ACKNOWLEDGEMENTS
Thisbookarose,asdidthepreviousoneswehavewritten,outofa momentofimprovisation,whichthensomehowhadtobecarried forwardstosomekindofconclusion.Theideawastotakeskeletons,the (mostly)hiddenframeworkofbiological life,which,inancientform,hasalso providedtheframework insomewhatmoreexposedandvisibleform of muchofourprofessionallives,andmakesenseoftheminthewaythat theyhaveunderpinnedthedevelopmentofcomplexlifeonEarth.
Inmakingthisstorytakeshape,wearedeeplygratefulfortheunceasingsupportandencouragementofLathaMenonandothersatOxford UniversityPress.Lathahasanunrivalledabilitytoseewhereanarrative linemightbeheadingfortherocks,andtogentlysteeritintomore productivewaters,whileJennyNugeeandhercolleagueshavebeen cheerfullypatientwithourperilouslycloseencounterswithdeadlines, andmarvellouslyefficientincompilingthe finishedproduct.
Astothesubjectmatter,thecontentherehasbeeninfluencedbyall thepeoplewhohaveinfluenced us aswehavecometogripswithfossil skeletons,oldandnew,inourstudiesandinourworkinglives.Thatisan awfullylonglist,butincludescolleaguessuchasDavidandDerekSiveter, RichardFortey,andthelateandmuchmissed figuresofDickAldridge andBarrieRickards.WededicatethisbooktoAdrianRushton,whohas influencedthesepagesbothdirectlyandindirectly;therangeoffossil skeletonshehascovered,expertly,hasnotbeenrivalledsincethedaysof thegreatVictorian-erapolymaths,andtheamountofhelpandsupport hehasgiventoothersthroughhiscareerhasbeenimmeasurable.We alsothankthefollowingfortheimagesthatwehaveusedinthisbook andforadviceonimages:JohnAhlgren,PeiyunCong,IvánCortijo,Jason Dunlop,DennisHansen,TomHarvey,SorayaMarali,DavidMartill,
GilesMiller,ChrisNedza,MarkPurnell,AdrianRushton,PaulSelden, DavidSiveter,DerekSiveter,VincentPerrier,UlrichSalzmann,Bernd Schöne,IanWilkinson,XianguangHou,XiaoyaMa,andJeremyYoung. Wethankourfamilies,too Asih,Kasia,Milana,Mat forthesupport andinspirationthattheycontinuallygive andinfinitepatience,too,as westealtimetowritethisbook.Andourparentstoo,Doreen,Les,Irena, andFeliks,whogaveustheinspirationtobeinquisitive.
PROLOGUE
WhatlinksthetumbleweedthathasinvadedtheNorthAmerican plains,thesingleoutstretchedcaninetoothofanarwhal,the fourth fingerofapterosaur,thecarapaceofabeetle,andtheancient submarinemountains,reaching5kilometrestall,atthetopofwhichthe coralislandsoftropicaloceansjustemergeattheoceansurface?Theseare allformsofskeleton,producedbylivingorganismsinanextraordinary late flurryofevolutiononaplanetthatwasalreadyeasingintomiddleage.
Imagine,then,aworldwhereskeletonshadnotevolved.Wewould notseebirds flyingthroughtheairtoperchonatreebranch,oracat leapinghighontoagardenwall(perhapstotrytocatchoneofthose birds),oracrabscuttlingonabeach,orachildrunningthrougha playground.Wemightbeforgivenforthinkingthataworldwithout suchfamiliarthingswouldbeabizarreplaceindeed.Butthatisjusthow itwasformostofthehistoryoflifeonEarth.
TheEarth’ssurfacenowholdsavastarrayofskeletons,frommicroscopictogigantic.Somepilesofskeletons,liketheGreatBarrierReefof Australia,aresolargethattheycanbeseenfromspace,whileothers, visibleonlythroughpowerfulmicroscopes,showexquisitepreservation ofminutestructuresfromhundredsofmillionsofyearsago.Specimens ofthemostspectacularskeletonshavebeenavidlysoughtbyhumans, eveninantiquity,andtheirpowertoaweremainsundimmed.Looking fromanotherperspective,skeletons enmasse,assembleddowntheyears andstoredwithinlayersofrock,havebeencrucialincontrollingsomeof theEarth’smostimportantchemicalcycles,andinmaintainingahabitableclimateonourplanet.
