Geologic Time Scale and Earth Her/History
Detailed notes
Precambrian Eon 4.5 billion – 544 million years ago
Its name means "before Cambrian." All geologic time before the beginning of the Paleozoic
era. This includes about 90% of all geologic time and spans the time from the beginning of the
earth.
Hadean Era 4500 to 3800 million years ago
The earliest subdivision of the Precambrian, spanning the time between the formation of the
Earth, about 4.5 billion years ago, and the start of the Archaean era, 3.8 billion years ago. This
interval predates the period of true geologic time since no rocks of this age are known on
Earth, with the exception of a few meteorites. Formation of the solar system and of earth.
Earth is in a molten state. Earth bombarded by planetoids. Formation of the crust. Primal
ocean starts to form with organic amino acids. No life forms in the fossil record.
Archaean Era 3800 to 2500 million years ago
Its name means "ancient." The middle era of Precambrian time, spanning the period between
3.8 and 2.5 billion years ago. There is a permanent crust similar to the structure we see today.
Canadian Shield forms. First signs of erosion as sediment begin to accumulate in the oceans.
Life appeared on Earth during the Archaean, as indicated by the appearance of fossil bacteria
(cyannobacteria or blue-green algae) in rocks thought to be about 3.5 billion years old.
Prokaryotes, single-celled micro-organisms with no distinct organs, are present.
Proterozoic Era 2.5 billion – 544 million
Its name means "early life." The final era of the Precambrian Eon, spanning the time between
2.5 billion and 544 million years ago. Fossils of both primitive single celled (prokaryotes) and
more advanced multi-cellular organisms (Eukaryotes) begin to appear in abundance in rocks
from this era.
Sturtian Period 800 million – 650 million
Vendian Period 650 million - 544 million years ago (AKA: Ediacaran)
The latest period of the Proterozoic era, spanning the time between 650 and 544 million years
ago. Sometimes referred to as the Ediacaran period, the Vendian is distinguished by fossils
representing a characteristic collection of complex soft-bodied organisms found at several
localities around the world. Mounds of blue-green algae form structures called stromatolites
are considered to be the first fossils. Photosynthesis creates oxygen in the atmosphere which
kills off many of the early living organisms. Examples of these early life forms are present in
water near volcanic vents. Some simple worm-like fossils. First cnidarians (Greek for
“stinging nettle”), which are animals armed with stinging cells called nematocysts. Precursors
to first arthropods (“jointed leg”). Some sponges also present.
Phanerozoic Eon 544 million years ago - present
Means “Revealed Life”. Though it only represents about 10% of earth her/history it is
arguably the most interesting with virtually all life found during this time.
Paleozoic Era 544 million – 248 million
The word Paleozoic is from Greek and means "ancient life." An era of geologic time, from the
end of the Precambrian to the beginning of the Mesozoic, spanning the time between 544 and
248 million years ago. Fossil record shows first shellfish, insects, plants, fish, spiders,
amphibians, and reptiles.
Cambrian Period 544 million – 505 million
It is named after Cambria, the Roman name for Wales, where rocks of this age were first
studied. The earliest period of the Paleozoic era, spanning the time between 544 and 505
million years ago. Major formation of sedimentary rock. Continents are flooded with shallow
seas. Mild climate. “North America” is tropical. Originally thought to be the first ‘explosion’
of life but has since been adjusted. Shelled (calcium carbonate) organisms in oceans. Every
phylum of invertebrate is represented in fossil record. Trilobites abundant. First active
predators/ hunters. Some brachiopods (“long foot”). Simple echinoderms (“spiny skin”),
which are ancestors of starfish. Possibly some early crinoids or sea lilies. A few simple jawless
vertebrates like lamprey. Early sponges called archaeocyathids present; their descendants
may have been the first reef-builders. Mass extinctions including oldest trilobites.
AKA: “Age of Marine Invertebrates” due to tremendous diversity of organisms.
