THE drab, grey world of fossils could soon be awash with colour. A 50-million-year-old brilliant blue beetle has given palaeontologists proof that they can determine the colours of a host of long-extinct species, including birds, butterflies and fish.
Iridescent creatures get their colour from specialised reflective structures rather than a chemical pigment. The shiny part of a beetle鈥檚 wing, for instance, is composed of fine layers of cuticle, each of which refracts and reflects light differently. As light hits the structure, it either passes through the layer or is reflected back, depending on its wavelength. The pattern of reflected light, which depends on the thickness of each layer and its refractive index, determines the colour that we see.
Using an electron microscope, scientists examine such fine structures to understand how individual species produce their unique shimmering displays. From the structure they see, they can infer a precise mathematical model that predicts and describes the rainbow of colour it will reflect. 鈥淚t鈥檚 a standard technique for beetles you pick up in the forest,鈥� says Roy Sambles, a physicist at the University of Exeter in the UK.
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But scientists have had no way of checking whether their estimations of the colours of extinct animals are correct. Now Andrew Parker, an optics specialist at the University of Oxford, has measured the colour spectrum of a unique beetle fossil collected from the oil shales of Messel, Germany. Some fossils in this area have a high water content, which allows the fossil to retain the refractive pattern 鈥� and therefore colour 鈥� of the live animal. Parker confirmed that the actual colour spectrum of the wet fossil matched the predicted spectrum he calculated from the structural layers of the wing (Proceedings of the Royal Society B, DOI: 10.98/rsbp.2003.0055).
Parker says the technique can now be used on other fossils that lack their original colours. For example, fine laminate structures have been preserved in trilobites from Wales and arthropods from Australia. And the wealth of living animals with structural colours 鈥� the green strip on a mallard鈥檚 wing, or the iridescent sheen of hummingbirds or snakes, for instance 鈥� suggests the technique could be applied to a wide range of animals. 鈥淣ow the trick will be finding the fossils,鈥� says Parker.