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1650s, "from or of Wales or the Welsh," from Cambria, variant of Cumbria, Latinized derivation of Cymry, the name of the Welsh for themselves, from Old Celtic Combroges "compatriots." Geological sense (of rocks first studied in Wales and Cumberland) is from 1836.
The first period of the Paleozoic Era, from about 540 to 505 million years ago. During this time warm seas and desert land areas were widespread, and animal life diversified rapidly during what is known as the Cambrian Explosion. See Chart at geologic time.