DEFINITION OF PHANEROZOIC

The eon of geological time since the base of

the Cambrian at 544 million years ago and extending to the

present. 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 Precambrian. Although paleontologists now recognize

that many forms of life existed prior to the Phanerozoic, 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 absolute ages, for the divisions are continuously

being revised based on new geochronological studies of

important fossil-bearing units. It is divided into three main

fundamental time divisions—eras—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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