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The Cambrian is the first geological period of the Phanerozoic eon, lasting from 542 ± 0.3 million years ago to 488.3 ± 1.7 million years ago ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for Wales, where Britain's Cambrian rocks are best exposed.

The Cambrian is unique in its unusually high proportion of lagerstatte. These are sites of exceptional preservation, where 'soft' parts of organisms are preserved as well as their more resistant shells. This means that, paradoxically, our understanding of the Cambrian biota surpasses that of later periods.

The Cambrian period marked a profound change in life on Earth. Before the Cambrian, life was on the whole small and simple. Complex organisms became gradually more common in the millions of years immediately preceding the Cambrian, but it wasn't until this period that mineralised - hence readily fossilised - organisms became common. This diversification of lifeforms was relatively rapid, and is termed the Cambrian explosion. This explosion produced the first representatives of most modern phyla, but on the whole, most Cambrian animals look alien to today's eyes, falling in the evolutionary stems of modern groups. While life prospered in the oceans, the land was barren - with nothing more than a microbial 'crud' gracing the soils. Apart from tentative evidence suggesting that some animals floundered around on land, most of the continents resembled deserts spanning from horizon to horizon. Shallow seas flanked the margins of several continents, which had resulted from the relatively recent breakup of the preceding supercontinent Pannotia. The seas were relatively warm, and polar ice was absent.

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