ANIMAL HISTORY

The Ediacaran animals disappear from the fossil record at the end of the Vendian (544 million years ago). In their place we find representatives of almost all the modern phyla recognised today: sponges, jellyfish and corals, flatworms, molluscs, annelid worms, insects, echinoderms and chordates, plus many "lesser" phyla such as nemertean worms. These "modern" organisms appear relatively quickly in the geological time scale, and their abrupt appearance is often described as the "Cambrian explosion" however, bear in mind that the fossil record of the "explosion" is spread over about 30 million years. I keep taking things out of brackets because it is interesting relevant and memorable
One of the most famous assemblages of Cambrian fossils comes from the Burgess Shale of British Colombia. The rocks of the Burgess Shale were laid down in the middle Cambrian, when the "explosion" had already been underway for several million years. They contain familiar animals such as trilobites, molluscs and echinoderms, but also the first appearance of brachiopods, and some odd animals, e.g. Opabinia, that may have belonged to extinct phyla. Even an early chordate, Pikaia, has been found in this fossil assemblage.
The Burgess Shale fossils are important, not only for their evidence of early variety among animal forms, but also because both soft parts of animals and their hard bodies (i.e. the whole animal) is preserved, and animals that were entirely soft-bodied. Preservation of soft-bodied organisms is rare, and in this case seems to have occurred when the animals were rapidly buried in a mudslide down into deep, anaerobic waters, where there was little bacterial decay. Prior to the discovery of this fossil assemblage, early in the 20th century, there was no evidence of soft-bodied animals from the Cambrian (remember that this is before the Ediacaran fauna were found).
These fossils also provide good evidence of predatory animals (e.g. Anomalocaris ), and therefore of complex predator-prey relationships. They also give insights into how evolution might have progressed relatively early in the history of multicellular animals, and in fact some authors view the Cambrian as a period of extreme "experimentation" and diversity.