The table and timeline of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. When used in formal writing, specific geological and historical periods and events are capitalized. Some examples: the Silurian Period, the Ice Age, the Neolithic, the Archean; however, when not specific, some historical events remain uncapitalized, as in ice ages.
The Earth is thought by geologists to be 4.6 billion years old. The geologic or "deep" time of Earth's past has been organized into various periods according to events which took place in each period.
Dates are slightly uncertain with differences of a few percent between various sources being common. This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where we would most like to have them. Dates with an * are radiometrically determined based on internationally agreed to GSSPs. The dates quoted above are according to the International Commission on Stratigraphy 2004 time scale. All dates given are for the end of the interval in question.
Paleontologists often refer to faunal stages rather than geologic Periods. The Stage Nomenclature is quite complex. See Harland (http://flatpebble.nceas.ucsb.edu/cgi-bin/bridge.pl?action=startScale) for an excellent time ordered list of faunal stages. Also see the article on GSSPs.
The time shown in the "Years Ago" column is that of the end of the interval named beside it.
The GSSP for the base of the Ediacaran, ratified in 2004, is a climatic/geochemical marker rather than biostratigraphic. It represents the end of a widespread glaciation event.
The geologictime scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth.
The table of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geologic Survey.
Different spans of time on the time scale are usually delimited by major geologic or paleontologic events, such as mass extinctions.
Geologictime is divided into units based on major geological and biological events, such as mountain building phases ('orogeny' in Geo-speak) or mass extinctions (relatively brief periods during which several - sometimes many - lineages die out).
Many of these blocks of time have strange, exotic-sounding names that are generally derived from the region where they were first clearly differentiated from other such blocks.
Familiarity with the sequence of the major units of geologictime will greatly enhance your understanding of the evolution of life in general and that of sharks in particular.
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