A paper out last week describes a strange fossil. It's a group of small shrimplike creatures attached head to tail. Nobody knows what they were doing 500 million years ago, but it must have been important for them to hold on to each other as they were buried in sediment. Feeding has been ruled out since that would have been nasty for all but the first one. Mating? Maybe it was a wedding that got out of hand.
Catalogue of Organisms, HMNH and Pharyngula have more insight into this mystery.