Macroscopic animals were mainly filterers or surface and sediment feeders. Base of the food chain were planctonic organisms and algal mats which were filtered out of the sea water or directly fed from the ground by larger animals. Teeth for hunting or armour plates for defense against predators were unknown.
The Precambrian ends with the first occurence of shelly organisms. The vast majority of all known classes of animals suddenly appeared at the beginning of the Cambrian. Before scientists recognized that it's more a matter of the sudden appearance of preservable exoskeletons, there was a long discussion about the "Cambrian Explosion". According to this theory, the ancestors of all known organisms have evolved from a few groups of organisms within an astonishingly short time. Today it seems much more simple: It is now believed that those ancestors have evolved much earlier and underwent a far longer evolutionary process than presumed, but they didn't have had any hard body parts and therefore could not be fossilized.
(after FAUPL 2000, abbreviated)
The casts have probably been caused by protomeduses that packed sand into their body orifices to achieve a more stable position on the sedimentary surface. Unlike fossils of later successors from the Cambrian they don't show any structures of the soft body.
Vendian, Mogilev Region (Ukraine)