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Phanerozoic The eon of geological time since the base of the Cambrian at 544 million years ago and extending to the present is called the Phanerozoic. Introduced by George H. Chadwick in 1930, the eon is characterized by the appearance of abundant visible life in the geological record, in contrast to the earlier eon referred to by Chadwick as the Cryptozoic, but now generally referred to as the Pre-cambrian. Although paleontologists now recognize that many forms of life existed prior to the Phanero-zoic, the first appearance of shelly fossils corresponds to the base of the Phanerozoic.
Phanerozoic time is divided mainly on the basis of fossil correlations, and new geochronological studies of important fossil-bearing units continuously force the revision of the absolute ages for the divisions. Its three main fundamental time divisions, called eras, include the Paleozoic, Mesozoic, and Cenozoic. These eras are in turn divided into smaller divisions known as periods, epochs, and ages.
See also Cenozoic; Mesozoic; Paleozoic.
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