The Geologic Time Scale Reference Table MillionsofYearsAgo Era Period MajorEvents
4600–540 Precambrian
Fossilsextremelyrare.Anaerobicheterotrophicprokaryotesappear3.5billionyearsago.Photosyntheticprokaryotesappear3.0billionyearsago.Firstaerobiceukaryoticcellsappear2.0billionyearsago.Multicellularalgaeappear1.2billionyearsago.Additionalmulticellularformsquicklyfollow.Lifeexistedonlyontheseas.
540–245
Paleozoic
Cambrian The“CambrianExplosion”bringsgreatdiversityininvertebratelife.Lifeincludes:(1)Soft-bodiedjellyUish,worms,andsponges,(2)Brachiopodswithtwo-shellssimilartoclams,and(3)Trilobites(arthropods)aredominant.
Ordovician Diversemarineinvertebratesaredominant.Ancestorsofmodernoctopiandsquidappear.FirstprimitivejawlessUisharepresent.
Silurian JawlessUishareabundant.JawedUishappear.Firstcolonizationoflandbyarthropodsandvascularplants,suchasferns.
Devonian Calledthe“AgeofFishes”becauseofabundantanddiverseformsintheseas.Fernsandhorsetailsarepresentonland,aswellaswinglessinsectsandarachnids.Firstamphibiansappear.
Carboniferous Richdepositsofcoalareformed.Firstseedplantsappear.Appearanceofamnioteegginearlyreptiles.Abundantsharks,amphibians,andwingedinsects.
Permian LargestmassextinctioninEarth’shistory.95%ofmarinespeciesdisappear.Onland,gymnospermsandconifersaredominant.
245–65
Mesozoic
TriassicSurvivorsofthelargestmassextinctionspreadandcolonizenewareas.Thesuper-continentPangaeaaltersclimatesandoceancirculation.Earliestdinosaursappear.Cycadsandconifersaredominantplantlife.Earlyrodent-likemammalsarepresentinlateTriassic.
Jurassic DinosaursruletheEarthasthedominantanimallifeonland.Somereptilesdevelopbird-likecharacteristicsleadingtotheevolutionofbirds.Pangaeabeginstobreakapart.Firstangiospermsappearonland.
CretaceousDinosaursaredominantearlyinthisperiod,butinanothermassextinctionattheendofthisperiod,50%ofallplantandanimalspeciesbecomeextinct,includingthedinosaurs.Angiospermsaredominant.BreakupofPangaeacontinues.
65–present
Cenozoic
Tertiary AngiospermsandinsectsUlourish.Earliestplacentalmammals.Rapidevolutionseeninmammalsincludeslargegrazingmammalsandmarinemammals.Earth’sclimateiswarmandmild.
QuaternaryClimatecoolsleadingtoaseriesoficeagesatthebeginningofthisperiod.Mastodons,mammoths,andlargecarnivoresappear.Earliesthumansappear4.5millionyearsago,andHomosapiensappear200,000yearsago.20,000yearsagoEarth’sclimatebegantowarm.
540
510
2.0
65
140
200
245
285
360
400
430
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