Ordovician Period 505 million – 440 million
It is named after a Celtic tribe called the Ordovices. The second earliest period of the
Paleozoic era, spanning the time between 505 and 440 million years ago. North America,
Europe and Africa colliding forming first highlands. Seas over North America at their
greatest extent. Rocks are mostly sedimentary. Super-continent called Gondwana in southern
hemisphere shifts toward south pole and much of it was submerged under shallow seas. In
the Late Ordovician massive glaciers dropped sea levels. Mature marine ecosystems develop
with some deep-water life. Graptolites- whose fossils look like saw blades. Fish with
notocords (early backbones) present. Jawless fish and other sea life widespread. Some plant
spores have been found suggesting primitive land plants may be in existence. Mass extinction
of 60% of all marine invertebrate genera and 25% of all families become extinct. Possible
reasons for extinction event are the cooling of the ocean and/or oxygen depletion.
Silurian Period 440 million – 417 million
It is named after a Celtic tribe called the Silures. A period of the Paleozoic, spanning the time
between 440 and 410 million years ago. Earth continues its mild climate as global temps
stabilize. Sea levels rise to previous levels. Mountains forming in Europe. Most of the world’s
salt (halite) deposits occur at this time. Earliest land plants. Extensive coral reefs present.
Jawless fish spread rapidly throughout fossil record. First fish with jaws as well as the first
sharks. Crinoids (sea lilies) abundant. Brachiopods widespread. Graptolites, conodonts,
corals, stromatoporoids and mollusks common. Land being invaded by arthropods (‘jointed
leg”) like relatives of spiders (arachnids) and centipedes. Earliest vascular plants (plants with
xylem and phloem; ferns) found in southern hemisphere; Most common plants belong to
genus Cooksonia; Controversial Baragwanathia (type of advanced vascular plant called a
lycophyte) found in Australia. Modern algae and fungus (ascomycete fossils).
Devonian Period 417 million – 354 million
A period of the Paleozoic era, spanning the time between 410 and 360 million years ago. It is
named after Devonshire, England, where rocks of this age were first studied. Extensive
volcanism in the eastern United States and Canada. Continents are becoming drier. Europe
and North America colliding near equator- orogeny continues. South Pole located in Central
Africa as South America, Africa, Antarctica, India, and Australia dominate southern
hemisphere. Fish are dominant life form- armored fish (ostracoderms), placoderms of huge
size, lungfish, Dunklosteous. First lobe-finned fish (sarcoterygiians) First land animals
develop near the end of Devonian; first amphibians. Brachiopods, echinoderms, corals
flourish. Psilotophyta (giant plants) present in southern hemisphere; Archaeopteris is an
example. Also evidence of first seed plants (spermatophytes), which are very modern-type
plants. Oldest known vascular plants found now in northern hemisphere are only one meter
tall. Brachiopods (arm- foot) are at their most diverse. Two major groups of animals colonize
land at end of Devonian: tetrapods (four legged) which are the first vertebrates and closely
related to lungfish and amphibians; and wingless insects and earliest true arachnids. AKA:
“Age of Fishes”.
Carboniferous Period 354 million – 290 million
The term ‘Carboniferous’ comes from England and refers to the large coal deposits there.A
period of time in the Paleozoic era that includes the Mississippian (Lower Carboniferus) and
the Pennsylvanian (upper Carboniferous) periods and extended from 354 to 290 million years
ago. In the United States the separation distinguishes between the mostly limestone
Mississippian and coal-bearing Pennsylvanian. The earth’s climate is still warm. Great coal
swamps cover much of lowlands on earth. Orogeny in eastern US (Appalchians), Texas,
Colorado, Britain (Hercynian Mountains), Eastern Europe/ Siberia (Ural Mountains).
Laurussia (Europe & North America) collides with Godwanaland (Africa & South America).
Though two massive ice sheets cover the South Pole, there is a decrease in seasonal variation
of temperatures. The milder climates caused the decrease of lycopods (tree-like plants) and
giant insects and the increase of tree ferns. Amphibians reach greatest number and diversity.
