Fossil burrows in ocean sediments from the Precambrian–Cambrian period about 540 million years ago are ubiquitous in the fossil record, but the creatures that created these Treptichnus burrows (pictured, left) remained a mystery. Now Jean Vannier at the University of Lyon, France, and his colleagues propose that the burrowers were priapulid worms, examples of which still exist today.
They collected modern priapulid worms (right) off the Swedish coast and watched them move about in sediment-filled trays in the lab. The traces made by these animals were similar to patterns of the fossil burrows. These included segmented branching structures, with alternating straight, curved and looped sections.
Priapulids seem to have been one of the earliest animal colonizers of the ocean bottom sediment, the authors say.