The Cambrian Period, 550-510 million years ago is often referred
to as the dawn of the explosion of life because it is a time
when great diversity of life forms were first recorded as fossils.
While multicelled organisms (metazoans) actually evolved much
earlier, the story is poorly documented in the fossil record
because they did not have protective outer coverings and soft
tissue is rarely preserved.
Most modern groups of invertebrates first appeared at the
beginning of the Cambrian Period. The major groups were arthropods
(animals with joined appendages such as insects, spiders, crabs),
echinoderms (spiny skinned animals such as starfish), cnidarians
(including corals, jellyfish and sea anemones), and mollusks
(clams, snails and squids).