geologic time - West Virginia Geological and Economic Survey

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Cryptozoic Eon
The prefix crypto- comes from the Greek word for “hidden”, referring to the fact that
very few fossil remains are found in Cryptozoic rocks. For reasons that will become
apparent in the following discussion, the entire Cryptozoic Eon is commonly referred
to as the pre-Cambrian.
Some geologists subdivide the Cryptozoic Eon into the Archean and Proterozoic
Eras. This arrangement is based primarily on the distinguishing characteristic
shown in the box below.
Proterozoic Era
2.5 billion to 550 million years ago
Distinguishing characteristic: shallow-water marine life forms
Archean Era
4.5 to 2.5 billion years ago
Distinguishing characteristic: no large continents exist
Phanerozoic Eon
The prefix phanero- comes from the Greek word meaning “apparent” and refers to
the abundant fossil remains found in these rocks. Historically, the dividing line
between the Cryptozoic and Phanerozoic eons has been placed at the first
appearance of abundant fossil remains about 550M years ago. As a result, the
Phanerozoic Eon represents the last 550M years of Earth history.
For reasons that will become apparent in the following discussion, the entire
Cryptozoic Eon is commonly referred to as the pre-Cambrian.
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