First reptiles appear in the late Carboniferous. This is perhaps the greatest ‘innovation’ of this
period- an Amniote egg. This is the design allowed tetrapods to lay eggs away from water- a
first in the fossil record in response to more arid conditions. Earliest amniote fossil was lizardlike Hylonomus. Other tetrapods include Amphibiamus (type of Temnospondyl),
Anthracosaurs, followed by Diapsids (most reptiles except turtles; dinosaurs and even birds
belong to this group), Synapsids (precursors to mammals, including Dimetrodon) Increase in
the diversity of insects; insects are huge. First land snails. Sharks are abundant. Great forests
of ferns, gymnosperms, horsetails. Sphenopsids (like today’s so-called Horse Tails or
Equisetum) become more prevalent. Continents are uplifted providing more terrestrial
environments but flooded areas on the continents host bryozoans (“Moss Animals”). Sea floor
dominated by brachiopods, foraminifers (single-celled protests). The Kingdom “Protista” is
one of the four kingdoms including animals, plants, and fungi. Freshwater clams appear.
Heavily armored fish from Devonian extinct. AKA: “Age of Amphibians”
Mississippian Epoch 360 million – 325 million
A period of the Paleozoic era, spanning the time between 360 and 325 million years ago. It is
named after the Mississippi River valley, which contains good exposures of rocks of this age.
Coal- forming swamps.
Pennsylvanian Epoch 325 million – 286 million
A period of the Paleozoic era, spanning the time between 325 and 286 million years ago. It is
named after the state of Pennsylvania where rocks of this age are widespread. Coal- forming
swamps at their fullest extent. All so-called fossil fuels (coal, oil, natural gas) are formed at
this time. First reptiles.
Permian Period 286 million – 248 million
The final period of the Paleozoic era, spanning the time between 286 and 248 million years
ago. It is named after the province of Perm, Russia, where rocks of this age were first studied.
Extensive glaciation of southern hemisphere. Appalachians are fully formed- about height of
Himalayas. Most of the shallow seas drain from North America. Reptiles diversify. First
coniferous trees; begin to out-compete earlier forests. Many modern insects appear. First
Archosaurs (“ruling reptiles”). Finback reptiles (Dimetridon) are major carnivores. Mass
extinction near end of Permian when 80%+ of all life goes extinct. Last trilobites extinct by 248
million years BP.
Mesozoic Era 248 million – 65 million
The word Mesozoic is from Greek meaning "middle life." An era of geologic time between the
Paleozoic and the Cenozoic, spanning the time between 248 and 65 million years ago.
Sometimes called “Age of Reptiles” but “Age of Dinosaur” more appropriate.
Triassic Period 248 million – 213 million
The earliest period of the Mesozoic era, spanning the time between 248 and 213 million years
ago. The name Triassic refers to the threefold division of rocks of this age in Germany.
Volcanism in eastern US. Overall climate cooler. Many areas are arid (dry) to semi-arid.
Pangaea (Supercontinent) is fully formed. Continents are mountainous. Appalachians broken
into basins due to uplift. Fossil record shows fewer species (decreased diversity) but
populations of each are great. Ammonites, clams, snails in oceans. First dinosaurs as well as
the first primitive mammals.
Jurassic Period 213 million – 145 million
The middle period of the Mesozoic era, spanning the time between 213 and 145 million years
ago. It is named after the Jura Mountains between France and Switzerland, where rocks of
this age were first studied. North America and Africa separate. Ocean basins open.
Appalachians eroding. Pacific and North American plates collide with subduction occurring
on the western coast. Orogeny from Alaska to Mexico. Climate warmer than today. Cycads,
ginkgo present. Reptiles dominate land, sea, and air. Archaeopteryx (transition from
theropod dinosaur to bird) lives.
Cretaceous Period 145 million – 65 million
The final period of the Mesozoic era, spanning the time between 145 and 65 million years ago.
The name is derived from the Latin word for chalk ("creta") and was first applied to extensive
deposits of this age that form white cliffs along the English Channel between Great Britain
and France. Lands are low and extensive. Rocky Mountains form near end of period. Africa
and South America Separate. Dinosaurs extinct by end of Cretaceous. Marsupials,
insectivores abundant and first true flowering plants appear. Another mass extinction event.
