Geologic Time Scale

• The geological time scale is divided into eons, eras, periods, epochs
• Eras are largest divisions in scale while periods are the smallest divisions of eras.
• Mass Extinction


  event that occurs when entire groups of organisms disappear (5 total in Earth's history)
• begins with formation of Earth about 4.6 (4.55 in LG) billion years ago with the "big bang"
• Precambrian Era


  oldest fossils found are 3.5 billion years old

  resemble photosynthetic bacteria called cyanobacteria

  also found dome-shaped structures called stromatolites

  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


  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% (96% in LG) marine life and 70% of land species disappeared
• Mesozoic Era 


  Three Periods
• Triassic Period

  251 mya - 200 mya

  mammals appeared in form of small, mouse-like creatures

  climate was hot and dry
• Jurassic Period

  200 mya - 146 mya

  referred to as "age of Dinosaurs"

  fossil record shows Archaeopteryx as 1st possible bird
• 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
• K-T Boundary 


  geological signature that represents the separation of the age of reptiles to 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 moved during the history of the earth and are still moving by 6cm per year

  245 mya continents joined in a landmass known as Pangaea
• 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