GEOLOGIC TIME SCALE RESEARCH INFORMATION
PRECAMBRIAN EON: BEGAN 4.6 BILLION YEARS (4600 MILLION YEARS AGO) AGO TO ~
600 MILLION YEARS AGO
 Precambrian makes up almost 80% of the total geologic time period!
 Precambrian Eon includes 3 ERAs: Hadean, Archean, and Proterozoic
 Precambrian time covers the vast bulk of the Earth's history, starting with the planet's creation about 4.5
billion years ago and ending with the emergence of complex, multicelled life-forms almost four billion
years later.
 The Earth was already more than 600 million years old when life began. The planet had cooled down
from its original molten state, developing a solid crust and oceans created from water vapor in the
atmosphere.
 The earliest living organisms were microscopic bacteria, which show up in the fossil record as early as
3.4 billion years ago
 Some three billion years ago the Earth's atmosphere was virtually devoid of oxygen. At about 2.4 billion
years ago, oxygen was released from the seas as a byproduct of photosynthesis by cyanobacteria
 About 800 million years ago, oxygen levels reached about 21 percent and began to breathe life into more
complex organisms. The oxygen-rich ozone layer was also established, shielding the Earth's surface
from harmful solar radiation.
Hadean Era 4.5-4 Billion Years Ago (bya):
Summary:
 Hadean comes from the Greek word “hellish” describing the conditions on Earth. Hadean Era is the first
period in Earth history.
 Collisions from meteorites and asteroids created extreme heat on the Earth, giving it the “hellish”
conditions of molten materials. It took almost a half billion years for the Earth to cool enough to form
the iron core, silicate mantle, and thin outer crust.
 The Atmosphere during Hadean Era: Early Earth’s atmosphere could have been made up of large
amounts of nitrogen or carbon dioxide and water vapor with volcanic gases
 The Earth’s surface: the Earth had water at the end of the Hadean Era.
any surface material was very thin and unstable as the interior was still very hot causing magma to rise to
the surface.
Archean Era 4.0-2.5bya:
Summary:
 Archean means “ancient” referring to the oldest known rocks
 Some sedimentary rocks contain the earliest fossils of life on Earth. Only a small amount of these rocks
survived erosion and subduction of the surface and are found in the central parts of continents.
 Paleoclimate: Volcanic activity produced gases rich in the basic ingredients of life: carbon, hydrogen,
oxygen, and nitrogen. Toxic gases like ammonia and methane were common and there was no free
oxygen.
 Details of life: Once the Earth’s surface cooled and solidified enough to form land masses where rain
fell forming seas where organic molecules became simple cells. These early bacteria began to produce
and release oxygen into oceans and atmosphere.
Proterozoic Era 2.5bya-542mya (million years ago)
Summary:
 Proterozoic means “first life”.
 This represents the last stage of the Precambrian Eon which lasted 3.7 billion years of the Earth’s 4.6
billion year history.
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 Paleoclimate: The end of the Precambrian Eon included 4 major mountain building episodes where
plate tectonics created mountains, volcanic activity that expanded the sizes of the continents, especially
North America.
 There were continental glaciations that occurred at the beginning and then again at the end of the
Proterozoic Era, each lasting several million years.
 During these times called the “Snowball Earth” phase life would have been restricted to hydrothermal
vents until volcanic activity increased carbon-dioxide levels high enough to warm the Earth from its
“deep freeze”.
 About 1.1 billion years ago, the supercontinent of Rodinia was assembled. North America formed the
core of this supercontinent.
 Rodinia split into 2 halves approximately 750 million years ago, opening the Panthalassic Ocean.
 Details of Life: Proterozoic cells are simple organisms with only DNA and RNA no Nucleus. They take
in materials from their environment which was mostly near hydrothermal vents in deep seas or shallow
hostile ponds rich in nutrients they needed to survive.
 2.7-203 bya cyanobacteria released enough free oxygen to cause iron rocks to rust into red beds known
as hematite.
PALEOZOIC ERA: BEGAN 552 mya – Ended 251 mya:
 Makes up 6 % of Earth’s Geologic Time Scale
 Paleozoic Era includes 6 Periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and
Permian.
 The 300 million years of the Paleozoic era included the development of most invertebrate groups,
organism’s movement onto land, the evolution of fish, reptiles, insects, and vascular plants, and the
formation of the supercontinent of Pangaea.