Inthisbookwelookatskeletonsfrommanyangles,andencompass allofthosemineralframeworksthathaveallowedlifetoengineerthe
planetweliveon.Welookattheskeletoninnovationsthatoccurredin deepantiquity,withintinycellsthatbuiltformsofscaffoldingforthemselves,andhowthisledtotheeventualevolutionofthefamiliarskeletonsoftoday.Weexplaintheverydifferentstrategiesbywhich skeletonscanbebuilt,andweshowhowtheseframeworkscanengineer anextraordinarydiversityofbodies,shapes,sizes,andsolutionstothe problemofliving.
LookingatlifeonEarthfromtheperspectiveofskeletonscanhelpto answersomebigquestions.Whyareskeletonsmadefromcertainmaterials?Whydosomeanimalshavetheirskeletonsontheoutside,and othersontheinside?Whatadvantageshaveskeletonsconferredto animalsandplants,andwhatlifestylepossibilitieshavetheyenabled? Andhowhavetheyre-engineeredourplanet,inprovidingtheframeworkforsophisticatednetworksoflifethatfashionedtheevolutionof Earth’soceans,land,andatmosphere.
Howhaveskeletons,too,helped human life?Thebonesofdinosaurs andmarinereptilesarenotonlyprizedasspectacularmuseumpieces,to giveusglimpsesoftheEarth’sancientanddramaticpast.Farmershave usedthem,too,inamuchmoreprosaicandfunctionalway,albeitina waythatwoulddismayapalaeontologist.And,now,asweenhanceour ownskeletonstechnologically,andbuildnewskeletonstoallowusto exploreourworldandotherworlds,scientistscanlooktothestructure ofboneandshellthathasevolvedovermillionsofyears,toprovide inspirationastheyconstructthenewmaterialfabricsofoursociety.
Tohumans,skeletonshavelongbeenmoreprofoundsymbols,too,of bothdeathandpermanence,andthisresonancehasundoubtedly increasedtheirfascination.Inthecourseofthisbook,wewillcomeacross someofthescientistswhohavediscoveredandstudiedspectacularskeletons,livingandfossil,andworkedtosolvethemanymysteriesoflifeand deathonEarth:ofhowthatlifecameto flourishanddevelop andhowit crashedwhenthoseskeleton-buildingpowersperiodicallyfailed.
WhatmightbethefutureforskeletonsastheEarthgrowsold?The inexorablecourseofourSun’sevolutionasastarmeansthatthereis
aboutanotherbillionyearsforthisparticularexperimentofEarth-bound lifetorun.Thereistime,therefore,forthestoryofskeletonsonEarthto developnewchaptersyet and,aswewrite,newchaptersseemtobe beginning.Weseemtobeonthevergeofrepeatingancientcatastrophes totheskeletalfabricofourworld,andrecreatingsomeofthenightmare worldsofthepast,asagifttoourchildren.
Yet,simultaneously,ourspeciesisproducingextraordinarynovelty. Whatmightbethefutureofaplanetwhereskeletonsarefashionedof metal,plastic,silicon,andre-engineeredtissueandbone?Perhapsour new,inventedframeworksforlifewillhelpustraveltowardsdistantstars andplanetstoencounterdifferentorganismswithdifferentskeletons? Afterthepastfewtumultuousyearsofdiscovery,suchodysseys,onthis worldandothers,mightnowberealisticallycontemplated,withboth forebodingandanticipation.Wherevertheywilltakeus,skeletonsofone sortoranotherwillestablishtheframeworkforfutureevolution and endurethereaftertotellthestory.
EXPLOSION’
Acritarchs Stromatolites
Million years ago
CENOZOIC EONS ERAS PERIODS MESOZOIC
QUATERNARY
EPOCHS
TERTIARY PLEISTOCENE HOLOCENE PLIOCENE
CRETACEOUS
JURASSIC TRIASSIC PERMIAN
CARBONIFEROUS
PHANEROZOIC
PALAEOZOIC
DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN
OLIGOCENE EOCENE
Technological skeletons appear Beginning of hominin bipedalism
PALEOCENE
Beginning of evolution of whale skeleton
First known bat skeleton Extinction of non-avian dinosaurs
MIOCENE
SKELETONSAPPEAR
CharlesDarwinsawaworldoftwohalves.Itperplexedhimdeeply. Helookedatthefamiliarworldaroundhimwherecatswalkedand leaped,insectsandpigeons flew, fishandwhalesswamintheseas,and shellfishaboundedonsea floorsandalongcoastlines.Asayoung scientistcircumnavigatingtheglobeonthe Beagle,hehadcollected fossilizedshellsintheCanaryIslandsandthebonesofgiganticextinct slothsandarmadillosonthecoastofArgentina.Evenbeforethat,asa Cambridgeundergraduatestudent,hehadbeeninthe fieldwiththat mightygeologistofVictoriandays,AdamSedgwick,amanwhohad extractedcountlessfossiltrilobiteskeletonsfromrockstrataformed duringtheCambrianPeriodthatlongpredatedtheappearanceofbony animalsonland.
