Geologic time scale

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This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before the fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).[1] Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

The geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy) in time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events in geologic history. The time scale was developed through the study and observation of layers of rock and relationships as well as the times when different organisms appeared, evolved and became extinct through the study of fossilized remains and imprints. The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).

Terminology[edit]

The largest catalogued divisions of time are intervals called eons. The first eon was the Hadean, starting with the formation of the Earth and lasting about 540 million years until the Archean eon, which is when the Earth had cooled enough for continents and the earliest known life to emerge. After about 1.5 billion years, oxygen generated by photosynthesizing single-celled organisms began to appear in the atmosphere, marking the beginning of the Proterozoic. Finally, the Phanerozoic eon encompasses 539 million years of diverse abundance of multicellular life, starting with the appearance of hard animal shells in the fossil record and continuing to the present. The first three eons (i.e. every eon but the Phanerozoic) can be referred to collectively as the Precambrian supereon. This is because of the significance of the Cambrian explosion, a massive diversification of multicellular life forms that took place in the Cambrian period at the start of the Phanerozoic. Eons are divided into eras, which are in turn divided into periods, epochs and ages.[2][3] A polarity chron or just "chron" can be used as a subdivision of an age, though this is not included in the ICS system.

Eon Era Period Extent, millions of
years ago
Duration
(millions
of years)
Phanerozoic Cenozoic Quaternary (Pleistocene/Holocene) 2.588 to 0 2.588+
Neogene (Miocene/Pliocene) 23.03 to 2.588 20.4
Paleogene (Paleocene/Eocene/Oligocene) 66.0 to 23.03 42.9
Mesozoic Cretaceous 145.0 to 66.0 79
Jurassic 201.3 to 145.0 56.3
Triassic 251.902 to 201.3 50.6
Paleozoic Permian 298.9 to 251.902 46.9
Carboniferous (Mississippian/Pennsylvanian) 358.9 to 298.9 60
Devonian 419.2 to 358.9 60.3
Silurian 443.4 to 419.2 24.2
Ordovician 485.4 to 443.4 42
Cambrian 538.8 to 485.4 53.4
Proterozoic Neoproterozoic Ediacaran 635.0 to 538.8 96.2
Cryogenian 720 to 635 85
Tonian 1,000 to 720 280
Mesoproterozoic Stenian 1,200 to 1,000 200
Ectasian 1,400 to 1,200 200
Calymmian 1,600 to 1,400 200
Paleoproterozoic Statherian 1,800 to 1,600 200
Orosirian 2,050 to 1,800 250
Rhyacian 2,300 to 2,050 250
Siderian 2,500 to 2,300 200
Archean Neoarchean Not officially divided into periods 2,800 to 2,500 300
Mesoarchean 3,200 to 2,800 400
Paleoarchean 3,600 to 3,200 400
Eoarchean 4,000 to 3,600 400
Hadean Not officially divided into eras Not officially divided into periods From formation of Earth
to 4,000
540
Units in geochronology and stratigraphy[4]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Eonothem Eon 4 total, half a billion years or more
Erathem Era 10 defined, several hundred million years
System Period 22 defined, tens to ~one hundred million years
Series Epoch 34 defined, tens of millions of years
Stage Age 99 defined, millions of years
Chronozone Chron subdivision of an age, not used by the ICS timescale
Visual timelines including ages

The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

SiderianRhyacianOrosirianStatherianCalymmianEctasianStenianTonianCryogenianEdiacaranEoarcheanPaleoarcheanMesoarcheanNeoarcheanPaleoproterozoicMesoproterozoicNeoproterozoicPaleozoicMesozoicCenozoicHadeanArcheanProterozoicPhanerozoicPrecambrian
CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogeneQuaternaryPaleozoicMesozoicCenozoicPhanerozoic
PaleoceneEoceneOligoceneMiocenePliocenePleistoceneHolocenePaleogeneNeogeneQuaternaryCenozoic
GelasianCalabrian (stage)ChibanianPleistocenePleistoceneHoloceneQuaternary
GreenlandianNorthgrippianMeghalayanHolocene
Millions of Years (1st, 2nd, 3rd, and 4th)
Thousands of years (5th)

Corresponding to eons, eras, periods, epochs and ages, the terms "eonothem", "erathem", "system", "series", "stage" are used to refer to the layers of rock that belong to these stretches of geologic time in Earth's history.

Geologists qualify these units as "early", "mid", and "late" when referring to time, and "lower", "middle", and "upper" when referring to the corresponding rocks. For example, the Lower Jurassic Series in chronostratigraphy corresponds to the Early Jurassic Epoch in geochronology.[1] The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic."

Era definitions[edit]

The Phanerozoic Eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries: the P-T boundary between the Paleozoic and the Mesozoic, and the K-Pg boundary between the Mesozoic and the Cenozoic.[5] There is evidence that the P-T boundary was caused by the eruption of the Siberian Traps, and the K-Pg boundary was caused by the meteorite impact that created the Chicxulub crater.[6]

The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, the Archean has been divided into four eras and the Proterozoic has been divided into three eras.

Period definitions[edit]

The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world.[7] In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion, and was the first such newly designated period in 130 years.[8]

A consequence of this approach to the Phanerozoic periods is that the ages of their beginnings and ends can change from time to time as the absolute age of the chosen rock sequences, which define them, is more precisely determined.[9]

The set of rocks (sedimentary, igneous or metamorphic) formed during a period belong to a chronostratigraphic unit called a system.[10] For example, the "Jurassic System" of rocks was formed during the "Jurassic Period" (between 201 and 145 million years ago).[10]

Principles[edit]

Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.[11][12] The geology or deep time of Earth's past has been organized into various units according to events that are thought to have taken place. Different spans of time on the GTS are usually marked by corresponding changes in the composition of strata which indicate major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs as well as many other groups of life. Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.

