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 Paleozoic is a major division of the geologic timescale, one of four geologiceras.
Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Rodinia and at the end of a global ice age.
At the start of the era, life was confined to bacteria, algae, sponges and a variety of somewhat enigmatic forms known collectively as the Ediacarian fauna.
The Eras are the next largest interval units into which the Geologic Time is divided and represented on the chart.
Eras encompass major intervals of Time and are defined based on the fossil life-forms found in the rock layers, and the Law of Superposition.
However, the Mesozoic Era landscape was also occupied by insects, early mammals, plants such as conifers and ferns, fish, and finally flowering plants and early birds.
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