Geologic time scale

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey

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 fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga ^[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 two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological 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 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–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.

Each era on the scale is separated from the next by a major event or change.

Graphical timelines

The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.

Millions of Years

The Holocene (the latest epoch) is too short to be shown clearly on this timeline.

Segments of rock (strata) in chronostratigraphy	Periods of time in geochronology	Notes
Units in geochronology and stratigraphy^[2]
Eonothem	Eon	4 total, half a billion years or more
Erathem	Era	12 total, several hundred million years
System	Period
Series	Epoch	tens of millions of years
Stage	Age	millions of years
Chronozone	Chron	smaller than an age/stage, not used by the ICS timescale

The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time.

Geologists qualify these units as Upper, Middle, and Lower. Examples are "Upper Jurassic" and "Middle Cambrian". The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." The names of subdivisions can vary when discussing either position in the geologic record or corresponding time period, by use of either upper/lower (for series) or late/early (for epochs). For example, the Lower Jurassic Series in geochronology corresponds to the Early Jurassic Epoch in chronostratigraphy.[3]

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same period 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 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.[4]

History of the time scale and names
Main articles: History of geology and History of paleontology
Animation showing Earth's palaeogeographic reconstruction beginning from early Cambrian period.
Diagram of geological time scale, where the past is toward the bottom of the spiral

In classical antiquity, Aristotle saw that fossil seashells from rocks were similar to those found on the beach and deduced that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods of time. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.[5]

The 11th-century Persian geologist Avicenna (Ibn Sina) examined various fossils and deduced that they originated from the petrifaction of plants and animals.[6] He 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 in 1027, he outlined the principle as follows:

It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, more layer`s was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimentary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains.

His contemporary, Abu Rayhan Biruni (973-1048), discovered the existence of shells and fossils in regions that once were seas and later became dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly evolving and proposed that the Earth had an age, but that its origin was too distant to measure.[citation needed] Later in the 11th century, the Chinese naturalist, Shen Kuo (1031–1095), also recognized the concept of 'deep time'.[7]

The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are 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 to real rocks proved complex. Over the course of the 18th century geologists realized that:

1. Sequences of strata were often 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 the Earth's long history.

A comparative geological timescale

The first serious attempts to formulate a geological 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 Abraham Werner, among others) divided the rocks of the 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 Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century.

The Neptunist theories popular at this time (expounded by Werner) 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 before the Royal Society of Edinburgh in March and April 1785. It has been said that "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology"[8] Hutton proposed that the interior of the 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 was dubbed "Plutonist" in contrast to the flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy, and Alexandre Brogniart 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 geological periods still used today.

The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales.[9] The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, 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 Brogniart 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[10] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates).

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

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 various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. 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 (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.

The first geologic time scale was eventually published in 1913 by the British geologist Arthur Holmes.[11] He greatly furthered the newly created discipline of geochronology and published the world renowned book The Age of the Earth in 1913 in which he estimated the Earth's age to be at least 1.6 billion years.[12]

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.[13]. A UML model for how the timescale is structured, relating it to the GSSP, is also available[14].

Table of geologic time

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. (See lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) This table is arranged with the most recent geologic periods at the top, and the most ancient 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 current official geologic time scale of the International Commission on Stratigraphy,[15] with the epoch names altered to the early/late format from lower/upper as recommended by the ICS when dealing with chronostratigraphy.[3]

