1. Geologic Time

2. 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)

3. Divisions of Geologic Time
Geologic time is broken down into four main divisions of time – Eons, Eras, Periods, Epochs.
Eon – the longest division of time.
4 eons to date
Last eon (Phanerozoic) started 544 million years ago (mya)
Era – A subdivision of an eon.
3 eras to date
Last era (Cenozoic) started 66.4 mya

4. Divisions of Geologic Time (cont.)
Period – Subdivision of an era.
11 periods to date
Last period (Quaternary) started 1.8 mya.
Epoch – subdivision of a period.
Last epoch (Holocene) started about 10,000 years ago.
Are sometimes divided into Ages (smallest time periods)

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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)

14. 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

15. 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

16. 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

17. 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.