Yet,DarwinknewthatbelowtheseCambrianrockstratathatheld fossilskeletonsofmanydifferentkinds,therewereenormousthicknesses ofyetolderrocksofPrecambriantimes,inwhichtherewerenobones andnoshells.Howhadthisancientworldwithoutskeletonschanged suddenly,itseemed toaworldaboundinginthesemostdurableand versatileframeworksforlife?
Itisadilemmathatscientistsstillpuzzleover,althoughtheproblemis nowframeddifferently.Perhaps,insomeways,itisevengreater.Darwin couldseenoevidenceforlifeofanykindintheancientrocksofthe Precambrian.Forallheknew,thatworldmighthavebeenadeadplanet. And,hehadnowayofmeasuringthetimespansinvolved,inorderto
assesshowlongagothisallwas.Becausewehaveleanttoreadthe atomicclocksbuiltintosomespecificmineralsthatcanbefoundin rocks,weknowthattheEarthis4.54billionyearsold,thattracesof microbiallifecanbefoundinrocks atleast3.8billionyearsoldand perhapsevenasoldas4.1billionyears andthattheskeleton-packed CambrianPeriodbeganjust541millionyearsago.Lifehasexisted thereforeforatleastfourth-fi fths andperhapsnine-tenths ofour planet ’ sduration.Yetthoroughgoing,familiar,skeletons,andthekind oflifethattheyliterallysupported,occupyjustthepast12%ofEarth time.Skeletonsmay,onaplanetary timescale,bealatecomerinnovation,buttheychangedtheworld fundamentally.Indeed,theyde fi ne thepresentgeologicaleon,thePhanerozoic,ofwhichtheCambrianis thelowestrung.
Theproblembecomesyetmorecuriousinthatwenowknowtoo,that foralmostallofthetimethatlifehasexistedonEarth,microbescould indeedproduceskeletonsofakindthatwereeithertootinyforDarwin toseeortoocrudeforhimtorecognize.Weknow,too,fromexceedingly rarefossils,thatsoft-bodiedmulticellularorganismshaveexistedon Earthforatleast1.2billionyears,andthatadistinctivearrayofmysterious,multicellularbutseeminglyskeleton-lessorganisms,the ‘Ediacara biota’,becamequitewidespreadintheoceansabout60millionyears beforethestartoftheCambrianPeriod.
Whatevertheskeletonfactoris,itisclearlyprettyspecial andwhen little Cloudina appearedonEarthabout550millionyearsago,withits brandnewshell,itwasthestartofarevolutionontheplanet.
TheFirstTrueSkeletons
PrestonCloudwouldbeincludedinanysensiblecatalogueofremarkable geologists.AsayoungmanintheUSNavyoftheearly1930s,he becamethatorganization ’ sbantamweightboxingchampion.Later, notabletoattenduniversityduetotheGreatDepression,hewentto
nightschoolanddidmanualworkattheUSNationalMuseumduring theday.Anenthusiasmforfossilsgothimajobasapreparatorinthe palaeontologylaboratoryatthemuseum,fromwhichopportunityhis careerblossomed.Amanwithawide-angleviewoftheEarthandits placeinthecosmos,heisbestknownfordeciphering,inmemorable terms,thelargerstructureofPrecambrianhistory hewastheoneto cointheterm ‘ HadeanEon ’ tomarktheearliest,mostmysterious partofPrecambriantime,forinstance.Soitis fi ttingthatoneofthe earliestknowntrulyskeleton-bearingorganismsonEarthisnamed afterhim(Figure1).
Cloudina (ofthefamily Cloudinidae,justtoaddfurtherlustretothe greatman)ishugelyimportant,butitisnotatallthemostspectacular offossils.Itisbasicallyaratherirregularcurvedtube,uptoafew millimetreswideandafewcentimetreslong.Thetubehasadistinctive structure,beingasetoflongcones,stackedoneinsidetheother;the firstconeisclosedatitsnarrowend,andalltheothersareopen.1 And
,lateEdiacaran(Precambrian), ElMembrillarolistostrome, HelechosadelosMontes,BadajozProvince,ExtremaduraRegion,Spain.Scalebar is5mm.
