Geologic Time Scale: a timeline of Earth’s history divided into eons, eras and periods.

Divisions of geologic time are determined by major changes in the fossil record.
Geologic time is based on both relative and radiometric dating.



o    Precambrian (4600-542 mya): Anaerobic forms evolved followed by photosynthetic life, aerobic forms and eukaryotes.  Life existed only in the sea.


At the end of the Precambrian Time: There was a mass extinction of single-celled life.


o    Paleozoic Era (542-251 mya):

a.     The Cambrian Period: Development of marine life such as trilobites, jellyfish, sponges and worms.




b.    Ordovician and Silurian Periods: Ancestral Octopi and Squid, aquatic arthropods, jawless fish, first land plants evolve (moss).


 [Peat moss]

c.     Devonian Period: Ferns adapted to drier areas, insects appear on land, bony fish, and first amphibians evolved from leg-like finned fish.


d.    Carboniferous and Permian Periods: Reptiles evolve from amphibians, winged insects evolve such as dragonflies and cockroaches, and plants formed in swampy forests.


At the end of the Paleozoic: There was a mass extinction of plants and animals.


o    Mesozoic Era (251-65.5 mya):

a.   Triassic Period: Increasing dominance of dinosaurs, mammals first appeared, and Coelophysis (a meat eating, light hollow boned dinosaur) ran on hind legs.


b.  Jurassic Period: Dinosaurs dominant land animal.  First bird (Archaeopteryx) appeared.

c.  Cretaceous Period: Dinosaurs such as T-Rex still dominate, crocodiles and sea turtles present in the water.



At the end of the Mesozoic: There was another mass extinction where more than half of all plants and animals groups were wiped out, including all the dinosaurs.


o    Cenozoic Era

a.     Paleogene Period:  Marine mammals such as whales and dolphins evolve, flowering plants and insects flourish on land, grasses evolved, providing a food source that encouraged the evolution of grazing mammals.




b. Neogene Period: Many mountain ranges were formed and ice started to develop in the Arctic. Climates continued a cooling and drying trend.

         c. Quaternary Period: Time of the ICE AGE!  OHHHH BURRRR!  In oceans, algae, coral,          mollusks, fishes, and mammals thrive; on land, mammals such as bats, dogs, cats, cattle, and mammoths were common, and about 6-7 million years ago an adaptive radiation began that lead to the ancestors of our own species Homo sapien.