Geologic units from the same time but different parts of the world often are not similar and contain different fossils, so the same time-span was historically given different names in different locales. For example, in North America, the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on the succession of trilobites. In East Asia and Siberia, the same unit is split into Alexian, Atdabanian, and Botomian stages. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.[13]

Some other planets and moons in the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus, Mars and the Earth's Moon. Dominantly fluid planets, such as the gas giants, do not comparably preserve their history. Apart from the Late Heavy Bombardment, events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets. Construction of a time scale that links the planets is, therefore, of only limited relevance to the Earth's time scale, except in a Solar System context. The existence, timing, and terrestrial effects of the Late Heavy Bombardment are still a matter of debate.[a]

History and nomenclature of the time scale[edit]

Graphical representation of Earth's history as a spiral

Early history[edit]

In Ancient Greece, Aristotle (384–322 BCE) observed that fossils of seashells in rocks resembled those found on beaches – he inferred that the fossils in rocks were formed by organisms, and he reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci (1452–1519) concurred with Aristotle's interpretation that fossils represented the remains of ancient life.[14]

The 11th-century Persian polymath Avicenna (Ibn Sina, died 1037) and the 13th-century Dominican bishop Albertus Magnus (died 1280) extended Aristotle's explanation into a theory of a petrifying fluid.[15] Avicenna also first proposed one of the principles underlying geologic time scales, the law of superposition of strata, while discussing the origins of mountains in The Book of Healing (1027).[16] The Chinese naturalist Shen Kuo (1031–1095) also recognized the concept of "deep time".[17]

Establishment of primary principles[edit]

In the late 17th century Nicolas Steno (1638–1686) pronounced the principles underlying geologic (geological) time scales. Steno argued that rock layers (or strata) were laid down in succession and that each represents a "slice" of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them proved challenging. Steno's ideas also lead to other important concepts geologists use today, such as relative dating. Over the course of the 18th-century geologists realized that:

  1. Sequences of strata often become eroded, distorted, tilted, or even inverted after deposition
  2. Strata laid down at the same time in different areas could have entirely different appearances
  3. The strata of any given area represented only part of Earth's long history

The Neptunist theories popular at this time (expounded by Abraham Werner (1749–1817) in the late 18th century) proposed that all rocks had precipitated out of a single enormous flood. A major shift in thinking came when James Hutton presented his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe[18] before the Royal Society of Edinburgh in March and April 1785. John McPhee asserts that "as things appear from the perspective of the 20th century, James Hutton in those readings became the founder of modern geology".[19]: 95–100  Hutton proposed that the interior of Earth was hot and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone and uplifted it into new lands. This theory, known as "Plutonism", stood in contrast to the "Neptunist" flood-oriented theory.

Formulation of geologic time scale[edit]

The first serious attempts to formulate a geologic time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Werner, among others) divided the rocks of Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleogene and Neogene) remained in use as the name of a geological period well into the 20th century and "Quaternary" remains in formal use as the name of the current period.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brongniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geologic periods still used today.

Naming of geologic periods, eras and epochs[edit]

Early work on developing the geologic time scale was dominated by British geologists, and the names of the geologic periods reflect that dominance. The "Cambrian", (the classical name for Wales) and the "Ordovician" and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales.[19]: 113–114  The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was an adaptation of "the Coal Measures", the old British geologists' term for the same set of strata. The "Permian" was named after the region of Perm in Russia, because it was defined using strata in that region by Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) – red beds, capped by chalk, followed by black shales – that are found throughout Germany and Northwest Europe, called the ‘Trias’. The "Jurassic" was named by a French geologist Alexandre Brongniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning ‘chalk’) as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin[20] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates) found in Western Europe.

British geologists were also responsible for the grouping of periods into eras and the subdivision of the Tertiary and Quaternary periods into epochs. In 1841 John Phillips published the first global geologic time scale based on the types of fossils found in each era. Phillips' scale helped standardize the use of terms like Paleozoic ("old life"), which he extended to cover a larger period than it had in previous usage, and Mesozoic ("middle life"), which he invented.[21]

Dating of time scales[edit]

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since estimates of rates of change were uncertain. While creationists had been proposing dates of around six or seven thousand years for the age of Earth based on the Bible, early geologists were suggesting millions of years for geologic periods, and some were even suggesting a virtually infinite age for Earth.[citation needed] Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century, the ages of various rock strata and the age of Earth were the subject of considerable debate.

The first geologic time scale that included absolute dates was published in 1913 by the British geologist Arthur Holmes.[22] He greatly furthered the newly created discipline of geochronology and published the world-renowned book The Age of the Earth in which he estimated Earth's age to be at least 1.6 billion years.[23]

In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system.[24]

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) began to define global references known as GSSP (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's work is described in the 2012 geologic time scale of Gradstein et al.[9] A UML model for how the timescale is structured, relating it to the GSSP, is also available.[25]

Correlation issues[edit]

American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right though the ICS now recognises them both as "subperiods" of the Carboniferous Period recognised by European geologists.[26] Cases like this in China, Russia and even New Zealand with other geological eras has slowed the uniform organization of the stratigraphic record.[27]

The Anthropocene[edit]

Popular culture and a growing number of scientists use the term "Anthropocene" informally to label the current epoch in which we are living.[28] The term was coined by Paul Crutzen and Eugene Stoermer in 2000 to describe the current time in which humans have had an enormous impact on the environment. It has evolved to describe an "epoch" starting some time in the past and on the whole defined by anthropogenic carbon emissions and production and consumption of plastic goods that are left in the ground.[29]

Critics of this term say that the term should not be used because it is difficult, if not nearly impossible, to define a specific time when humans started influencing the rock strata – defining the start of an epoch.[30]

