Life's Early Glimmers

Fossils aren't very colorful. But some of them used to be: A zoologist has found that some Cambrian creatures that roamed the sea floor 515 million years ago were iridescent, a development he believes was one of many triggered by the emergence of vision. The find, described in the 7 June Proceedings: Biological Sciences of the Royal Society, comes from the fossil-rich Burgess Shale of British Columbia.

Wiwaxia's iridescent spines might have warned off predators.

AUSTRALIAN MUSEUM

The creatures' color came not from pigments but diffraction gratings--parallel ridges spaced at about the wavelength of visible light. In 1995, invertebrate zoologist Andrew Parker of the Australian Museum in Sydney found diffraction gratings in living crustaceans called ostracodes, which flash iridescent hues while courting. Later, he noticed lines similar to those on ostracodes on reconstructions of Burgess Shale organisms. Although these were too coarse to be diffraction gratings, inspection of actual fossils with an electron microscope revealed traces of the gratings on the spines, scales, and hairs of several creatures: the armored Wiwaxia, the worm-like Canadia, and the swimmer Marrella. Because all gratings split incoming light into its component wavelengths, Parker was able to use optical equations to calculate that the creatures could flash a wide range of colors--perhaps to warn off predators.

Paleontologist James Valentine of the University of California, Berkeley, says the finding helps bring the Burgess Shale animals to life: "It really makes you think about the animals running around flickering and shining in the light."

Science 1998; 281: 169.