# Paleontology/Geologic Time

Geologic time is the term used by geologists, paleontologists, and other Earth scientists to relate the relationships and timing of events in the history of Earth. The Geologic Time Scale is the representation of geologic time, listing the different groupings.
Originally in the study of Earth history, it was not possible to give specific dates to when things happened in the past. Even the total age of the Earth was debated. It is now known that the Earth is about 4.5 billion years old.
Because of that vast span of time, and the nature of the preservation of events in geology, we can't use a regular calendar. It's impossible to say, for instance, that a particular fossil fish died on February 12, 29,000,000 years ago. Using the geologic time scale allows us to group things together as to when they happened. Sometimes this grouping can be fairly precise, but even two fossils laying next to each other may not mean the organisms died together, or even within the same century. But we can say, probably, that they both lived (and died) in the Miocene Epoch (5.3-1.8 million years ago). It's a coarse system, but it allows us to follow the history of the Earth fairly well.
Modern dating techniques have given us better ideas regarding the timing of events over the course of the planet's history, and further refinements are being worked on all the time.

## About the Geologic Time Scale

The geologic time scale is governed by international agreement among scientists by the International Commission on Stratigraphy. The commission periodically releases updates and corrections as new information becomes available and accepted as accurate.
The time scale is divided into three Times, the earliest being Hadean Time which is the time before life evolved, one is Precambrian Time which was originally part of Hadean Time until fossils from it were found, the third has no name and includes everything since the Precambrian. Not the
The Times may be divided into Eras. The Eras may be divided into Periods. The Periods may be divided into Epochs. Not every division on the scale has subdivisions.
As you look over the scale below, note that the length of each section is not equal. The boundaries of each is set by some major geological event. In the case, for instance, of the Mesozoic Era, the ending is considered to be the extinction of the last of the non-avian dinosaurs about 65 million years ago.
The scale is presented from most recent to earliest. The earliest
One further note. The Anthropocene Epoch (1945 to present) is newly added to the time scale. This is the time of major human impact on the Earth. The timing of it's start is still widely debated. Some argue for it to have begun with wide-scaled agriculture abut 12,000 years ago, others place it beginning with the explosion of the first atomic bomb. Most scientists seem to currently favor the latter, so that is the date given here.

## The Geologic Time Scale

* Cenozoic Era (65 million years ago to present)
** Quaternary Period (1.8 million years ago to 1945)
*** Anthropocene Epoch (1945 to present)
*** Holocene Epoch (8,000 years ago to present)
*** Pleistocene Epoch (1.8 million to 8,000 years ago)
** Tertiary Period (65 to 1.8 million years ago)
*** Pliocene Epoch (5.3 to 1.8 million years ago)
*** Miocene Epoch (23.8 to 5.3 million years ago)
*** Oligocene Epoch (33.7 to 23.8 million years ago)
*** Eocene Epoch (55.5 to 33.7 million years ago)
*** Paleocene Epoch (65 to 55.5 million years ago)
* Mesozoic Era (248 to 65 million years ago)
** Cretaceous Period (145 to 65 million years ago)
** Jurassic Period (213 to 145 million years ago)
** Triassic Period (248 to 213 million years ago)
* Paleozoic Era (544 to 248 million years ago)
** Permian Period (286 to 248 million years ago)
** Carboniferous Period (360 to 286 million years ago)
** Pennsylvanian Period (325 to 286 million years ago)
** Mississippian Period (360 to 325 million years ago)
** Devonian Period (410 to 360 million years ago)
** Silurian Period (440 to 410 million years ago)
** Ordovician Period (505 to 440 million years ago)
** Cambrian Period (544 to 505 million years ago)
* Precambrian Time (4500 to 544 million years ago)
** Proterozoic Era (2500 to 544 million years ago)
*** Vendian Period (544 to 650 million years ago)
** Archaean Era (3800 to 2500 million years ago)
* Hadean Time (4500 to 3800 million years ago)

## External Links

* International Commission on Stratigraphy
* Geologic Time Scale Foundation
* The Anthropocene at Wikipedia