 The Paleozoic was ended by the greatest mass extinction event in geologic history, the Permian/Triassic
extinction, when some 90% of all marine species met extinction.
CAMBRIAN PERIOD: 544-505 mya “Cambrian Explosion” or “Age of Trilobites”
Summary:
 Hard body shelled animals appear: Arthropods known as Trilobites and primitive reef-forms like coral.
 Multi-celled forms appear like sponges, and ancestors to vertebrates called Chrodates.
 Trilobites were most successful during this period. They developed eyes, antennae, jointed legs and a
hard exoskeleton like other arthropods.
 No life on land as yet.
 Supercontinent GONDWANA forms.
 Shallow seas are filled with trilobites, arthropods, sponges, mollusks, and early chrodates
Paleoclimate: (Cambrian)
 Warm and stable which triggered an “explosion” in sea life and animal diversity.
 Ocean oxygen levels were high enough where multicelled organism could evolve.
 Laurentia was along the equator with Baltica south of equator. The first supercontinent- Gondwana was
drifting towards the South Pole.
Details of life: (Cambrian)
 Most abundant and diverse animals were the Trilobites. They developed eyes, long antennae, jointed
legs, and a hard exoskeleton-like other arthropods. “Age of Trilobites”
 Other animals evolved included: snails, clams, cephalopods and corals.
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ORDOVICIAN PERIOD: 505-438 mya
Summary:
 Development of Shallow-water Reef Systems. The first land organisms appear (lichen and bryophytes or
simple mosses)
 Early jawless fish appear
 Sea levels rise and fall
 Period ends with a Major Ice Age and 2nd largest Mass Extinction
Paleoclimate (Ordovician)
 Mountain building creates barren, cold land regions as Gondwana drifts toward the South Pole.
 Laurentia and Baltica are surrounded by shallow seas in tropical zones along the equator.
 Major Ice Age causes mass extinctions of some trilobites and reef systems
 The Earth was divided between mostly warm shallow oceans and tropical zones at the equator and cold
barren land regions on Gondwana in southern hemisphere.
Details of Life (Ordovician)
 New type of marine animals appear in deeper ocean waters-Jawless Fish (the first backbone ancestor)
 Cephalopods, snails, clams appear in shallow reef systems.
 Jawless fish appear at end of Period
 Major Ice Age with shallow oceans cause the 2nd largest extinction to occur at end of Ordovician.
SILURIAN PERIOD: 438-408 mya (Plants on land)
Summary:
 Began with stable and warmer climate and rise of sea levels as glaciers melted
 Laurentia and Baltica collide as they drift into each other at the equator.
 Gondwana has drifted over the South Pole.
 Sea levels rise causes flooding along coastline creating shallow seas and expansions of coral reef
systems.
 First fossils of simple plant life appear. These did not have stems or leave structures, more like mosses.
Paleoclimate (Silurian)
 Stable warmer climate with shallow seas helped to established large reef systems and expansion of corals
and shelled life forms.
 Mountain building occurred as Laurentia and Baltica collide as Gondwana drifted towards the South
Pole.
 Flooding on low continental regions led to shallow oceans and expansion of coral reef systems.
 Iapetus Sea closes
Details of Life: (Silurian)
 Shallow seas show oldest fossils of coral reefs, expansion of brachiopods (trilobite forms).
 Evolving species of Jawless fish.
 Plants begin to appear on land. Simple moss like plants non vascular (no stems or leaves).
 Arthropods move onto land at end of period- early arachnids (spiders) and wingless insects.
DEVONIAN: 408-360 mya (Age of fishes)
Summary:
 Called “Age of Fishes”.
 Increased varieties of coral reef systems, shelled and jawless fish.
 First fish with bones- vertebrate fishes appear. Life was adapting to land as Tetrapods (4 legged land
vertebrates) amphibians appear.
 Arthropods like wingless insects and arachnids evolve.
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 Plants develop vascular systems with stems and leaves. (Devonian forests)
 Laurentia and Baltica form the continent Euramerica as Gondwana begins to drift north towards
Euramerica.
Paleoclimate : (Devonian)
 Laurentia and Baltica collide forming Euramerica and Appalachian Mountains.
 Gondwana begins to drift towards the equator. (Pangaea will eventually form in +100 my)
 A new ocean appeared the Rheic.
 The climate was relatively warm and dry (arid). No glaciers until the end of Devonian where ice began
to cover the South Pole.
Details of life: (Devonian)
 Three major forms of life appear: vertebrate fish, cartilaginous fish (sharks and rays) and Plants evolve
to wooded plants like trees.
