Geologic time scale

The geologic time 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.

The Earth is thought by geologists to be about 4,570 million years old. The geologic or "deep" time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geologic or paleontologic events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species.

Contents
1 Terminology
2 History of the timescale
3 Table of geologic time
4 Graphical timeline
5 References
6 See also
7 External link

Terminology

The largest defined unit is the Eon. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time, paleontologists define a system of faunal stages, of varying lengths, based changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geologic nomenclature, though in general there are far more recognized faunal stages than defined geologic time units.

Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units -- e.g. "Upper Jurassic", "Middle Cambrian". Because geologic units occuring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period of time was historically given different names in different locales. For example, in North America the Early Cambrian is refered to as the Waucoban series that is then subdivided into zones based on trilobites. The same timespan is split into Tommotian, Atdabanian and Botomian stages in East Asia and Siberia. It is a key aspect of the work of the International Commission on Stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world.

History of the timescale

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, there was no way to determine what time scale they represented. Creationists proposed dates of only a few thousand years, while others suggested large (and even infinite) ages. For over 100 years, the age of the Earth and of the rock strata was the subject of considerable debate until advances in the latter part of the 20th century allowed radioactive dating to provide relatively firm dates to geologic horizons. In the intervening century and a half, geologists and paleontologists constructed time scales based solely on the relative positions of different strata and fossils.

In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references (GSSPs) for geologic periods and faunal stages. Their most recent work is described in the 2004 geologic time scale of Gradstein et al. (ISBN 0521786738), and used as the foundation of this page.

