Geologic Time Scale Notes


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 jellyfish, sponges, trilobites, jawless fishes.


trilobite jellyfish Tube%20Sponge%20640



b.    Ordovician and Silurian Periods: Octopi and squid, aquatic arthropods, starfish, coral, jawed fish, first land plants evolve (moss).

Octopus squid_600

lamprey [Peat moss]

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

Ferns grouper amphibians

d.    Carboniferous and Permian Periods: Reptiles evolve from amphibians, winged insects evolve such as dragonflies and cockroaches, and cone-bearing plants (conifers) formed in swampy forests.

reptiles Dragonfly%2010


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

     50% of land organisms and 95% of marine organisms went extinct.



o  Mesozoic Era (251-65.5 mya)::

a.   Triassic Period: Fishes, insects, reptiles and cone-bearing plants, first dinosaurs evolved, mammals first appeared about the size of a mouse.                   Coelophysis

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. Birds fly in the sky.

trex sea_turtle_live


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


o  Cenozoic Era (65.5 mya-present):

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 large grazing mammals on land.

4 Dolphin

Beepollinating pe-05-alpamayo-14-grazing-animals



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.