 Armored fish appear grower larger and become fearsome predators.
 Lung fish expand on land evolving into four legged vertebrates (first amphibians).
 Fish with cartilage structures evolve. Sharks and rays become diverse and larger.
 Plants evolve to vascular structures with stems and leaves- including taller trees and ferns.
 First land arthropods appear. They were wingless insects. First arachnids-spiders appear.
Carboniferous: 360-286 mya (Age of Plants: Coal Forests)
Summary: Two Epochs- Mississippian 354-323 mya and Pennsylvanian 323-290 mya,
 Climate changes and warms as Euramerica begins to collide with Gondwana.
 Higher oxygen levels allow arthropods to grow larger.
 Ice covers the southern hemisphere and coal swamps form at the warmer equator regions.
 Coal forests where First Amphibians and Large air breathing winged insects (dragonflies and
cockroaches) dominant carboniferous swamps.
 Conifers, ferns and other flowerless plants appear.
Paleoclimate: (Carboniferous)
 Northern hemisphere has shallow seas covering much of Euramerica with carboniferous (coal) swamps
in the warm climate.
 Giant glaciers cover the southern hemisphere where Gondwana begins its movement towards
Euramerica.
 The Appalachian Mountains are formed in Euramerica as continents continue to collide in plate
movements.
 There were no distinct seasons as the Earth was uniformly warm and humid throughout the year.
 Oxygen levels increased as towering trees and forests filled the swamp lands taking in Carbon dioxide
and releasing huge amounts of oxygen into the atmosphere (35% compared to today’s 21%)
 Global cooling occurred as glaciers covered more of the South Pole ending the Carboniferous Period’s
warm climate.
Details of Life (Carboniferous)
 Marine life included the now scarce Trilobites. Shelled fish, corals, and sponges were abundant as coral
reef systems and shallow water species expanded their range in the warm shallow seas along the equator.
 The new land species (tetrapods) or amphibians and reptiles were not as well established but found the
warm shallow swamps habitable.
 Seedless plants like ferns and early trees grew up to 15 feet in height and formed large forests.
 Arthropods and other invertebrates (millipedes, spiders, scorpions, snails, dragon flies over a foot in
length, and cockroaches) grew enormous in size roaming the swamplands.
 Sharks and other cartilaginous fish were dominant large marine animals.
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Permian Period: 286-245 mya
Summary:
 Pangaea “all land” began to take shape as Gondwana collided with Euramerica
 Panthalassic and Tethys Sea were established.
 Monsoon seasons occurred along coastlines of the large continent with dry desert regions in the central
part of continent.
 Gymnosperm plants appear during this time.
 Explosion of diversity of insects occur during this time.
Paleoclimate: (Permian)
 The global cooling or ice age began Permian Period where both North and South Poles were covered in
ice and glaciers. But as Pangaea formed, the Earth warmed again to average temp higher than today.
 Atmosphere oxygen levels were still high and climate was warm along the equator and cool at the poles
 The Middle Permian extinction occurred that may have been caused by a dramatic drop in sea level.
 The major extinction that ends the Permian Period was probably caused by a series of volcanic eruptions
in Siberia, which produced massive outpourings of lava called flood basalts. This volcanism covered an
area about two-thirds the size of the United States and erupted very rapidly just at the time of the
extinction.
 Significant atmospheric disturbances caused by volcanism caused global warming and anoxic (lowoxygen) ocean waters which forced the majority of sea life and land organisms to die off in great
numbers. Causing the 90% extinction.
Details of life: (Permian)
 Insects grew larger with the increased oxygen levels.
 Dragonfly-like palaeodictyopterans and early grasshoppers and cockroaches, as well as newly evolved
forms such as beetles and scorpion-flies that are the earliest known members of insect groups that
undergo true metamorphosis.
 The two major groups, the synapsids (often called mammal-like reptiles) and diapsids (true reptiles), had
diversified into many different types.
 Some diapsids, like the armored pareiasaurs were some of the world’s first large vertebrate herbivores.
 Plants began to evolve to survive the onslaught of new, abundant vertebrate herbivores.
 Seed plants become diversified to survive the new seasonal rainy and dry periods.
 Early Bark bearing trees including conifers and flowerless spore-forming plants like ferns.
 The end of the Permian was marked by the greatest mass extinction of the last 600 million years of Earth
history, during which perhaps 90% of marine animal species disappeared. Major groups such as
trilobites became extinct.
