The Cambrian explosion is a radiation of animals with hard parts that marks the start of a new eon in geologic time.
The Cambrian Period is the first geologic period of the Paleozoic Era ("Time of Ancient Life"). This period lasted from 541 million to 485.4 million years ago, more than 55 million years ago, and marked a dramatic burst of evolutionary change in life on Earth, known as the "Cambrian Explosion". Among the animals that evolved during this period were chordates — animals with a dorsal nerve cord; brachiopods with hard bodies that resembled clams; and arthropods—the ancestors of spiders, insects, and crustaceans.
Although there is some scientific debate as to which fossil strata should mark the beginning of this period, the International Commission on Stratigraphy places the lower limit of the period at 541 million years ago, when worms making horizontal burrows first appeared in the fossil record. The end of the Cambrian period is marked by evidence in the fossil record of a mass extinction about 485.4 million years ago. The Cambrian period was followed by the Ordovician period.
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