Geologic time scale

Supereon	Eon	Era	Period^[16]	Epoch	Age^[17]	Major events	Start, million years ago^[17]
	Phanerozoic	Cenozoic^[18]
			Quaternary	Holocene	Atlantic / Subboreal / Subatlantic	The last glacial period ends; rise of human civilization. Quaternary Ice Age recedes, and the current interglacial begins. Younger Dryas cold spell occurs, Sahara forms from savannah, and agriculture begins, allowing humans to build cities. Paleolithic/Neolithic (Stone Age) cultures begin around 10000 BC, giving way to Copper Age (3500 BC) and Bronze Age (2500 BC). Cultures continue to grow in complexity and technical advancement through the Iron Age (1200 BC), giving rise to many pre-historic cultures throughout the world, eventually leading into Classical Antiquity, such as the Roman Empire and even to the Middle Ages and present day. Little Ice Age (stadial) causes brief cooling in Northern Hemisphere from 1400 to 1850. Also refer to the List of archaeological periods for clarification on early cultures and ages. Mount Tambora erupts in 1815, causing the Year Without a Summer (1816) in Europe and North America from a volcanic winter. Atmospheric CO₂ levels start creeping from 100 parts per million volume (ppmv) at the end of the last glaciation to the current level of 385 ppmv, possibly causing global warming and/or climate change, likely from anthropogenic sources, such as the Industrial Revolution^[19]	0.011430 ± 0.00013^[18]^[20]
				Holocene	Boreal		0.011430 ± 0.00013^[18]^[20]
				Pleistocene	Late/Tyrrhenian Stage/Eemian/Sangamonian	Flourishing and then extinction of many large mammals (Pleistocene megafauna). Evolution of anatomically modern humans. Quaternary Ice Age continues with glaciations and interstadials (and the accompanying fluctuations from 100 to 300 ppmv in atmospheric CO₂ levels^[19]), further intensification of Icehouse Earth conditions, roughly 1.6 Ma. Last glacial maximum (30000 years ago), last glacial period (18000–15000 years ago). Dawn of human stone-age cultures, with increasing technical complexity relative to previous ice age cultures, such as engravings and clay statues (e.g. Venus of Lespugue), particularly in the Mediterranean and Europe. Lake Toba supervolcano erupts 75000 years before present, causing a volcanic winter that pushes humanity to the brink of extinction. Pleistocene ends with Oldest Dryas, Older Dryas/Allerød and Younger Dryas climate events, with Younger Dryas forming the boundary with the Holocene.	0.126 ± 0.005^*
					Middle		0.500?
					Early		1.806 ± 0.005^*
					Gelasian		2.588 ± 0.005^*
			Neogene	Pliocene	Piacenzian/Blancan	Intensification of present Icehouse conditions, present (Quaternary) ice age begins roughly 2.58 Ma; cool and dry climate. Australopithecines, many of the existing genera of mammals, and recent mollusks appear. Homo habilis appears.	3.600 ± 0.005^*
				Pliocene	Zanclean		5.332 ± 0.005^*
				Miocene	Messinian	Moderate Icehouse climate, puncuated by ice ages; Orogeny in northern hemisphere. Modern mammal and bird families become recognizable. Horses and mastodons diverse. Grasses become ubiquitous. First apes appear (for reference see the article: "Sahelanthropus tchadensis"). Kaikoura Orogeny forms Southern Alps in New Zealand, continues today. Orogeny of the Alps in Europe slows, but continues to this day. Carpathian orogeny forms Carpathian Mountains in Central and Eastern Europe. Hellenic orogeny in Greece and Aegean Sea slows, but continues to this day. Middle Miocene Disruption occurs. Widespread forests slowly draw in massive amounts of CO₂, gradually lowering the level of atmospheric CO₂ from 650 ppmv down to around 100 ppmv^[19].	7.246 ± 0.05^*
