Transcript Geologic Time

Geologic Time Scale

The geologic time scale is a record of
the life forms and geologic events in
Earth’s history.
 First developed by studying rock layers and
index fossils worldwide.

Different divisions of the geologic time
scale are separated by events in the
history of life on Earth. (aka – major life
changes)
Divisions of Geologic Time

Geologic time is broken down into four
main divisions of time – Eons, Eras,
Periods, Epochs.
1. Eon – the longest division of time.
○ 4 eons to date
○ Last eon (Phanerozoic) started 544 million
years ago (mya)
2. Era – A subdivision of an eon.
○ 3 eras to date
○ Last era (Cenozoic) started 66.4 mya
Divisions of Geologic Time (cont.)
Period – Subdivision of an era.
3.
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11 periods to date
Last period (Quaternary) started 1.8 mya.
Epoch – subdivision of a period.
4.
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•
Last epoch (Holocene) started about 10,000
years ago.
Are sometimes divided into Ages (smallest time
periods)
Precambrian
Precambrian time starts when Earth was
created 4.6 billion years ago.
 Scientists think Earth began as a ball of dust,
rock, and ice in space that was pulled
together by gravity.
 During Precambrian Time the atmosphere,
oceans, and continents all began to form.

 Oceans form and cover Earth about 4 billion
years ago.
Precambrian (cont.)
Earth’s second atmosphere formed, making
ozone layer.
 Life is thought to have developed during
Precambrian time, about 3.5 billion years ago.

 Fossils similar to modern day bacteria
2.5 Billion Years ago first Photosynthesis by
organisms
 First rocks form about 4 billion years ago

 Continental drift begins

First multi-cellular organisms develop in late
Precambrian.
 Jellyfish-like animals
Paleozoic Era
(Cambrian Period)
544 – 505 mya
 “Cambrian Explosion”

 LOTS of different organisms evolved.

All animals lived in the sea
 Most were invertebrates (no backbone)
 Some had shells (trilobites)

Most of land covered by shallow seas
Paleozoic Era
(Ordovician Period)
505 – 438 mya
 Invertebrates dominate the oceans
 Jawless fish evolves

 First vertebrates (animal with a backbone) to
appear.

Ancestors of modern octopus and squid
appear
Paleozoic Era
(Silurian Period)
438 – 408 mya
 Coral Reefs start to develop
 Plants become abundant

 Ancestors of ferns
Fish with jaws develop
 Insects first appear

Paleozoic Era
(Devonian period)
408 – 360 mya
 Known as “Age of Fishes”

 Sharks first appear, fish develop scales, and fish
develop bony skeletons
 Fish developed Jaws

Animals begin to invade land
 First vertebrates to crawl on land were lungfish
○ Amphibians evolved from lungfish
Paleozoic Era
(Carboniferous Period)

360 – 286 mya
 Divided into two segments (Mississippian 360 – 320
mya and Pennsylvanian 320 – 286 mya)
Appalachian Mountains begin to form.
 Winged insects evolved

 Cockroaches and dragonflies
Small reptiles developed
 Giant ferns and cone bearing plants form
swamps, making “Coal forests”

 Where we get our coal from now
Paleozoic Era
(Permian Period)
286 – 245 mya
 Pangaea forms (~ 260 mya)

 All continents together as one land mass
 Deserts expand and ice covers poles
Reptiles rule the land
 First warm-blooded reptiles appear
 First mass extinction occurs.

 Trilobites, most marine organisms
Mesozoic Era
(Triassic Period)
245 – 208 mya
 Beginning of Mesozoic Era (sometimes called

Age of the Reptiles)
Some fish, reptiles, insects and conifers (cone
bearing plants) survived first mass extinction
 First dinosaurs appear
 First turtles and crocodiles appear
 First mammals appear

 Very small (mouse)
Mesozoic Era
(Jurassic Period)
208 – 144 mya
 Dinosaurs dominant land

 Mostly large dinosaurs
Pangaea begins to break apart
 First flying reptiles appear
 First birds appear

 Archaeopteryx: means “ancient wing thing”
○ Thought to be link between birds and
dinosaurs
Mesozoic Era
(Cretaceous Period)
144 – 66 mya
 First flowering plants appear

 Like modern day plants – have a fruit to protect
seeds
Flying reptiles go extinct after competition
with “new birds”
 Second mass extinction eliminates over half
of all plants and animals on Earth at end of
Cretaceous (~ 65 mya) including all dinosaurs.

 Most think it was due to an asteroid hitting Earth
Cenozoic Era
(Tertiary Period)
66 – 1.8 mya
 Often called Age of Mammals
 Continents move into modern day positions
 First grasses appear

 Let grazing animals thrive (ancestors to cattle,
sheep, deer etc.)

Modern day like groups appear
 Horses, elephants, bears, primates, whales, dolphins
Cenozoic Era
(Quaternary Period)
1.8 mya – present
 Earth’s climate cooled during a series of ice
ages
 20,000 years ago climate began to warm

 Coral, fish, algae, and mammals thrive in ocean
 Smaller land mammals become common
○ Cats, dogs, cattle, humans
Giant mammals go extinct when ice age
ends about 10,000 years ago.
 Modern humans may have evolved as early
as 100,000 years ago.
