Traces of soft-bodied beasties.

If you're a soft-bodied animal, chances are you won't leave a fossil legacy. The fossil record is made up mostly of shells, bones and other hard parts that are able to withstand decay and weathering long enough to become immortalized as fossils. This leaves paleontologists with a rather narrow view of the diversity and evolution of past life. So when scientists uncover fossils of the soft body parts -- as some did recently in a quarry near Milwaukee -- it's an event to write home about.

Donald Mikulic of the Illinois State Geological Survey in Champaign and co-workers report in the May 10 SCIENCE on the discovery of 15 different types of soft-bodied animals dating back 430 million years to the Silurian period. The Wisconsin find is important because it partially fills the gap between two previous major discoveries of fossilized soft animals: one dating from the middle Cambrian (about 530 million years ago) and the other from the Lower Devonian (about 400 million years ago).

Among the rather bizarre-looking creatures unveiled by Mikulic's group are worms with multiple-lensed compound eyes and two arthropods (invertebrates having jointed limbs and segmented covering) with limbs that look well adapted for snatching prey -- suggesting that these arthropods, unlike most other Silurian animals found before, were predatory. There is also a worm with a sucker disk, making it perhaps the earliest leech. "Leeches have only been found in the Jurassic in Germany," says Mikulic. "So this occurrence extends the range of leeches back over 200 million years."

The scientists also discovered a fossil of what may be the oldest uniramian (a type of arthropod that includes centipedes)--a possible candidate for the marine ancestor of terrestrial forms that show up some 30 million years later. Also included in the assemblage is the earliest, best preserved xiphosure (an animal that remotely resembles a horseshoe crab) and an arthropod that doesn't appear to be related to anything ever described before.

An important find is a 1-centimeter-long conodont animal, the oldest and fourth example ever found. Conodonts--usually 1- to 3-millimeter-long fossils that look like, but are not necessarily, relics of teeth--have puzzled paleontologists for over a century because alone they cannot be linked to any known animal phyla. Only recently have three arrays of conodonts been discovered in Scotland in conjunction with three whole fossilized animals. The Wisconsin find confirms these earlier discoveries.

Conspicuously absent from the Wisconsin assemblage are mollusks, corals, echinoderms and shelly animals that usually flood the Silurian record. "This means that there was something different about either the bottom conditions or the water chemistry itself that excluded those organisms from living in that particular spot," says Mikulic. This environment was also probably responsible for the unusual preservation of the soft-bodied animals. Mikulic suggests, for example, that the bottom waters lacked oxygen. This anoxic environment would have kept out not only the shelly animals, but also aerobic bacteria, which would have decomposed any soft animals that happened to float into the region. The lack of such bacteria would have given the soft animals a shot at being buried and fossilized.