					Tortonian		11.608 ± 0.05^*
					Serravallian		13.65 ± 0.05^*
					Langhian		15.97 ± 0.05^*
					Burdigalian		20.43 ± 0.05^*
					Aquitanian		23.03 ± 0.05^*
			Paleogene	Oligocene	Chattian	Warm but cooling climate, moving towards Icehouse; Rapid evolution and diversification of fauna, especially mammals. Major evolution and dispersal of modern types of flowering plants	28.4 ± 0.1^*
				Oligocene	Rupelian		33.9 ± 0.1^*
				Eocene	Priabonian	Moderate, cooling climate. 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 and formation of its ice cap; Azolla event triggers ice age, and the Icehouse Earth climate that would follow it to this day, from the settlement and decay of seafloor algae drawing in massive amounts of atmospheric carbon dioxide^[19], lowering it from 3800 ppmv down to 650 ppmv. End of Laramide and Sevier Orogenies of the Rocky Mountains in North America. Orogeny of the Alps in Europe begins. Hellenic Orogeny begins in Greece and Aegean Sea.	37.2 ± 0.1^*
					Bartonian		40.4 ± 0.2^*
					Lutetian		48.6 ± 0.2^*
					Ypresian		55.8 ± 0.2^*
				Paleocene	Thanetian	Climate tropical. Modern plants appear; Mammals diversify into a number of primitive lineages following the extinction of the dinosaurs. First large mammals (up to bear or small hippo size). Alpine orogeny in Europe and Asia begins. Indian Subcontinent collides with Asia 55 Ma, Himalayan Orogeny starts between 52 and 48 Ma.	58.7 ± 0.2^*
					Selandian		61.7 ± 0.3^*
					Danian		65.5 ± 0.3^*
		Mesozoic	Cretaceous	Late	Maastrichtian	Flowering plants proliferate, along with new types of insects. More modern teleost fish begin to appear. Ammonites, belemnites, rudist bivalves, echinoids and sponges all common. Many new types of dinosaurs (e.g. Tyrannosaurs, Titanosaurs, duck bills, and horned dinosaurs) evolve on land, as do Eusuchia (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. Beginning of Laramide and Sevier Orogenies of the Rocky Mountains. Atmospheric CO₂ close to present-day levels.	70.6 ± 0.6^*
					Campanian		83.5 ± 0.7^*
					Santonian		85.8 ± 0.7^*
					Coniacian		89.3 ± 1.0^*
					Turonian		93.5 ± 0.8^*
					Cenomanian		99.6 ± 0.9^*
				Early	Albian		112.0 ± 1.0^*
					Aptian		125.0 ± 1.0^*
					Barremian		130.0 ± 1.5^*
					Hauterivian		136.4 ± 2.0^*
					Valanginian		140.2 ± 3.0^*
					Berriasian		145.5 ± 4.0^*
			Jurassic	Late	Tithonian	Gymnosperms (especially conifers, Bennettitales and cycads) 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. Bivalves, Ammonites and belemnites abundant. Sea urchins very common, along with crinoids, starfish, sponges, and terebratulid and rhynchonellid brachiopods. Breakup of Pangaea into Gondwana and Laurasia. Nevadan orogeny in North America. Rantigata and Cimmerian Orogenies taper off. Atmospheric CO₂ levels 4–5 times the present day levels (1200–1500 ppmv, compared to today's 385 ppmv^[19]).	150.8 ± 4.0^*
					Kimmeridgian		155.7 ± 4.0^*
					Oxfordian		161.2 ± 4.0^*
				Middle	Callovian		164.7 ± 4.0
					Bathonian		167.7 ± 3.5^*
					Bajocian		171.6 ± 3.0^*
					Aalenian		175.6 ± 2.0^*
				Early	Toarcian		183.0 ± 1.5^*
					Pliensbachian		189.6 ± 1.5^*
					Sinemurian		196.5 ± 1.0^*
					Hettangian		199.6 ± 0.6^*
			Triassic	Late	Rhaetian	Archosaurs dominant on land as dinosaurs, in the oceans as Ichthyosaurs and nothosaurs, and in the air as pterosaurs. Cynodonts become smaller and more mammal-like, while first mammals and crocodilia appear. Dicroidium flora common on land. Many large aquatic temnospondyl amphibians. Ceratitic ammonoids extremely common. Modern corals and teleost fish appear, as do many modern insect clades. 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)	203.6 ± 1.5^*