The ICS has not officially approved the term as of September 2015.[31] The Anthropocene Working Group met in Oslo in April 2016 to consolidate evidence supporting the argument for the Anthropocene as a true geologic epoch.[31] Evidence was evaluated and the group voted to recommend "Anthropocene" as the new geological age in August 2016.[32] Should the International Commission on Stratigraphy approve the recommendation, the proposal to adopt the term will have to be ratified by the International Union of Geological Sciences before its formal adoption as part of the geologic time scale.[33]

Notable period changes[edit]

  • Changes in recent years have included the abandonment of the former Tertiary Period in favour of the Paleogene and succeeding Neogene periods. This remains controversial.[34]
  • The abandonment of the Quaternary period was also considered but it has been retained for continuity reasons.[35]
  • Even earlier in the history of the science, the Tertiary was considered to be an "era" and its subdivisions (Paleocene, Eocene, Oligocene, Miocene and Pliocene) were themselves referred to as "periods"[36] but they now enjoy the status of "epochs" within the more recently delineated Paleogene and Neogene periods.[7]

Table of geologic time[edit]

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. This table is arranged with the most recent geologic periods at the top, and the oldest at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time.

The content of the table is based on the official geologic time scale of the International Commission on Stratigraphy (ICS).[1] The current version is provided by the ICS online.[37] The ICS provides an online interactive version of this chart, ics-chart, based on a service delivering a machine-readable Resource Description Framework/Web Ontology Language representation of the timescale, which is available through the Commission for the Management and Application of Geoscience Information GeoSciML project as a service[38] and at a SPARQL end-point.[39][40]

The chronostratrigraphic epoch/subepoch names are altered to the early/late format from lower/upper of the equivalent geochronologic series/subseries as is recommended by the ICS.[2] Subseries/subepochs for the Neogene have been ratified as of 13 October 2021.[41]

This table is not to scale, and even though the Phanerozoic eon looks longer than the rest, it merely spans 539 million years, whilst the previous three eons (or the Precambrian supereon) collectively span over 3.5 billion years. This bias toward the most recent eon is due to the relative lack of information about events that occurred during the first three eons (or supereon) compared to the current eon (the Phanerozoic).[42][43]

The proposed Anthropocene epoch is not included.