MESOZOIC ERA: 245-65 mya 6% of Geologic Time
 Known as the Age of Dinosaurs Mesozoic means “middle life”
Includes
 Triassic Period 245-208mya
 Jurassic Period 208-144mya
 Cretaceous Period 144-65mya
During the Mesozoic Era Triassic Period: 245-208 mya
Summary: Age of Reptiles
 Life on Earth was sparse. Only a few types of animals and plants were left after the great mass extinction
at the end of the Permian, and sea life in particular was severely depleted.
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 The repopulation of Pangaea had meager beginnings, and the rediversification of life took 4–6 million
years.
 Pangaea had completely formed and the climate was generally hot and dry with polar regions moist and
temperate. (not very cold)
 Middle of Triassic mammals and dinosaurs appear.
 Pangaea was formed with warm temperate climates and monsoon seasons.
Paleoclimate: (Triassic)
 Two oceans surrounded Pangaea, Panthalassic and the Tethys oceans.
 The climate was very warm and moist river regions close to coastlines. Inland it was dry and arid.
 Monsoons were seasonal and the North and South Poles were moist and temperate.
 Pangaea would begin to separate at end of Triassic. The North America and European plates began to
diverge initiating what would be the new Atlantic Ocean.
Details of life: (Triassic)
 New forms of coral formed smaller reef systems.
 Sea urchins and Star fish appear
 New families of conifers only appeared in mid Triassic period
 Many species were greatly reduced leaving space for new diversity of species, which took the better part
of Triassic Period, about 4-6 million years.
 The first mammals and dinosaurs originated almost simultaneously, in the middle to late Triassic period.
 The Triassic ended with a marine extinction with ichthyosaurs, reached lengths of 23 meters (75 feet)
and plesiosaurs surviving into the Jurassic Period.
 Turtles, crocodyliforms, and pterosaurs all made their debuts, along with frogs and sphenodontians.
 By the end of the Triassic, many of the animal groups we see today had made their first appearances on
Earth.
Jurassic Period: 208-144
Summary:
 Ocean toxicity causes a minor mass extinction.
 Pangaea still centered along equator separates.
 Flowering plants appear
 Dinosaurs are the dominant species while mammals stay small-the size of a rat.
 Early birds appear “Archaeopteryx”
Paleoclimate: (Jurassic)
 Sea levels began to rise as Pangaea separated.
 Atlantic Ocean was born as plate tectonics caused increase in volcanic activity and Pangaea separated.
 North America had great rivers and most of the Pangaea was covered in lush green forests with mild and
warm climates.
Details of life (Jurassic)
 Plants and animals were widespread and found throughout Pangaea, until the continent broke apart
completely separating species. Isolated regions saw speciation as groups adapted to new environment.
 Dinosaurs dominated the continents and oceans becoming larger. Larger plant eating dinosaurs fed on
lush ferns and palm-like cycads while smaller vicious carnivores stalked them.
 Warm shallow seas inland saw diverse organisms in phytoplankton and other microscopic organisms.
 Reefs continued to flourish in the shallow oceans and oceans saw ichthyosaurs, long-necked plesiosaurs,
and larger shark species. The crocodile ancestor appeared.
 Wasps and other winged insects like butterflies that took in sap or plant liquids appeared.
 Vertebrates like the pterosaurs and the first birds, took to the air.
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Cretaceous Period: 144-65 mya
Summary:
 New forms of dinosaurs continued to appear along with modern mammals and birds and flowering plants
(angiosperms). Pangaea’s breaking apart (Atlantic Oceans’ beginning) saw many organisms becoming
isolated and speciation occurring with plants and animals.
 The Mesozoic Era, Cretaceous period ended with a great extinction called the K-T boundary event.
 Climate change, volcanic eruptions, and asteroid impact(s) caused major changing on the planet which
led to the a mass extinction of the dinosaurs.
Paleoclimate: (Cretaceous)
 Supercontinent Pangaea was already rifting apart, and by the mid-Cretaceous, it had split into several
smaller continents. Plate movements increased volcanic activities, which threw up soot into the air,
causing short term acid rain, emission of poisonous gases, and cooling.
 Seasons began to grow more pronounced as the global climate became cooler
 An asteroid hit Earth in the Yucatan Peninsula, Mexico, forming what is today called the Chicxulub
impact crater.
 Half of the world's species went extinct at about this time. It is thought that many of the species became
extinct before the impact, perhaps because of environmental changes occurring at this time.
