What hermit crabs can do, the discreetly penis-shaped priaps or proboscis worms were apparently able to use around 500 million years ago: mussel and snail shells as housing. The proof has been provided by an international team of researchers in the precise evaluation of fossils from the Guanshan deposit site in China from the Cambrian. They report in the journal »Current Biology« of four strikingly similar imprints of the worms in mollusc shells.
The proboscis worms of the genus Eximipriapulus had obviously lived in the conically shaped shells of hyoliths during their lifetime, an extinct group of snail-like creatures from the Cambrian with a rather unclear position in the animal kingdom. Today they are thrown with mollusks and armfeet into the large kinship group of lophotrochozoa. So far, it has been assumed that the animals in the Cambrian had their greatest enemy in proboscis worms – among other things, because both groups often appear together in fossil sites.
Martin Smith and his colleagues now write that the proboscis worms may not only have hunted and eaten the primordial snails, but above all recycled them as armor against their own enemies. This is at least the best explanation for the finding situation: "The animals always sit neatly in identical positions in the shells," says Smith. It is astonishing that this type of interaction was observed shortly after the Cambrian explosion, in which many of the animal phyla began. Until now, shell recycling had only been found in the era of the dinosaurs: after the surge of new marine species in the Mesozoic, when early hermit crabs began to use shells as homes.