Supereon Eon Era Period[b] Epoch Age[c] Major events Start, million years ago[c]
n/a[d] Phanerozoic Cenozoic[e] Quaternary Holocene Meghalayan 4.2-kiloyear event, Austronesian expansion (finally occupying Madagascar and Remote Oceania), increasing industrial CO2. 0.0042*
Northgrippian 8.2-kiloyear event, Holocene climatic optimum. Sea level flooding of Doggerland and Sundaland. Desertification of the Sahara and Arabia. End of Stone Age and start of recorded history. Humans finally expand into the Arctic Archipelago and Greenland. 0.0082*
Greenlandian Climate stabilizes. Current interglacial and Holocene extinction begins. Agriculture begins. Humans spread across the wet Sahara and Arabia, the Extreme North, and the Americas (mainland and the Caribbean). 0.0117*
Pleistocene Late ('Tarantian') Eemian interglacial, last glacial period, ending with Younger Dryas. Toba eruption. Pleistocene megafauna (including the last terror birds) extinction. Humans expand into Near Oceania and the Americas. 0.129
Chibanian Mid-Pleistocene Transition occurs, high amplitude 100 ka glacial cycles. Rise of Homo sapiens. 0.774*
Calabrian Further cooling of the climate. Giant terror birds go extinct. Spread of Homo erectus across Afro-Eurasia. 1.8*
Gelasian Start of Quaternary glaciations and unstable climate.[46] Rise of the Pleistocene megafauna and Homo habilis. 2.58*
Neogene Pliocene Piacenzian Greenland ice sheet develops[47] as the cold slowly intensifies towards the Pleistocene. Atmospheric O2 and CO2 content reaches present day levels while landmasses also reach their current locations (e.g. the Isthmus of Panama joins the North and South Americas, while allowing a faunal interchange). The last non-marsupial metatherians go extinct. Australopithecus common in East Africa; Stone Age begins.[48] 3.6*
Zanclean Zanclean flooding of the Mediterranean Basin. Cooling climate continues from the Miocene. First equines and elephantines. Ardipithecus in Africa.[48] 5.333*
Miocene Messinian Messinian Event with hypersaline lakes in empty Mediterranean Basin. Moderate icehouse climate, punctuated by ice ages and re-establishment of East Antarctic Ice Sheet. Choristoderes, the last non-crocodilian crocodylomorphs and creodonts go extinct. After separating from gorilla ancestors, chimpanzee and human ancestors gradually separate; Sahelanthropus and Orrorin in Africa. 7.246*
Tortonian 11.63*
Serravallian Middle Miocene climate optimum temporarily provides a warm climate.[49] Extinctions in middle Miocene disruption, decreasing shark diversity. First hippos. Ancestor of great apes. 13.82*
Langhian 15.97
Burdigalian Orogeny in Northern Hemisphere. Start of Kaikoura Orogeny forming Southern Alps in New Zealand. Widespread forests slowly draw in massive amounts of CO2, gradually lowering the level of atmospheric CO2 from 650 ppmv down to around 100 ppmv during the Miocene.[50][f] Modern bird and mammal families become recognizable. The last of the primitive whales go extinct. Grasses become ubiquitous. Ancestor of apes, including humans.[51] Afro-Arabia collides with Eurasia, fully forming the Alpide Belt and closing the Tethys Ocean, while allowing a faunal interchange. At the same time, Afro-Arabia splits into Africa and West Asia. 20.44
Aquitanian 23.03*
Paleogene Oligocene Chattian Grande Coupure extinction. Start of widespread Antarctic glaciation.[52] Rapid evolution and diversification of fauna, especially mammals (e.g. first macropods and seals). Major evolution and dispersal of modern types of flowering plants. Cimolestans, miacoids and condylarths go extinct. First neocetes (modern, fully aquatic whales) appear. 27.82
Rupelian 33.9*
Eocene Priabonian Moderate, cooling climate. Archaic mammals (e.g. creodonts, miacoids, "condylarths" etc.) flourish and continue to develop during the epoch. Appearance of several "modern" mammal families. Primitive whales and sea cows diversify after returning to water. Birds continue to diversify. First kelp, diprotodonts, bears and simians. The multituberculates and leptictidans go extinct by the end of the epoch. Reglaciation of Antarctica and formation of its ice cap; End of Laramide and Sevier Orogenies of the Rocky Mountains in North America. Hellenic Orogeny begins in Greece and Aegean Sea. 37.71*
Bartonian 41.2
Lutetian 47.8*
Ypresian Two transient events of global warming (PETM and ETM-2) and warming climate until the Eocene Climatic Optimum. The Azolla event decreased CO2 levels from 3500 ppm to 650 ppm, setting the stage for a long period of cooling.[50][f] Greater India collides with Eurasia and starts Himalayan Orogeny (allowing a biotic interchange) while Eurasia completely separates from North America, creating the North Atlantic Ocean. Maritime Southeast Asia diverges from the rest of Eurasia. First passerines, ruminants, pangolins, bats and true primates. 56*
Paleocene Thanetian Starts with Chicxulub impact and the K-Pg extinction event, wiping out all non-avian dinosaurs and pterosaurs, most marine reptiles, many other vertebrates (e.g. many Laurasian metatherians), most cephalopods (only Nautilidae and Coleoidea survived) and many other invertebrates. Climate tropical. Mammals and birds (avians) diversify rapidly into a number of lineages following the extinction event (while the marine revolution stops). Multituberculates and the first rodents widespread. First large birds (e.g. ratites and terror birds) and mammals (up to bear or small hippo size). Alpine orogeny in Europe and Asia begins. First proboscideans and plesiadapiformes (stem primates) appear. Some marsupials migrate to Australia. 59.2*
Selandian 61.6*
Danian 66*
Mesozoic Cretaceous Late Maastrichtian Flowering plants proliferate (after developing many features since the Carboniferous), along with new types of insects, while other seed plants (gymnosperms and seed ferns) decline. More modern teleost fish begin to appear. Ammonoids, belemnites, rudist bivalves, sea urchins and sponges all common. Many new types of dinosaurs (e.g. tyrannosaurs, titanosaurs, hadrosaurs, and ceratopsids) evolve on land, while crocodilians appear in water and probably cause the last temnospondyls to die out; and mosasaurs and modern types of sharks appear in the sea. The revolution started by marine reptiles and sharks reaches its peak, though ichthyosaurs vanish few million years after being heavily reduced at the Bonarelli Event. Toothed and toothless avian birds coexist with pterosaurs. Modern monotremes, metatherian (including marsupials, who migrate to South America) and eutherian (including placentals, leptictidans and cimolestans) mammals appear while the last non-mammalian cynodonts die out. First terrestrial crabs. Many snails become terrestrial. Further breakup of Gondwana creates South America, Afro-Arabia, Antarctica, Oceania, Madagascar, Greater India, and the South Atlantic, Indian and Antarctic Oceans and the islands of the Indian (and some of the Atlantic) Ocean. Beginning of Laramide and Sevier Orogenies of the Rocky Mountains. Atmospheric oxygen and carbon dioxide levels similar to present day. Acritarchs disappear. Climate initially warm, but later it cools. 72.1 ± 0.2*
