Trace fossil associations in the Swedish Mickwitzia sandstone (Lower Cambrian): Did trilobites really hunt for worms?

The spatial and temporal associations between Cruziana, trace fossils dug by foraging trilobites, and back filled burrows made by infaunal “worms” in a Lower Cambrian sandstone are described and discussed. In three large slabs and several smaller pieces, all from the same bedding plane in the lowermost part of the Mickwitzia sandstone (File Haidar Formation) at Kinnekulle, Västergötland, Sweden, Cruziana and worm burrows coincided more frequently than would be expected by chance. Overall, however, the association was weak and independent of the size of the worm trace. In 24 out of 29 cases, the worm burrow had deformed the Cruziana, implying that the arthropod was first on the spot. No clear case of the opposite situation was found. It is concluded that for an unknown reason the infaunal worms preferentially dug in places which had been visited earlier by foraging trilobites, or, possibly, that worms and trilobites preferred to dig in the same spots independently of each other. There is no evidence from this surface that the trilobites had been searching for worms or predated on them.