The geological period approximately 495-545 million years ago. Its name derives from the Roman name for Wales. The richly fossil-strewn sandstone and shale in north Wales is the 'type area' for this period.
Originally there was some overlap in the time period claimed as Cambrian and Silurian -- later the Ordovician period was added in between to clearly separate the periods.
The older boundary of the Cambrian separates it from the preceding Precambrain periods, based on the first appearance of certain distinctive fossil types in the in the 'type area' for the Ediacaran period at Mistaken Point in south-east Newfoundland. The newer boundary is demarcated by the mass extinciton of trilobites.
During the Cambrian, the Cambrian Explosion filled the seas with complex animals, and indeed
representatives of all known animal phyla appear somewhere in the rocks that form the Cambrian record. First among them was the trilobite, in its many varieties.
The period is thought to have been mild in climate, with high sea levels and no ice ages. About 505 million years ago, a massive mudslide in Laurentia buried a great number of creatures, creating the fossil-rich Burgess Shale.
Back: The Ediacaran period
On: The Ordovician period