Campanian 83.6 ± 0.2
Santonian 86.3 ± 0.5*
Coniacian 89.8 ± 0.3
Turonian 93.9*
Cenomanian 100.5*
Early Albian ~113*
Aptian ~121.4
Barremian ~129.4
Hauterivian ~132.6*
Valanginian ~139.8
Berriasian ~145
Jurassic Late Tithonian Climate becomes humid again. Gymnosperms (especially conifers, cycads and cycadeoids) and ferns common. Dinosaurs, including sauropods, carnosaurs, stegosaurs and coelurosaurs, become the dominant land vertebrates. Mammals diversify into shuotheriids, australosphenidans, eutriconodonts, multituberculates, symmetrodonts, dryolestids and boreosphenidans but mostly remain small. First birds, lizards, snakes and turtles. First brown algae, rays, shrimps, crabs and lobsters. Parvipelvian ichthyosaurs and plesiosaurs diverse. Rhynchocephalians throughout the world. Bivalves, ammonoids and belemnites abundant. Sea urchins very common, along with crinoids, starfish, sponges, and terebratulid and rhynchonellid brachiopods. Breakup of Pangaea into Laurasia and Gondwana, with the latter also breaking into two main parts; the Pacific and Arctic Oceans form. Tethys Ocean forms. Nevadan orogeny in North America. Rangitata and Cimmerian orogenies taper off. Atmospheric CO2 levels 3–4 times the present day levels (1200–1500 ppmv, compared to today's 400 ppmv[50][f]). Crocodylomorphs (last pseudosuchians) seek out an aquatic lifestyle. Mesozoic marine revolution continues from late Triassic. Tentaculitans disappear. 152.1 ± 0.9
Kimmeridgian 157.3 ± 1.0*
Oxfordian 163.5 ± 1.0
Middle Callovian 166.1 ± 1.2
Bathonian 168.3 ± 1.3*
Bajocian 170.3 ± 1.4*
Aalenian 174.1 ± 1.0*
Early Toarcian 182.7 ± 0.7*
Pliensbachian 190.8 ± 1.0*
Sinemurian 199.3 ± 0.3*
Hettangian 201.3 ± 0.2*
Triassic Late Rhaetian Archosaurs dominant on land as pseudosuchians and in the air as pterosaurs. Dinosaurs also arise from bipedal archosaurs. Ichthyosaurs and nothosaurs (a group of sauropterygians) dominate large marine fauna. Cynodonts become smaller and nocturnal, eventually becoming the first true mammals, while other remaining synapsids die out. Rhynchosaurs (archosaur relatives) also common. Seed ferns called Dicroidium remained common in Gondwana, before being replaced by advanced gymnosperms. Many large aquatic temnospondyl amphibians. Ceratitidan ammonoids extremely common. Modern corals and teleost fish appear, as do many modern insect orders and suborders. First starfish. Andean Orogeny in South America. Cimmerian Orogeny in Asia. Rangitata Orogeny begins in New Zealand. Hunter-Bowen Orogeny in Northern Australia, Queensland and New South Wales ends, (c. 260–225 Ma). Carnian pluvial event occurs around 234-232 Ma, allowing the first dinosaurs and lepidosaurs (including rhynchocephalians) to radiate. Triassic-Jurassic extinction event occurs 201 Ma, wiping out all conodonts and the last parareptiles, many marine reptiles (e.g. all sauropterygians except plesiosaurs and all ichthyosaurs except parvipelvians), all crocopodans except crocodylomorphs, pterosaurs, and dinosaurs, and many ammonoids (including the whole Ceratitida), bivalves, brachiopods, corals and sponges. First diatoms.[53] ~208.5
Norian ~227
Carnian ~237*
Middle Ladinian ~242*
Anisian 247.2
Early Olenekian 251.2
Induan 251.902 ± 0.024*
Paleozoic Permian Lopingian Changhsingian Landmasses unite into supercontinent Pangaea, creating the Urals, Ouachitas and Appalachians, among other mountain ranges (the superocean Panthalassa or Proto-Pacific also forms). End of Permo-Carboniferous glaciation. Hot and dry climate. A possible drop in oxygen levels. Synapsids (pelycosaurs and therapsids) become widespread and dominant, while parareptiles and temnospondyl amphibians remain common, with the latter probably giving rise to modern amphibians in this period. In the mid-Permian, lycophytes are heavily replaced by ferns and seed plants. Beetles and flies evolve. The very large arthropods and non-tetrapod tetrapodomorphs go extinct. Marine life flourishes in warm shallow reefs; productid and spiriferid brachiopods, bivalves, forams, ammonoids (including goniatites), and orthoceridans all abundant. Crown reptiles arise from earlier diapsids, and split into the ancestors of lepidosaurs, kuehneosaurids, choristoderes, archosaurs, testudinatans, ichthyosaurs, thalattosaurs, and sauropterygians. Cynodonts evolve from larger therapsids. Olson's Extinction (273 Ma), End-Capitanian extinction (260 Ma), and Permian-Triassic extinction event (252 Ma) occur one after another: more than 80% of life on Earth becomes extinct in the lattermost, including most retarian plankton, corals (Tabulata and Rugosa die out fully), brachiopods, bryozoans, gastropods, ammonoids (the goniatites die off fully), insects, parareptiles, synapsids, amphibians, and crinoids (only articulates survived), and all eurypterids, trilobites, graptolites, hyoliths, edrioasteroid crinozoans, blastoids and acanthodians. Ouachita and Innuitian orogenies in North America. Uralian orogeny in Europe/Asia tapers off. Altaid orogeny in Asia. Hunter-Bowen Orogeny on Australian continent begins (c. 260–225 Ma), forming the MacDonnell Ranges. 254.14 ± 0.07*
Wuchiapingian 259.51 ± 0.21*
Guadalupian Capitanian 264.28 ± 0.16*
Wordian 266.9 ± 0.4*
Roadian 273.01 ± 0.14*
Cisuralian Kungurian 283.5 ± 0.6
Artinskian 290.1 ± 0.26*
Sakmarian 293.52 ± 0.17*
Asselian 298.9 ± 0.15*
Carbon-
iferous
[g]
Pennsylvanian Gzhelian Winged insects radiate suddenly; some (esp. Protodonata and Palaeodictyoptera) of them as well some millipedes and scorpions become very large. First coal forests (scale trees, ferns, club trees, giant horsetails, Cordaites, etc.). Higher atmospheric oxygen levels. Ice Age continues to the Early Permian. Goniatites, brachiopods, bryozoa, bivalves, and corals plentiful in the seas and oceans. First woodlice. Testate forams proliferate. Euramerica collides with Gondwana and Siberia-Kazakhstania, the latter of which forms Laurasia and the Uralian orogeny. Variscan orogeny continues (these collisions created orogenies, and ultimately Pangaea). Amphibians (e.g. temnospondyls) spread in Euramerica, with some becoming the first amniotes. Carboniferous Rainforest Collapse occurs, initiating a dry climate which favors amniotes over amphibians. Amniotes diversify rapidly into synapsids, parareptiles, cotylosaurs, protorothyridids and diapsids. Rhizodonts remained common before they died out by the end of the period. First sharks. 303.7 ± 0.1
