• Examining layers of sedimentary rock, scientists have put together a chronology of Earth’s history.

Divided into 4 Eras:

1. Precambrian (3.5 bya - 540 mya)

2. Paleozoic (540 mya - 245 mya)

3. Mesozoic (245 mya - 66 mya)

4. Cenozoic (66 mya - present)

p. 560-563

- Eras are largest divisions in scale while periods are smaller divisions of eras.

- mass extinction is an event that occurs when entire groups of organisms disappear. (Five total in Earth’s history)

- begins with formation of Earth about 4.6 billion years ago with the “big bang”.

Precambrian Era:

• oldest fossils found in Precambrian rocks that are 3.5 billion years old. They resemble photosynthetic bacteria called cyanobacteria.

• also found dome-shaped structures called stromatolites.

Precambrian Era:

• unicellular prokaryotes flourished in beginning of era while multi-cellular eukaryotes flourished at the end of the era.

• accounts for 87% of Earth’s history.

Paleozoic Era:

Divided into 6 periods:

1. Cambrian (542 - 488 mya)

2. Ordovician (488 - 444 mya)

3. Silurian (444 - 416 mya)

4. Devonian (416 - 359 mya)

5. Carboniferous (359 - 299 mya)

6. Permian (299 - 251 mya)

Paleozoic Era:

• explosion of life during Cambrian Period, increase in diversity of life.

• oceans teemed with life, fish appeared. Ferns and early seed plants formed on land. Amphibians and reptiles appeared during last half of Paleozoic Era.

•ended with largest mass extinction recorded by fossil record (Permian Extinction). 90% of marine life and 70% of land species disappeared.

Mesozoic Era:

Three Periods:

1. Triassic Period (251 mya - 200 mya)

- mammals appeared in form of small, mouse-like creatures

- Climate was hot and dry

Mesozoic Era:

2. Jurassic Period (200 mya - 146 mya)

- referred to as “Age of Dinosaurs”

- fossil record shows Archaeopteryx as 1st possible bird

Mesozoic Era:

3. Cretaceous Period (146 mya – 65.5 mya)

- first flowering plants developed

- mass extinction 65.5 mya marked end of period. Dinosaurs wiped off Earth and 2/3 of living species extinct.

- caused by meteor colliding with Earth (112 mile wide Chicxulub Crater on Yucatan Penninsula)

K-T Boundary

- geological signature that represents the separation of the age of reptiles and the age of mammals.

- can be seen by a thin line in sedimentary rock due to high levels of iridium (a metal found mostly in meteors).

Changes During Mesozoic Era

- Theory of continental drift suggests that the continents have moved during the history of Earth and are still moving at a rate of 6 cm per year. (p. 387)

• 245 mya continents joined in a landmass known as Pangaea.

• 135 mya Pangaea broke apart resulting in 2 large landmasses.

• 66 mya most continents have taken modern shape.

Cenozoic Era:

1. Tertiary Period (66 mya - 1.6 mya)

- mammals that survived extinction flourished.

2. Quarternary Period (1.6 mya - present day)

- primates appeared 30 mya and diversified greatly. First humans appeared as early as 200,000 years ago.

January 1st = First evidence of life (Precambrian Era)

October 15th = Start of Paleozoic Era

December 10th = Start of Mesozoic Era

December 15th = Dinosaurs roam Earth

December 26th = K-T extinction and start of Cenozoic Era

11:30 PM December 31st = Humans appear