In a study published in 2008, Siveter and his colleagues reported fossil evidence of similar behaviour from 525 million years ago. In this case, peculiar free- swimming arthropods had formed chains as they swam through the seas. Nowadays, strange invertebrates called tunicates form superficially similar-looking chains. However, not everyone is convinced that the trilobites were really migrating in groups. It is certainly possible that the Polish trilobite fossils are the remains of ancient seafloor migrations, but the question is far from settled, says Brian Chatterton at the University of Alberta in Edmonton, Canada. "I would prefer a verdict somewhere between unlikely and the Scottish verdict of not proven'.