Kasimovian 307 ± 0.1
Moscovian 315.2 ± 0.2
Bashkirian 323.2 ± 0.4*
Mississippian Serpukhovian Large lycopodian primitive trees flourish and amphibious eurypterids live amid coal-forming coastal swamps, radiating significantly one last time. First gymnosperms. First holometabolous, paraneopteran, polyneopteran, odonatopteran and ephemeropteran insects and first barnacles. First five-digited tetrapods (amphibians) and land snails. In the oceans, bony and cartilaginous fishes are dominant and diverse; echinoderms (especially crinoids and blastoids) abundant. Corals, bryozoans, orthoceridans, goniatites and brachiopods (Productida, Spiriferida, etc.) recover and become very common again, but trilobites and nautiloids decline. Glaciation in East Gondwana continues from Late Devonian. Tuhua Orogeny in New Zealand tapers off. Some lobe finned fish called rhizodonts become abundant and dominant in freshwaters. Siberia collides with a different small continent, Kazakhstania. 330.9 ± 0.2
Viséan 346.7 ± 0.4*
Tournaisian 358.9 ± 0.4*
Devonian Late Famennian First lycopods, ferns, seed plants (seed ferns, from earlier progymnosperms), first trees (the progymnosperm Archaeopteris), and first winged insects (palaeoptera and neoptera). Strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids are all abundant in the oceans. First fully coiled cephalopods (Ammonoidea and Nautilida, independently) with the former group very abundant (especially goniatites). Trilobites and ostracoderms decline, while jawed fishes (placoderms, lobe-finned and ray-finned bony fish, and acanthodians and early cartilaginous fish) proliferate. Some lobe finned fish transform into digited fishapods, slowly becoming amphibious. The last non-trilobite artiopods die off. First decapods (like prawns) and isopods. Pressure from jawed fishes cause eurypterids to decline and some cephalopods to lose their shells while anomalocarids vanish. "Old Red Continent" of Euramerica persists after forming in the Caledonian orogeny. Beginning of Acadian Orogeny for Anti-Atlas Mountains of North Africa, and Appalachian Mountains of North America, also the Antler, Variscan, and Tuhua orogenies in New Zealand. A series of extinction events, including the massive Kellwasser and Hangenberg ones, wipe out many acritarchs, corals, sponges, molluscs, trilobites, eurypterids, graptolites, brachiopods, crinozoans (e.g. all cystoids), and fish, including all placoderms and ostracoderms. 372.2 ± 1.6*
Frasnian 382.7 ± 1.6*
Middle Givetian 387.7 ± 0.8*
Eifelian 393.3 ± 1.2*
Early Emsian 407.6 ± 2.6*
Pragian 410.8 ± 2.8*
Lochkovian 419.2 ± 3.2*
Silurian Pridoli Ozone layer thickens. First vascular plants and fully terrestrialized arthropods: myriapods, hexapods (including insects), and arachnids. Eurypterids diversify rapidly, becoming widespread and dominant. Cephalopods continue to flourish. True jawed fishes, along with ostracoderms, also roam the seas. Tabulate and rugose corals, brachiopods (Pentamerida, Rhynchonellida, etc.), cystoids and crinoids all abundant. Trilobites and molluscs diverse; graptolites not as varied. Three minor extinction events. Some echinoderms go extinct. Beginning of Caledonian Orogeny (collision between Laurentia, Baltica and one of the formerly small Gondwanan terranes) for hills in England, Ireland, Wales, Scotland, and the Scandinavian Mountains. Also continued into Devonian period as the Acadian Orogeny, above (thus Euramerica forms). Taconic Orogeny tapers off. Icehouse period ends late in this period after starting in Late Ordovician. Lachlan Orogeny on Australian continent tapers off. 423 ± 2.3*
Ludlow Ludfordian 425.6 ± 0.9*
Gorstian 427.4 ± 0.5*
Wenlock Homerian 430.5 ± 0.7*
Sheinwoodian 433.4 ± 0.8*
Llandovery Telychian 438.5 ± 1.1*
Aeronian 440.8 ± 1.2*
Rhuddanian 443.8 ± 1.5*
Ordovician Late Hirnantian The Great Ordovician Biodiversification Event occurs as plankton increase in number: invertebrates diversify into many new types (especially brachiopods and molluscs; e.g. long straight-shelled cephalopods like the long lasting and diverse Orthocerida). Early corals, articulate brachiopods (Orthida, Strophomenida, etc.), bivalves, cephalopods (nautiloids), trilobites, ostracods, bryozoans, many types of echinoderms (blastoids, cystoids, crinoids, sea urchins, sea cucumbers, and star-like forms, etc.), branched graptolites, and other taxa all common. Acritarchs still persist and common. Cephalopods become dominant and common, with some trending toward a coiled shell. Anomalocarids decline. Mysterious tentaculitans appear. First eurypterids and ostracoderm fish appear, the latter probably giving rise to the jawed fish at the end of the period. First uncontroversial terrestrial fungi and fully terrestrialized plants. Ice age at the end of this period, as well as a series of mass extinction events, killing off some cephalopods and many brachiopods, bryozoans, echinoderms, graptolites, trilobites, bivalves, corals and conodonts. 445.2 ± 1.4*
Katian 453 ± 0.7*
Sandbian 458.4 ± 0.9*
Middle Darriwilian 467.3 ± 1.1*
Dapingian 470 ± 1.4*
Early Floian
(formerly Arenig)
477.7 ± 1.4*
Tremadocian 485.4 ± 1.9*
Cambrian Furongian Stage 10 Major diversification of (fossils mainly show bilaterian) life in the Cambrian Explosion as oxygen levels increase. Numerous fossils; most modern animal phyla (including arthropods, molluscs, annelids, echinoderms, hemichordates and chordates) appear. Reef-building archaeocyathan sponges initially abundant, then vanish. Stromatolites replace them, but quickly fall prey to the Agronomic revolution, when some animals started burrowing through the microbial mats (affecting some other animals as well). First artiopods (including trilobites), priapulid worms, inarticulate brachiopods (unhinged lampshells), hyoliths, bryozoans, graptolites, pentaradial echinoderms (e.g. blastozoans, crinozoans and eleutherozoans), and numerous other animals. Anomalocarids are dominant and giant predators, while many Ediacaran fauna die out. Crustaceans and molluscs diversify rapidly. Prokaryotes, protists (e.g., forams), algae and fungi continue to present day. First vertebrates from earlier chordates. Petermann Orogeny on the Australian continent tapers off (550–535 Ma). Ross Orogeny in Antarctica. Delamerian Orogeny (c. 514–490 Ma) and Lachlan Orogeny (c. 540–440 Ma) on Australian continent. Some small terranes split off from Gondwana. Atmospheric CO2 content roughly 15 times present-day (Holocene) levels (6000 ppmv compared to today's 400 ppmv)[50][f] Arthropods and streptophyta start colonizing land. 3 extinction events occur 517, 502 & 488 Ma, the first and last of which wipe out many of the anomalocarids, artiopods, hyoliths, brachiopods, molluscs, and conodonts (early jawless vertebrates). ~489.5