					Norian		216.5 ± 2.0^*
					Carnian		228.0 ± 2.0^*
				Middle	Ladinian		237.0 ± 2.0^*
				Middle	Anisian		245.0 ± 1.5^*
				Early	Olenekian		249.7 ± 1.5^*
				Early	Induan		251.0 ± 0.7^*
		Paleozoic	Permian	Lopingian	Changhsingian	Landmasses unite into supercontinent Pangaea, creating the Appalachians. End of Permo-Carboniferous glaciation. Synapsid reptiles (pelycosaurs and therapsids) become plentiful, while parareptiles and temnospondyl amphibians remain common. In the mid-Permian, coal-age flora are replaced by cone-bearing gymnosperms (the first true seed plants) and by the first true mosses. Beetles and flies evolve. Marine life flourishes in warm shallow reefs; productid and spiriferid brachiopods, bivalves, forams, and ammonoids all abundant. Permian-Triassic extinction event occurs 251 Ma: 95% of life on Earth becomes extinct, including all trilobites, graptolites, and blastoids. 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.	253.8 ± 0.7^*
				Lopingian	Wuchiapingian		260.4 ± 0.7^*
				Guadalupian	Capitanian		265.8 ± 0.7^*
					Wordian/Kazanian		268.4 ± 0.7^*
					Roadian/Ufimian		270.6 ± 0.7^*
				Cisuralian	Kungurian		275.6 ± 0.7^*
					Artinskian		284.4 ± 0.7^*
					Sakmarian		294.6 ± 0.8^*
					Asselian		299.0 ± 0.8^*
			Carbon- iferous^[21]/ Pennsyl- vanian	Late	Gzhelian	Winged insects radiate suddenly; some (esp. Protodonata and Palaeodictyoptera) are quite large. Amphibians common and diverse. First reptiles and coal forests (scale trees, ferns, club trees, giant horsetails, Cordaites, etc.). Highest-ever atmospheric oxygen levels. Goniatites, brachiopods, bryozoa, bivalves, and corals plentiful in the seas and oceans. Testate forams proliferate. Uralian orogeny in Europe and Asia. Variscan orogeny occurs towards middle and late Mississippian Periods.	303.9 ± 0.9^*
Kasimovian	306.5 ± 1.0^*			Late
Middle	Moscovian			311.7 ± 1.1^*
Early	Bashkirian			318.1 ± 1.3^*
Carbon- iferous^[21]/ Missis- sippian	Late		Serpukhovian	Large primitive trees, first land vertebrates, and amphibious sea-scorpions live amid coal-forming coastal swamps. Lobe-finned rhizodonts are dominant big fresh-water predators. In the oceans, early sharks are common and quite diverse; echinoderms (especially crinoids and blastoids) abundant. Corals, bryozoa, goniatites and brachiopods (Productida, Spiriferida, etc.) very common, but trilobites and nautiloids decline. Glaciation in East Gondwana. Tuhua Orogeny in New Zealand tapers off.	326.4 ± 1.6^*
	Middle		Viséan		345.3 ± 2.1^*
	Early		Tournaisian		359.2 ± 2.5^*
Devonian	Late		Famennian	First clubmosses, horsetails and ferns appear, as do the first seed-bearing plants (progymnosperms), first trees (the progymnosperm Archaeopteris), and first (wingless) insects. Strophomenid and atrypid brachiopods, rugose and tabulate corals, and crinoids are all abundant in the oceans. Goniatite ammonoids are plentiful, while squid-like coleoids arise. Trilobites and armoured agnaths decline, while jawed fishes (placoderms, lobe-finned and ray-finned fish, and early sharks) rule the seas. First amphibians still aquatic. "Old Red Continent" of Euramerica. Beginning of Acadian Orogeny for Anti-Atlas Mountains of North Africa, and Appalachian Mountains of North America, also the Antler, Variscan, and Tuhua Orogeny in New Zealand.	374.5 ± 2.6^*
	Late		Frasnian		385.3 ± 2.6^*
	Middle		Givetian		391.8 ± 2.7^*
