Most fossils look rather dull: Like old bones, of course, or dark lines and smudges imprinted in rock. Now a zoologist has discovered that some fossils, while drab themselves, have tiny grooves that would have made the original creatures sparkle with iridescent colors. What's more, he's using the shiny scales and spines, described in the 7 June Proceedings of the Royal Society London--B, to back a controversial idea: That the appearance of eyes triggered a dramatic pulse of evolution that led to a rich diversity of animals.
The evidence comes from the Burgess Shale of British Columbia, a trove of bizarre fossils some 515 million years old. The ancient color that some of these creatures appear to have had comes not from pigments, but diffraction gratings--parallel ridges that are 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. Then, while visiting the Smithsonian Institution in Washington, D.C., he noticed similar parallel lines on reconstructions of Burgess Shale organisms. Although these were too coarse to be diffraction gratings, he wondered if they might hint at finer detail.
After a close-up inspection with an electron microscope, Parker found traces of diffraction gratings on the spines and scales of Wiwaxia, an armored animal that crawled on the muddy seafloor. He also spotted gratings on the stiff and pointed hairs that lined the sides of the wormlike Canadia. The gratings also grace two projections from a head shield of a swimming beast called Marrella. Although the gratings aren't preserved well enough for the fossils to shine, Parker calculates that the creatures would have flashed a wide range of colors--perhaps to warn predators of their spines.
Such a signal would have been one way to deal with predators that had evolved eyes--a tremendous advantage over their prey, Parker says. He believes that this escalation might have triggered the stunning array of new body plans, which arose during a rapid diversification of life-forms at the start of the Cambrian Era. "Selection pressure of this magnitude might have been just the thing to set off the Cambrian explosion," says Parker. "Animals had to adapt dramatically."
Most experts agree that the appearance of eyes probably altered the course of evolution, but they aren't ready to say that eyes started the Cambrian explosion. "If a whole lot of new complex animals appear, eyes may be associated with them--but so are kidneys," says James Valentine, a paleontologist at the University of California, Berkeley. "It's hard to separate cause and effect." Still, the finding helps bring the Burgess Shale animals to life, says Valentine: "It really makes you think about the animals running around flickering and shining in the light."