Periods and
Systems

GEOLOGIC TIME
Eon	Era	Epochs and Series	Beginning of Interval*	Biological Forms
Phanerozoic	Cenozoic	Quaternary	Holocene Pleistocene	0.01 1.8	Earliest humans
	Cenozoic	Tertiary	Pliocene Milocene Oligocene Eocene Paleocene	5 24 34 55 65	Earliest hominids Earliest grasses Earliest large mammals
	Cretaceous-Tertiary boundary (65 million years ago):extinction of dinosaurs
	Mesozoic	Cretaceous	Upper Lower	98 144	Earliest flowering plants; dinosaurs in ascendancy
	Mesozoic	Jurassic Triassic		208 248	Earliest birds Earliest dinosaurs & mammals
	Paleozoic	Permian Carboniferous Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian		286 320 360 410 438 505 544	Earliest reptiles Earliest winged insects Earliest amphibians & bony fish Earliest land plants & insects Earliest corals Earliest fish
Proterozoic	Precambrian			2500	Earliest colonial algae & soft-bodied invertebrates
Archean				3800	Earliest surviving microfossils of primitive single-celled organisms
Hadean				4600	No surviving fossils

*Approximate, in millions of years before the present