Details of life: (Cretaceous)
 The first flowering plants appeared. The angiosperms thrived in a variety of environments such as areas
with damper climates, habitats favored by cycads and cycadeoids.
 Forests evolved to look similar to present day forests, with oaks, hickories, and magnolias becoming
common in North America by the end of the Cretaceous.
 The "marine reptiles" — the ichthyosaurs, plesiosaurs, and mosasaurs were extinct by the end of the
Cretaceous, as were the flying pterosaurs, but some, like the ichthyosaurs, were probably extinct a little
before the end of the Cretaceous.
 Many groups of organisms, such as flowering plants, snails and clams, amphibians, lizards and snakes,
crocodilians, and mammals survived the mass extinction.
 Many modern groups of insects were beginning to diversify, and we find the oldest known ants and
butterflies. Aphids, grasshoppers, and gall wasps appear in the Cretaceous.
 Theories to what caused the mass extinction of dinosaurs include more than one factor.
 Pangaea’s break-up and tectonic activity including increased volcanism around the world would have
caused a major climate change.
 There may have been multiple asteroid impacts occurring over millions of years, which would have
caused massive destruction of habitats and major climate changes.
Cenozoic Era: 65mya-present time
During (the beginning of Cenozoic Era) the mass extinction of dinosaurs left the Earth open to the smaller
organisms who survived the major changes. Mammals increased in numbers and diversity as did the birds,
reptiles, fish, insects, trees, grasses, and other forms of life. The continents move to their current positions as
Pangaea continued to separate. The Tertiary Period shows major evolutionary changes in mammals, became
larger, fish, birds, and plants also became specialized as they adapted to new environments.
Tertiary Period 65-1.5mya
Summary
 The warm climates, even in Polar Regions gives way to the first Ice Ages as continents move to present
day positions. Primates appear and evolve to separate species of ape and early man. Mammals expand
around the world living in all environments, including whales in the ocean.
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Paleoclimate: (Tertiary)
 Abrupt warming at the end of the Paleocene followed the release of a large volume of methane contained
in seafloor sediments. This led to the extinction of over 50% of the species of deep-sea single celled
organisms.
 Continents move to present day positions, this changed ocean currents and the climate becomes dryer
and colder creating Polar Ice Caps which lowers ocean levels creating land bridges.
 Mountain building occurs as Andes, Cascade, and Himalayas mountains are created.
 By the end of the period the first Ice Age occurs.
Details of Life: (Tertiary)
 Angiosperms continued their expansion throughout the world, as did grasses.
 Temperate deciduous woodlands mostly replaced tropical and sub-tropical forests, while plains
(grasslands) and deserts became more commonplace.
 Mammals diversified markedly, and marine fauna evolved to forms closely resembling those extant
today.
 Ancestors of modern elephants and rhinoceros grew to large size in Africa, where
 The first apes primate including monkeys, apes, and humans, also appeared.
Quaternary Period: 1.5 mya to present
Summary:
 Huge glaciers advanced and retreated over much of North America and Europe, parts of South America
and Asia, and over all of Antarctica. The Great Lakes formed and giant mammals thrived in parts of
North America and Eurasia not covered in ice. Giant mammals became extinct when the last Ice Age
ended some 11,700 years ago. Modern humans evolved about 190,000 years ago. During the Quaternary
period, mammals, flowering plants, and insects dominated the land.
Paleoclimate: (Quaternary)
 The continents had moved to their current place.
 Global cooling, or ice ages, took place. Much of the world's temperate zones were alternately covered by
glaciers during cool periods and uncovered during the warmer interglacial periods when the glaciers
retreated.
 The dramatic climatic changes during the ice age impacted fauna and flora in major ways.
 When glaciers advanced south, so did the plants and animals as they were forced to find available living
space and food.
Details of life: (Quaternary)
 Native horses and camels galloped across the plains of North America. Great teratorn birds with 25-foot
wingspans stalked prey.
 A major extinction event of large mammals (megafauna) that included mammoths, mastodons, sabertoothed cats, glyptodons, ground sloths, Irish elk, and cave bears began in the middle of Quaternary
Period.
 Neanderthals also became extinct during this period.
 Early humans were developing larger brains and evidence of hunting and cave drawings have been found
about 600,000 years ago.
 By the end of the last ice age, smaller and swifter mammals and migratory birds, had displaced the
megafauna and then returned northward.
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