	Middle		Eifelian		397.5 ± 2.7^*
	Early		Emsian		407.0 ± 2.8^*
			Pragian		411.2 ± 2.8^*
			Lochkovian		416.0 ± 2.8^*
Silurian	Pridoli		no faunal stages defined	First Vascular plants (the rhyniophytes and their relatives), first millipedes and arthropleurids on land. First jawed fishes, as well as many armoured jawless fish, populate the seas. Sea-scorpions reach large size. Tabulate and rugose corals, brachiopods (Pentamerida, Rhynchonellida, etc.), and crinoids all abundant. Trilobites and mollusks diverse; graptolites not as varied. Beginning of Caledonian Orogeny for hills in England, Ireland, Wales, Scotland, and the Scandinavian Mountains. Also continued into Devonian period as the Acadian Orogeny, above. Taconic Orogeny tapers off. Lachlan Orogeny on Australian Continent tapers off.	418.7 ± 2.7^*
	Ludlow/Cayugan		Ludfordian		421.3 ± 2.6^*
	Ludlow/Cayugan		Gorstian		422.9 ± 2.5^*
	Wenlock		Homerian/Lockportian		426.2 ± 2.4^*
	Wenlock		Sheinwoodian/Tonawandan		428.2 ± 2.3^*
	Llandovery/ Alexandrian		Telychian/Ontarian		436.0 ± 1.9^*
			Aeronian		439.0 ± 1.8^*
			Rhuddanian		443.7 ± 1.5^*
Ordovician	Late		Hirnantian	Invertebrates diversify into many new types (e.g., long straight-shelled cephalopods). Early corals, articulate brachiopods (Orthida, Strophomenida, etc.), bivalves, nautiloids, trilobites, ostracods, bryozoa, many types of echinoderms (crinoids, cystoids, starfish, etc.), branched graptolites, and other taxa all common. Conodonts (early planktonic vertebrates) appear. First green plants and fungi on land. Ice age at end of period.	445.6 ± 1.5^*
	Late		other faunal stages		460.9 ± 1.6^*
	Middle		Darriwilian		468.1 ± 1.6^*
	Middle		other faunal stages		471.8 ± 1.6^*
	Early		Arenig		478.6 ± 1.7^*
	Early		Tremadocian		488.3 ± 1.7^*
Cambrian	Furongian		other faunal stages	Major diversification of life in the Cambrian Explosion. Many fossils; most modern animal phyla appear. First chordates appear, along with a number of extinct, problematic phyla. Reef-building Archaeocyatha abundant; then vanish. Trilobites, priapulid worms, sponges, inarticulate brachiopods (unhinged lampshells), and many other animals numerous. Anomalocarids are giant predators, while many Ediacaran fauna die out. Prokaryotes, protists (e.g., forams), fungi and algae continue to present day. Gondwana emerges. Petermann Orogeny on the Australian Continent tapers off (550–535 Ma). Ross Orogeny in Antarctica. Adelaide Geosyncline (Delamerian Orogeny), majority of orogenic activity from 514–500 Ma. Lachlan Orogeny on Australian Continent, c. 540–440 Ma. Atmospheric CO₂ content roughly 20–35 times present-day (Holocene) levels (6000 ppmv compared to today's 385 ppmv)^[19]	496.0 ± 2.0^*
	Furongian		Paibian/Ibexian/ Ayusokkanian/Sakian/ Aksayan		501.0 ± 2.0^*
	Middle		other faunal stages/Albertan		513.0 ± 2.0
	Early		other faunal stages/ Waucoban/Tommotian/ Atdabanian/Botomian		542.0 ± 1.0^*
Precam- brian^[22]	Proter- ozoic^[23]	Neo- proterozoic^[23]	Ediacaran	Good fossils of the first multi-celled animals. Ediacaran biota flourish worldwide in seas. Simple trace fossils of possible worm-like Trichophycus, etc. First sponges and trilobitomorphs. Enigmatic forms include many soft-jellied creatures shaped like bags, disks, or quilts (like Dickinsonia). Taconic Orogeny in North America. Aravalli Range orogeny in Indian Subcontinent. Beginning of Petermann Orogeny on Australian Continent. Beardmore Orogeny in Antarctica, 633–620 Ma.			630 +5/-30^*
			Cryogenian	Possible "Snowball Earth" period. Fossils still rare. Rodinia landmass begins to break up. Late Ruker / Nimrod Orogeny in Antarctica tapers off.			850^[24]