Table of geologic time

Eon Era Period1 Series/
Epoch 		Major Events End, Million
Years Ago2
Phanerozoic Cenozoic Neogene3 Holocene End of recent glaciation and rise of modern civilization Ongoing
Pleistocene Flourishing and then extinction of many large mammals (Pleistocene megafauna); Evolution of fully modern humans 0.011430 ± 0.00013
Pliocene Intensification of present ice age. Cool and dry climate; Australopithecines appear, many of the existing genera of mammals, and recent molluscs appear 1.806 ± 0.005 *
Miocene Moderate climate; Mountain building in northern hemisphere; Modern mammal and bird families became recognizable. Horses and mastodonts diverse. Grasses become ubiquitous. First hominoids appear. 5.332 ± 0.005 *
Paleogene3 Oligocene Warm climate; Rapid evolution and diversification of fauna, especially mammals. Major evolution and dispersal of modern types of angiosperms 23.03 ± 0.05 *
Eocene Archaic mammals (e.g. Creodonts, Condylarths, Uintatheres, etc) flourish and continue to develop during the epoch. Appearance of several "modern" mammal families. Primitive whales diversify. First grasses. Reglaciation of Antarctica; start of current ice age. 33.9 ± 0.1 *
Paleocene Climate tropical. Modern plants; Mammals diversify into a number of primitive lineages following the extinction of the dinosaurs. First large mammals (up to bear or small hippo size) 55.8 ± 0.2 *
Mesozoic Cretaceous Upper/Late Flowering plants appear, along with new types of insects. More modern teleost fish begin to appear. Ammonites, belemnites, rudists, and sponges common. Many new types of dinosaurs (e.g. Tyrannosaurs, Titanosaurs, duck bills, and horned dinosaurs) evolve on land, as do modern crocodilians; and mosasaurs and modern sharks appear in the sea. Primitive birds gradually replace pterosaurs. Monotremes, marsupials and placental mammals appear. Break up of Gondwana. 65.5 ± 0.3 *
Lower/Early 99.6 ± 0.9 *
Jurassic Upper/Late Gymnosperms and ferns common. Many types of dinosaurs, such as sauropods, carnosaurs, and stegosaurs. Mammals common but small. First birds and lizards. Ichthyosaurs and plesiosaurs diverse. Ammonites and belemnites abundant. Breakup of Pangea into Gondwana and Laurasia 145.5 ± 4.0
Middle 161.2 ± 4.0
Lower/Early 175.6 ± 2.0 *
Triassic Upper/Late Archosaurs dominant and diverse on land; cynodonts become smaller and more mammal-like. Many large aquatic amphibians. Ichthyosaurs and ceratites common in the seas. First dinosaurs, mammals, teleosts, and crocodylia. 199.6 ± 0.6
Middle 228.0 ± 2.0
Lower/Early 245.0 ± 1.5
Paleozoic Permian Lopingian Landmass unites in the supercontinent of Pangea. Synapsid reptiles become common (Pelycosaurs and Therapsids), amphibians also remain common. Carboniferous flora replaced by gymnosperms in the middle of the period. Beetles and flies evolve. Marine life flourishes in the warm shallow reefs. End of Permo-carboniferous ice age. At the end of the period the Permian extinction event- 95% of life on Earth becomes extinct 251.0 ± 0.4 *
Guadalupian 260.4 ± 0.7 *
Cisuralian 270.6 ± 0.7 *
Carbon-
iferous4 		Pennsyl-
vanian 		Upper/Late Winged insects appear and are abundant, some growing to large size. Amphibians common and diverse. First reptiles, coal forests (Lepidodendron, Sigillaria, Calamites, Cordaites, etc), very high atmospheric oxygen content. 299.0 ± 0.8 *
Middle 306.5 ± 1.0
Lower/Early 311.7 ± 1.1
Missis-
sippian 		Upper/Late Large primitive trees, first land vertebrates, brackish water and amphibious eurypterids; rhizodonts dominant fresh-water predators. In the seas primitive sharks common and very diverse, echinoderms (especially crinoids and blastoids) abundant, brachiopods (Productida, Spriferida, etc) very common; trilobites and nautiloids in decline. Glaciation in East Gondwana. 318.1 ± 1.3 *
Middle 326.4 ± 1.6
Lower/Early 345.3 ± 2.1
Devonian Upper/Late First clubmosses and horsetails appear, progymnosperms (first seed bearing plants) appear, first trees (Archaeopteris). First Amphibians (but still aquatic). In the sea strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids abundant; ammonoids and coleoids appear; jawed fish (Placoderms, lobe-finned and ray-finned fish, and early sharks) important life in the sea. "Old Red Continent" (Euramerica) 359.2 ± 2.5 *
Middle 385.3 ± 2.6 *
Lower/Early 397.5 ± 2.7 *
Silurian Pridoli First vascular land plants, millipedes and arthropleurids, first jawed fish, sea-scorpions reach large size, tabulate and rugose corals and brachiopods very common; trilobites and molluscs diverse. Graptolites not as varied. 416.0 ± 2.8 *
Ludlow 418.7 ± 2.7 *
Wenlock 422.9 ± 2.5 *
Llandovery 428.2 ± 2.3 *
Ordovician Upper/Late Invertebrates dominant; they include brachiopods (Orthida, Strophomenida, etc), bivalves, nautiloids, trilobites, bryozoa, many types of echinoderms (cystoids, crinoids, starfish, etc), graptolites, and other taxa. Conodonts were primitive planktonic vertebrates that appear at the start of the Ordovician. Ice age at the end of the period. First land plants appear. 443.7 ± 1.5 *
Middle 460.9 ± 1.6 *
Lower/Early 471.8 ± 1.6
Cambrian Furongian Major diversification of life in the Cambrian Explosion; more than half of modern animal phyla appear, along with a number of extinct and problematic forms. Trilobites, Priapulida, sponges, inarticulate brachiopods, anomalocarids, and many other forms common. 488.3 ± 1.7 *
Middle 501.0 ± 2.0 *
Lower/Early 513.0 ± 2.0
Proterozoic5 Neo-
proterozoic 		Ediacaran First multi-celled animals 542.0 ± 1.0 *
Cryogenian Possible snowball Earth period, Rodinia begins to break up 630 +5/-30 *
Tonian First acritarch radiation 850 6
Meso-
proterozoic 		Stennian Formation of Rodinia 1000 6
Ectasian   1200 6
Calymmian   1400 6
Paleo-
proterozoic 		Statherian First complex single-celled life 1600 6
Orosirian Transition to oxygen atmosphere 1800 6
Rhyacian   2050 6
Siderian   2300 6
Archaean5 Neoarchean Stabilization of most modern cratons, possible mantle overturn event 2500 6
Mesoarchean First stromatolites 2800 6
Paleoarchean First known oxygen producing bacteria 3200 6
Eoarchean Simple single-celled life 3600 6
Hadean5,7 4100 MYA - Oldest known rock;
4400 MYA - Oldest known mineral;
4570 MYA - Formation of Earth 		3800
  1. Paleontologists often refer to faunal stages rather than geologic Periods. The stage nomenclature is quite complex. See Harland for an excellent time ordered list of faunal stages.
  2. 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 they would be most useful. The dates and errors quoted above are according to the International Commission on Stratigraphy 2004 time scale. Dates labeled with a * indicate boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed upon.
  3. Historically, the Cenozoic has been divided up into the Quaternary and Tertiary sub-eras, as well as the Neogene and Paleogene periods. However, the International Commission on Stratigraphy has recently decided to stop endorsing the terms Quaternary and Tertiary as part of the formal nomenclature.
  4. In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
  5. The Proterozoic, Archean and Hadean are often collectively referred to as Precambrian Time, and sometimes also as the Cryptozoic.
  6. Defined by absolute age (Global Standard Stratigraphic Age).
  7. Though commonly used, the Hadean is not a formal eon and no lower bound for the Eoarchean has been agreed upon. The Hadean has also sometimes been called the Priscoan.

Graphical timeline

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  from: -488.3   till: -443.7  color:ordovician
  from: -542   till: -488.3    color:cambrian
 
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  from: -1800  till:  -1600 text:Stath-~erian color:paleoproterozoic shift:(0,0.5)
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  from: -2300  till:  -2050 text:Rhy-~acian color:paleoproterozoic shift:(0,0.5)
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Millions of Years</center>

References

See also

External link

See also: Geologic time scale, Acritarch, Age of the Earth, Ammonite, Ammonoid, Amphibian, Angiosperm, Anomalocaris, Antarctica