Jiangshanian ~494*
Paibian ~497*
Miaolingian Guzhangian ~500.5*
Drumian ~504.5*
Wuliuan ~509
Series 2 Stage 4 ~514
Stage 3 ~521
Terreneuvian Stage 2 ~529
Fortunian ~538.8 ± 1.0*
Precambrian[h] Proterozoic[i] Neoproterozoic[i] Ediacaran Good fossils of primitive animals. Ediacaran biota flourish worldwide in seas, possibly appearing after an explosion, possibly caused by a large-scale oxidation event.[54] First vendozoans (unknown affinity among animals), cnidarians and bilaterians. Enigmatic vendozoans include many soft-jellied creatures shaped like bags, disks, or quilts (like Dickinsonia). Simple trace fossils of possible worm-like Trichophycus, etc.Taconic Orogeny in North America. Aravalli Range orogeny in Indian subcontinent. Beginning of Pan-African Orogeny, leading to the formation of the short-lived Ediacaran supercontinent Pannotia, which by the end of the period breaks up into Laurentia, Baltica, Siberia and Gondwana. Petermann Orogeny forms on Australian continent. Beardmore Orogeny in Antarctica, 633–620 Ma. Ozone layer forms. An increase in oceanic mineral levels. ~635*
Cryogenian Possible "Snowball Earth" period. Fossils still rare. Rodinia landmass begins to break up. Late Ruker / Nimrod Orogeny in Antarctica tapers off. First uncontroversial animal fossils. First hypothetical terrestrial fungi[55] and streptophyta.[56] ~720[j]
Tonian Rodinia supercontinent persists. Sveconorwegian orogeny ends. Grenville Orogeny tapers off in North America. Lake Ruker / Nimrod Orogeny in Antarctica, 1,000 ± 150 Ma. Edmundian Orogeny (c. 920 – 850 Ma), Gascoyne Complex, Western Australia. Deposition of Adelaide Superbasin and Centralian Superbasin begins on Australian continent. First hypothetical animals (from holozoans) and terrestrial algal mats. Many endosymbiotic events concerning red and green algae occur, transferring plastids to ochrophyta (e.g. diatoms, brown algae), dinoflagellates, cryptophyta, haptophyta, and euglenids (the events may have begun in the Mesoproterozoic)[57] while the first retarians (e.g. forams) also appear: eukaryotes diversify rapidly, including algal, eukaryovoric and biomineralized forms. Trace fossils of simple multi-celled eukaryotes. 1000[j]
Mesoproterozoic[i] Stenian Narrow highly metamorphic belts due to orogeny as Rodinia forms, surrounded by the Pan-African Ocean. Sveconorwegian orogeny starts. Late Ruker / Nimrod Orogeny in Antarctica possibly begins. Musgrave Orogeny (c. 1,080 Ma), Musgrave Block, Central Australia. Stromatolites decline as algae proliferate. 1200[j]
Ectasian Platform covers continue to expand. Algal colonies in the seas. Grenville Orogeny in North America. Columbia breaks up. 1400[j]
Calymmian Platform covers expand. Barramundi Orogeny, McArthur Basin, Northern Australia, and Isan Orogeny, c. 1,600 Ma, Mount Isa Block, Queensland. First archaeplastidans (the first eukaryotes with plastids from cyanobacteria; e.g. red and green algae) and opisthokonts (giving rise to the first fungi and holozoans). Acritarchs (remains of marine algae possibly) start appearing in the fossil record. 1600[j]
Paleoproterozoic[i] Statherian First uncontroversial eukaryotes: protists with nuclei and endomembrane system. Columbia forms as the second undisputed earliest supercontinent. Kimban Orogeny in Australian continent ends. Yapungku Orogeny on Yilgarn craton, in Western Australia. Mangaroon Orogeny, 1,680–1,620 Ma, on the Gascoyne Complex in Western Australia. Kararan Orogeny (1,650 Ma), Gawler Craton, South Australia. Oxygen levels drop again. 1800[j]
Orosirian The atmosphere becomes much more oxygenic while more cyanobacterial stromatolites appear. Vredefort and Sudbury Basin asteroid impacts. Much orogeny. Penokean and Trans-Hudsonian Orogenies in North America. Early Ruker Orogeny in Antarctica, 2,000–1,700 Ma. Glenburgh Orogeny, Glenburgh Terrane, Australian continent c. 2,005–1,920 Ma. Kimban Orogeny, Gawler craton in Australian continent begins. 2050[j]
Rhyacian Bushveld Igneous Complex forms. Huronian glaciation. First hypothetical eukaryotes. Multicellular Francevillian biota. Kenorland disassembles. 2300[j]
Siderian Great Oxidation Event (due to cyanobacteria) increases oxygen. Sleaford Orogeny on Australian continent, Gawler Craton 2,440–2,420 Ma. 2500[j]
Archean[i] Neoarchean[i] Stabilization of most modern cratons; possible mantle overturn event. Insell Orogeny, 2,650 ± 150 Ma. Abitibi greenstone belt in present-day Ontario and Quebec begins to form, stabilizes by 2,600 Ma. First uncontroversial supercontinent, Kenorland, and first terrestrial prokaryotes. 2800[j]
Mesoarchean[i] First stromatolites (probably colonial phototrophic bacteria, like cyanobacteria). Oldest macrofossils. Humboldt Orogeny in Antarctica. Blake River Megacaldera Complex begins to form in present-day Ontario and Quebec, ends by roughly 2,696 Ma. 3200[j]
Paleoarchean[i] Prokaryotic archaea (e.g. methanogens) and bacteria (e.g. cyanobacteria) diversify rapidly, along with early viruses. First known phototrophic bacteria. Oldest definitive microfossils. First microbial mats. Oldest cratons on Earth (such as the Canadian Shield and the Pilbara Craton) may have formed during this period.[k] Rayner Orogeny in Antarctica. 3600[j]
Eoarchean[i] First uncontroversial living organisms: at first protocells with RNA-based genes around 4000 Ma, after which true cells (prokaryotes) evolve along with proteins and DNA-based genes around 3800 Ma. The end of the Late Heavy Bombardment. Napier Orogeny in Antarctica, 4,000 ± 200 Ma. ~4000
Hadean[i][l] Early Imbrian (Neohadean) (unofficial)[i][m] This era overlaps the beginning of the Late Heavy Bombardment of the Inner Solar System, produced possibly by the planetary migration of Neptune into the Kuiper belt as a result of orbital resonances between Jupiter and Saturn. Oldest known rock (4,031 to 3,580 Ma).[59] 4130[60]
Nectarian (Mesohadean) (unofficial)[i][m] Possible first appearance of plate tectonics. This unit gets its name from the lunar geologic timescale when the Nectaris Basin and other greater lunar basins form by big impact events. First hypothetical life forms. 4280[60]
Basin Groups (Paleohadean) (unofficial)[i][m] End of the Early Bombardment Phase. Oldest known mineral (Zircon, 4,404 ± 8 Ma).[61] Asteroids and comets bring water to Earth, forming the first oceans.[62] 4533[60]
Cryptic (Eohadean) (unofficial)[i][m] Formation of Moon (4,533 to 4,527 Ma), probably from giant impact, since the end of this era. Formation of Earth (4,570 to 4,567.17 Ma), Early Bombardment Phase begins. Formation of Sun (4,680 to 4,630 Ma). 4600

