n. A period of geologic time when the vast majority of animal body plans appeared that extends from approximately 545 to 490 million years ago.

First period of the Paleozoic Era. Main biological events include the first widespread appearance of organisms with the mineralised skeleton. Like during the Vendian, this period was the time of active «experiments»in the body plan of the multicellular animals. Alongwith the sponge-like archeocyathids, molluscs, brachiopods, echinoderms and arthropods there existed some forms of uncertain nature (so called «problematics»). Most of these taxa were short living groups which did not have descendants. Some of the problematics may have been the «hopeful monsters» which gave rise to the later widespread groups of animals.

A geological period 0.6-0.5 billion years ago during which life on Earth became diverse and complex.Cambrian rocks caontain the oldest easily identifiable fossiles.