GEOLOGIC TIME SCALE

Hadean Eon

The eon of initial formation of Earth. This eon has no fossil records.

key words: constant asteroid impacts, planetary accretion, formation of the moon, atmosphere, water

Archean Eon

The eon involving the first formation of life. Fossils involve stromatolites (from algae).

key words: late heavy bombardment of ice comets, landmass formations, cyanobacteria

Proterozoic Eon

The Eon involving the formation of multi-cellular animals. Fossils involve trilobites which are the first vertebrates.

key words: oxygenated atmosphere, global glaciations, forming and breaking of continents

Phanerozoic Eon

The Eon separated to the Paleozoic, Mesozoic, and Cenozoic Eras.

Cambrian Period

Era: Paleozoic Era

key words: sudden diversification, first trilobites and chordates

Ordovician Period

Era: Paleozoic Era

key words: first primitive plants, diversification of marine invertebrates, grapolites are common, mass extinction.

Silurian Period

Era: Paleozoic Era

key words: first vascular plants, first jawed fish, coral reefs, arthropods move onto land

Devonian Period

Era: Paleozoic Era

key words: major diversification of fishes, first amphibians, and insects

Carboniferous Period

Era: Paleozoic Era

key words: giant swamp forests form coal deposits, first reptiles, high oxygen levels and giant insects

Permian Period

Era: Paleozoic Era

key words: pangea forms, reptiles diversify, ends with largest mass extinction

Triassic Period

Era: Mesozoic Era

key words: First dinosaurs, first mammals, breaking of pangea

Jurassic Period

Era: Mesozoic Era

key words: domination of dinosaurs, first flowering plants, large sauropods

Cretaceous Period

Era: Mesozoic Era

key words: flowering plants spread, insects diversify, mammals and birds continue to evolve, extinction of dinosaurs

Paleogene Period

Era: Cenozoic Era

key words: recovery from extinction, first primates, warm global climates, first horses, whales, and bats, first true carnivores, early apes evolve

Neogene Period

Era: Cenozoic Era

key words: mammals dominate, kelp forests develop, early hominins appear, ice sheets expand, genus Homo emerges, modern animals evolve

Quarternary Period

Era: Cenozoic Era

key words: ice ages, mammoths and saber-toothed cats, homo sapiens emerged, eventual end of ice age, agriculture and human civilization