Two sides. Fossil “bilaterian" or natural mineral formation? Experts disagree.

The Roots of Symmetry?

An oval blob less than a fifth of a millimeter long, discovered in ancient Chinese rocks, is the oldest known fossil of an animal with a mirror-image symmetry, a team of paleontologists reports. If true, the find would suggest a new, simpler kind of "bilaterian" ancestor. But other experts suspect that the specimens might not be fossils at all.

For decades, paleontologists have been eagerly looking for the roots of the so-called Cambrian Explosion--an apparent eruption of many-celled animals about 540 million years ago in which all the major animal body plans appeared. One key question: When did animals first develop mirror-image symmetry and other features (such as a digestive tract) that go with it? The authors of the new study, published online 3 June in Science, say their find shows that key features of bilaterians are preserved in rocks some 580 million to 600 million years old--and thus that the genetic tool kit used to assemble more sophisticated body plans was present long before the Cambrian Period.

The fossils come from the Doushantuo Formation in southwestern China, rocks famous for well-preserved fossils of microscopic sponges and other simple animals. Jun-Yuan Chen of the Nanjing Institute of Geology and Palaeontology in China and colleagues have collected thousands of specimens from the deposit. To examine them, Chen teamed up with paleontologist David Bottjer of the University of Southern California in Los Angeles and Eric Davidson of the California Institute of Technology in Pasadena. In nine cross sections of similar specimens, they identified several key features of bilaterians. These include what appear to be a mouth, pharynx, and gut; three layers of tissue; body cavities, called coeloms, on either side of the gut; and pits in the soft outer surface that might have contained sensory organs. The researchers dubbed the creature Vernanimalcula guizhouena, or "Small Spring Animal," because the Doushantuo Formation was deposited after a glacial period.

Skeptics see it differently. "These may well have started out as fossils, but we can't say much about their morphology," says Stefan Bengtson of the Swedish Museum of Natural History in Stockholm. For example, Bengtson suspects that the presumed tissue layers are really thin, banded mineral crusts. But if Vernanimalcula is real, it might mean primitive bilaterians were smaller and simpler than many researchers had imagined, says Doug Erwin of the National Museum of Natural History in Washington, D.C.

Related sites
The Sciencexpress paper describing the find
More about the Cambrian Explosion