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

Copyright©AmyBrownScience