Finding fossils of large-boned dinosaurs is hard enough, but imagine searching for fossilized jellyfish. Good specimens of such soft-tissue organisms are extremely rare. As a result, scientists have very little information about their role in early ecosystems. But a recent discovery of hundreds of jellyfish impressions on what were once the shores of an inland sea in central Wisconsin suggests that they occupied an important position on the food chain.

In a report appearing in the February issue of Geology, James W. Hagadorn of the California Institute of Technology and colleagues conclude that the fossilized jellyfish, which are similar in size to their modern descendants but much bigger than previously uncovered specimens from the same time, were some of the largest and most important predators during the Cambrian period. In other words, Hagadorn says, "we may have been inadvertently omitting a huge amount of information about all of the soft-bodied animals that were swimming around in the water column, munching on other organisms, but which were rarely fossilized."

The discovery represents one of only two jellyfish mass-strandings known from the fossil record. The authors hypothesize that either wind blew the jellyfish into shallower water or the creatures swam there in search of prey. In any case, the ebbing tide left them ashore. Analysis of the fossils shows that some of the jellyfish tried to keep moving, ingesting sediment in the process and essentially anchoring themselves to the floor. Several tidal cycles later, loose sediments had entombed them. Fortunately for scientists, not much seems to have disturbed their remains since then.