What it was
A flat, ribbed oval with no head, no mouth and no shell. The smallest are a few millimeters across. The largest is over a meter; stand one on end and it comes up to your waist. Its ribs never quite meet down the middle: each sits half a step off the one opposite.
How we know
For seventy years nobody could prove what it was. Then scientists sampled a thin film of its own carbon, preserved on slabs from the White Sea in northern Russia. Animals build their cell skins with cholesterol, a molecule with 27 carbon atoms. In the film, 93 percent was that animal kind. In the rock a few millimeters away, only 11 percent.
Still a mystery
The film also held a molecule that usually forms inside a gut, and nobody has ever found a gut in Dickinsonia. Scientists are still arguing about why.