Cenozoic Era 65 million - present
Its name is from Greek and means "new life” or “recent life”. An era of geologic time from the
beginning of the Tertiary period (65 million years ago) to the present.
Tertiary Period 65 million – 1.8 million
The first period of the Cenozoic era (after the Mesozoic era and before the Quaternary
period), spanning the time between 65 and 1.8 million years ago.
Paleocene Epoch 65 million – 55.5 million
It is named after the Greek words "palaois" (old) and "ceno" (new). Earliest epoch of the
Tertiary period, spanning the time between 65 and 55.5 million years ago. Climate is mild to
cool. Wide, shallow continental seas mostly gone. Volcanic activity begins to form Rocky
Mountains. Greenland and North America separate. First known primitive primates and
mammalian carnivores. Some modern plants are present.
Eocene Epoch 55.5 million – 33.7 million
Its name is from the Greek words "eos" (dawn) and "ceno" (new). An epoch of the lower
Tertiary period, spanning the time between 55.5 and 33.7 million years ago. Climate is now
mild to tropical. Many lakes in western North America. Australia separates from Antarctica.
India collides with Asia. Primitive horses, tiny camels, modern birds and gigantic birds
present. Hardwoods and redwoods in western US. Grasslands expand.
Oligocene Epoch 33.7 million – 23.8 million
It is named after the Greek words "oligos" (little, few) and "ceno" (new). An epoch of the early
Tertiary period, spanning the time between 33.7 and 23.8 million years ago. Himalayas and
Alps rise from the subduction of India and Asia. Red Sea forming due to separation of Africa
and Arabia. Rockies are volcanic. California collides with mid-Pacific Ridge. Large browsing
mammals. True apes appear. Origin of many modern families of flowering plants. Species of
feline (cats) and canine (wolves) appear. Early rhinoceros and elephant present in fossil
record.
Miocene Epoch 23.8 million – 5.3 million
It is named after the Greek words "meion" (less) and "ceno" (new). A epoch of the upper
Tertiary period, spanning the time between 23.8 and 5.3 million years ago. Climate is
moderate. Southern hemisphere experiences another round of extensive glaciation. Rockies
continue to uplift. Volcanic activity produces the Cascade Mountains. South America and
Antarctica separate. Andes Mountains forming. Whales, apes, and grazing animals
widespread. Forests diminish while grasslands continue to expand. Hyena, bear, seal and
raccoon present.
Pliocene Epoch 5.3 million – 1.8 million
It is named after the Greek words "pleion" (more) and "ceno" (new). Final epoch of the
Tertiary period, spanning the time between 5.3 and 1.8 million years ago. Climate cooler.
Uplift and mountain-building continue and many species of animals become extinct. North
and south America connect as Panama uplifts. Large carnivores prowl grasslands. First
appearance of upright hominids (Australipithicenes)
Quaternary Period 1.8 million - present
It is named after the Latin word "quatern" (four at a time). The second period of the Cenozoic
era, spanning the time between 1.8 million years ago and the present. It contains two epochs:
the Pleistocene and the Holocene.
Pleistocene Epoch 1.8 million – 8,000
It is named after the Greek words "pleistos" (most) and "ceno" (new). An epoch of the
Quaternary period, spanning the time between 1.8 million years ago and the beginning of the
Holocene at 8,000 years ago. Climate fluctuates between mild and cool. Four major glacial
advances and retreats (AKA: Ice Age). Sierra Nevada Mountains uplift. Large numbers of
mammal species (giant mammoth, mastodon, saber tooth cats) begin to disappear by the end
of Pleistocene.
Holocene Epoch 8,000 - present
It is named after the Greek words "holos" (entire) and "ceno" (new). An epoch of the
Quaternary period, spanning the time from the end of the Pleistocene (8,000 years ago) to the
present. Glaciers retreat; warming; another interglacial period. Deserts spread on a large scale
Planetary spread of modern humans Homo sapiens (AKA: “Wise Man”). Extinction of many
of the large mammals.