A geologic period is a subdivision of geologic time that divides an era into smaller timeframes. The equivalent term used to demarcate rock layers and the fossil record is the system; thus the rocks of the Devonian System were laid down during the Devonian Period. While paleontologists often refer to faunal stages rather than geologic periods, they are often used in popular presentations of paleontology. Such references include the book and movie Jurassic Park.

As illustrated in the article on the geologic time scale, most periods are subdivided into smaller units called epochs. In 2004 the International Union of Geological Sciences (I.U.G.S.) recognized the Ediacaran period of the Neoproterozoic era, the first such newly-designated period in 130 years.

Eon Era Period Start, Million
Years Ago
Phanerozoic Cenozoic Neogene* (Miocene/Pliocene/Pleistocene/Holocene) 23.0
Paleogene (Paleocene/Eocene/Oligocene) 65.5
Mesozoic Cretaceous 145.5
Jurassic 200
Triassic 251
Paleozoic Permian 300
Carboniferous (Mississippian/Pennsylvanian) 359
Devonian 416
Silurian 444
Ordovician 488
Cambrian 542
Proterozoic Neoproterozoic Ediacaran 630
  • Some Geologists still designate a third period, or "sub-era, of the Cenozoic Era called the Quaternary, comprising the last two epochs of the Neogene Period and extending up to the present.
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