Scientists have long puzzled over how to classify Ediacarans, little floppy sea creatures that lived about 600 million years ago, because the only evidence they left are impressions in sandstone.

Now two paleontologists, Kenneth Schopf of Harvard and Tomasz Baumiller of the University of Michigan, Ann Arbor, have done a novel modeling experiment that they believe will help with Ediacaran taxonomy by showing that at least one type, the flat, oval-shaped Dickinsonia, may not have lived on the seabed surface, as is generally supposed.

Dickinsonid P. CRIMES

Schopf and Baumiller modeled the 13-cm-long creature in various densities of Jell-O-like Polygel, then subjected it to flowing water at different speeds. A dickinsonid as dense as a soft-bodied worm is unstable at currents fast enough to deposit the sands in which they are often preserved, the scientists report in the summer issue of Lethaia. To be stable, it would have to be as dense as a flounder or live buried in the seabed. Either option would rule out Dickinsonia's "conventional construction as a free-living, flattened worm," says Schopf.

Soren Jensen of Cambridge University says modeling Ediacarans in gel has "great potential"--but he still favors a scenario that places Dickinsonia in quiet locales where sticky microbial mats helped them adhere to the seabed.

Science 1998; 281: 511.