			Tonian	Rodinia supercontinent persists. Trace fossils of simple multi-celled eukaryotes. First radiation of dinoflagellate-like acritarchs. Grenville Orogeny tapers off in North America. Pan-African orogeny in Africa. Lake Ruker / Nimrod Orogeny in Antarctica, 1000 ± 150 Ma. Edmundian Orogeny (c. 920 - 850 Ma), Gascoyne Complex, Western Australia. Adelaide Geosyncline laid down on Australian Continent, beginning of Adelaide Geosyncline (Delamerian Orogeny) in that continent.			1000^[24]
		Meso- proterozoic^[23]	Stenian	Narrow highly metamorphic belts due to orogeny as Rodinia forms. Late Ruker / Nimrod Orogeny in Antarctica possibly begins. Musgrave Orogeny (c. 1080 Ma), Musgrave Block, Central Australia.			1200^[24]
			Ectasian	Platform covers continue to expand. Green algae colonies in the seas. Grenville Orogeny in North America.			1400^[24]
			Calymmian	Platform covers expand. Barramundi Orogeny, McArthur Basin, Northern Australia, and Isan Orogeny, c. 1600 Ma, Mount Isa Block, Queensland			1600^[24]
		Paleo- proterozoic^[23]	Statherian	First complex single-celled life: protists with nuclei. Columbia is the primordial supercontinent. Kimban Orogeny in Australian Continent ends. Yapungku Orogeny on Yilgarn craton, in Western Australia. Mangaroon Orogeny, 1680–1620 Ma, on the Gascoyne Complex in Western Australia. Kararan Orogeny (1650-Ma), Gawler Craton, South Australia.			1800^[24]
			Orosirian	The atmosphere become oxygenic. Vredefort and Sudbury Basin asteroid impacts. Much orogeny. Penokean and Trans-Hudsonian Orogenies in North America. Early Ruker Orogeny in Antarctica, 2000 - 1700 Ma. Glenburgh Orogeny, Glenburgh Terrane, Australian Continent c. 2005 - 1920 Ma. Kimban Orogeny, Gawler craton in Australian Continent begins.			2050^[24]
			Rhyacian	Bushveld Formation forms. Huronian glaciation.			2300^[24]
			Siderian	Oxygen catastrophe: banded iron formations forms. Sleaford Orogeny on Australian Continent, Gawler Craton 2440–2420 Ma.			2500^[24]
	Archean^[23]	Neoarchean^[23]	Stabilization of most modern cratons; possible mantle overturn event. Insell Orogeny, 2650 ± 150 Ma. Abitibi greenstone belt in present-day Ontario and Quebec begins to form, stablizes by 2600 Ma.				2800^[24]
		Mesoarchean^[23]	First stromatolites (probably colonial cyanobacteria). Oldest macrofossils. Humboldt Orogeny in Antarctica. Blake River Megacaldera Complex begins to form in present-day Ontario and Quebec, ends by roughly 2696 Ma.				3200^[24]
		Paleoarchean^[23]	First known oxygen-producing bacteria. Oldest definitive microfossils. Oldest cratons on earth (such as the Canadian Shield and the Pilbara Craton) may have formed during this period^[25]. Rayner Orogeny in Antarctica.				3600^[24]
		Eoarchean^[23]	Simple single-celled life (probably bacteria and perhaps archaea). Oldest probable microfossils.				3800
	Hadean ^[23]^[26]	Early Imbrian^[23]^[27]	This era overlaps the end of the Late Heavy Bombardment of the inner solar system.				c.3850
		Nectarian^[23]^[27]	This unit gets its name from the lunar geologic timescale when the Nectaris Basin and other major lunar basins form by large impact events.				c.3920
		Basin Groups^[23]^[27]	Oldest known rock (4030 Ma)^[28]. The first lifeforms and self-replicating RNA molecules may have evolved on earth around 4000 Ma during this era. Napier Orogeny in Antarctica, 4000 ± 200 Ma.				c.4150
		Cryptic^[23]^[27]	Oldest known mineral (Zircon, 4406 ± 8 Ma^[29]). Formation of Moon (4533 Ma), probably from giant impact. Formation of Earth (4567.17 to 4570 Ma)				c.4570