Cambrian Explosion

The Cambrian Explosion is the commonly used term to denote the radiation of animal phyla that started about 570 million years ago which is 30 million years before beginning of the Cambrian geologic period and proceeded through the Cambrian. The Varangian glaciation, which preceded this radiation, along with subsequent greenhouse warming of the Earth, is theorized to have provided the evolutionary pressure. It is thought that severities in climate led to the "invention" of sexual reproduction which increased the rate of evolutionary change.

Evidence for the earlier animal forms that may have been the precursors of this radiation date from 600 million years. Notable among these are trace fossils[?] in the form of imprints of odd animals and their activity which paleontologists call the Ediacaran fauna[?]. These organisms were soft-bodied and are found with various strange body forms. Small shelly organisms of this period also included cloudinia.

However, the original source of fossils from the actual radiation period is the Burgess Shale in British Columbia. Some Burgess Shale organisms display strikingly unusual body plans.

A popular account of the paleontological analysis of the Burgess Shale is given in Wonderful Life[?] by Stephen Jay Gould.