscope (SEM). This was first used in a fossil feather from the Lower Cretaceous [1], while the theropod Sinosauropteryx was the first dinosaur to have its colouration described by looking at melanosomes. The fossils showed a strange banding of the primitive feathers in a dark, and light pattern. When these different sections were analysed under SEM, pheomelanosomes were discovered in the darker banded areas, and no melanosomes (i.e. no pigment, and therefore white) were found in the lighter areas [2]. The authors suggested that to mean this animal had a tail banded in white and reddish colours in life. 

Since that first feathered dinosaur discovery, several other extinct feathered animals have been analysed using this method. This includes the four winged dinosaur Microraptor, and the possible early bird Anchiornis. In fact, one study on Microraptor has actually suggested that you can see iridescence from looking at the pattern of melanosomes within the fossils.
Artists impression of Anchiornis, colouration patterns known from fossilised feathers. Image by Nobuyuki Tamura.
 For anyone interested in more information about colouration in fossils and feathers, check out the blog Prehistoric Colours by a group at the University of Bristol in the UK. They are currently working on this issue and looking at different pigments found in fossils and how you can identify them. They are also working on identifying deformation in these structures in the fossil record. It's really neat research!

1. Vinter, J., et al. 2008. The colour of fossil feathers. Biology Letters 4: 522-525.
2. Zhang, F., et al. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463: 1075-1078.

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