Proposed Precambrian timeline[edit]

The ICS's Geologic Time Scale 2012 book which includes the new approved time scale also displays a proposal to substantially revise the Precambrian time scale to reflect important events such as the formation of the Earth or the Great Oxidation Event, among others, while at the same time maintaining most of the previous chronostratigraphic nomenclature for the pertinent time span.[63] (See also Period (geology)#Structure.)

Shown to scale:

AcastanIsuanVaalbaranPongolanMethanianSiderianOxygenianEukaryianColumbianRodinianCryogenianEdiacaranChaotianPaleoarcheanMesoarcheanNeoarcheanPaleoproterozoicMesoproterozoicNeoproterozoicHadeanArcheanProterozoicPrecambrian

Compare with the current official timeline, shown to scale:

SiderianRhyacianOrosirianStatherianCalymmianEctasianStenianTonianCryogenianEdiacaranEoarcheanPaleoarcheanMesoarcheanNeoarcheanPaleoproterozoicMesoproterozoicNeoproterozoicHadeanArcheanProterozoicPrecambrian

See also[edit]

Notes[edit]

  1. ^ Not enough is known about extra-solar planets for worthwhile speculation.
  2. ^ Paleontologists often refer to faunal stages rather than geologic (geological) periods. The stage nomenclature is quite complex. For a time-ordered list of faunal stages, see.[44]
  3. ^ a b 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 v2022/02 time scale except the Hadean eon. Where errors are not quoted, errors are less than the precision of the age given.

    * indicates boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed upon.
  4. ^ References to the "Post-Cambrian Supereon" are not universally accepted, and therefore must be considered unofficial.
  5. ^ Historically, the Cenozoic has been divided up into the Quaternary and Tertiary sub-eras, as well as the Neogene and Paleogene periods. The 2009 version of the ICS time chart[45] recognizes a slightly extended Quaternary as well as the Paleogene and a truncated Neogene, the Tertiary having been demoted to informal status.
  6. ^ a b c d For more information on this, see Atmosphere of Earth#Evolution of Earth's atmosphere, Carbon dioxide in the Earth's atmosphere, and climate change. Specific graphs of reconstructed CO2 levels over the past ~550, 65, and 5 million years can be seen at File:Phanerozoic Carbon Dioxide.png, File:65 Myr Climate Change.png, File:Five Myr Climate Change.png, respectively.
  7. ^ In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods.
  8. ^ The Precambrian is also known as Cryptozoic.
  9. ^ a b c d e f g h i j k l m n The Proterozoic, Archean and Hadean are often collectively referred to as the Precambrian or, sometimes, the Cryptozoic.
  10. ^ a b c d e f g h i j k l Defined by absolute age (Global Standard Stratigraphic Age).
  11. ^ The age of the oldest measurable craton, or continental crust, is dated to 3,600–3,800 Ma.
  12. ^ Though commonly used, the Hadean is not a formal eon[58] and no lower bound for the Archean and Eoarchean have been agreed upon. The Hadean has also sometimes been called the Priscoan or the Azoic. Sometimes, the Hadean can be found to be subdivided according to the lunar geologic timescale. These eras include the Cryptic and Basin Groups (which are subdivisions of the Pre-Nectarian era), Nectarian, and Early Imbrian units.
  13. ^ a b c d These unit names were taken from the lunar geologic timescale and refer to geologic events that did not occur on Earth. Their use for Earth geology is unofficial. Note that their start times do not dovetail perfectly with the later, terrestrially defined boundaries.

References[edit]

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