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Cambrian period a geological period which began about 515 million years ago and ended 445 million years ago. At this time the British Isles lay in the southern hemisphere and the Sahara was at the South Pole and subject to extensive glaciation. Trilobites and Brachiopods flourished and most invertebrate phyla had evolved. The first graptolites appeared in the mid-Cambrian, but few traces of plants have been found and these are largely limited to simple algae. see GEOLOGICAL TIME.