Figure1. Cloudinacarinata
that’sit:therearenostructuressuchasholdfasts,nointernalpartitions, andnolid. Cloudina ismoderatelycommoninstrataoftheverylatest Precambrian,fromabout550millionyearsago.Itcoexistedwiththelast oftheenigmaticEdiacaranorganisms buttheyarenotfoundtogether inthesamerockstrata,soperhapstheylivedondifferentpartsofthesea floor. Cloudina hasareasonableclaimtorepresentthebeginningof skeletonformationaswecommonlyunderstandit.
Thetubesseemtohavebeenconstructedofcalcite,thecommon formofcalciumcarbonate.Discerningtheoriginalcompositionof theseveryearlyskeletons,of Cloudina andtheformsthatwereto shortlysucceedit,isnotalwayseasy.Perhapsbecauseofthechemical makeupoflatePrecambrianseas,theshellmaterialwasproneto beingalteredafterdeath,onthesea fl oororjustfollowingburialby sediment.Thereplacingmaterialisoftencalciumphosphate,though inotherexamplessilicaisinvolved,oralternativelycalciumcarbonate.Inthecaseof Cloudina ,well-preservedexamplesseemtoshow thattheoriginalshellmaterialwasofcalcite,intiny,micrometre-sized crystals.Thesecrystalsseemtohaveoriginallybeenembeddedin somesortoftoughexteriororganicmaterial,forthesomewhat corrugatedappearanceofthetubessuggeststhattheyweremoderately fl exibleinlife,thoughtheybecamebrittleafterdeathand recrystallization.
Whatkindofbiochemicaltrickdid Cloudina usetomakeitsskeleton?
Thebasicmechanismofallsuch biomineralization inallskeleton-builders hingesonmodifyingconditionsinsidethetissuestoencouragechemicals thataregenerallydissolvedin fluidinandaroundtheorganismto crystallizeoutashardstructures.Thesea,ingeneral,issosaturated withionsofcalcium(Ca2+)andcarbonate(CO3 2 )thatcalciumcarbonate (CaCO3)caneasilycrystallizeoutofit withnohelpfrombiologyatall. Organismscanthenencouragethisprocessbytweakingthechemical conditionsaroundthem,byconcentratingtheingredients.Ortheycan changefactorsthataffectcrystallization,ofwhichoneofthemost importantistheacidity,whichismeasuredonthepHscale,ameasure
oftheconcentrationofhydrogenions(H+).ThelowerthepH thatis themorehydrogenions,asit’saninverserelation themoreacidarethe conditions,discouragingcrystallization.RaisethepHinthetissues, though,togivemorealkalineconditions,andcrystallizationisencouraged. Cloudina hadclearlyevolvedaneffectivebiomineralizingmechanism.Ithadamakeshiftskeleton,comparedwithsomeofthemarvelsthat wewillconsiderlater.Nevertheless,athoroughgoingskeletonitwas.
Whygototheefforttobuildit?
AttackoftheSoftAnimals
Cloudina appearedataboutthesametimeasanothermajorinnovation intheEarth’ sbiosphere.About550millionyearsago,strataalso begintoshowthe fi rstsignsofsubstantialburrowing,indicatingthe evolutionofanimalssufficientlymuscularandactivetopushthrough thesedimentsofthesea floor.Theseanimalspuncturedorchurnedthe surfaceofthesedimentinaprocessthatgeologistscallbioturbation,to produceamottled,disturbedtextureintherocks.Thereareoccasionallya fewsimplertracesinolderPrecambrianrocks,butnothingofthisscale andcomplexity.FromthebeginningoftheCambrian,though,thiskindof bioturbatedrocktexturebecomescommon.Topushthroughthesea floorsedimentsinthisway,aspecialkindofskeletonwasneeded,one witharecognizableheadandtailend,ofa bilaterian animal.
Moreprimitiveanimals,suchasjellyfish,possessasimple,softhydrostaticskeleton,a fluid-filledinterior,surroundedbymuscularlayers underthecontrolofanervoussystem.Asthe fluidhere seawater is incompressible,contractionofmusclesthatmaybearrangedeither aroundtheanimal,oralongitslength,willchangeitsshape.Thatis onlyabeginningofsuchmuscularity.Ajellyfishcanmoveinthisway throughseawater butitcannotburrowthroughsediment.2
Withamoreadvancedbodyplan onewitha flexiblebuttough externalcuticle theanimalcandojustthat,asanyglanceatan
earthwormingardensoilwillshow(Darwin,whomadeadetailedstudy ofearthworms,hadasgoodaninsightintotheseprocessesasanyone). Thatkindofadvancewasonlypossiblewiththeevolutionofamore sophisticatedandrobustarrayofstructuraltissues,inthebodyplanofa bilateriananimal.Bilateriansincludemostmajoranimalgroups,andthe appearanceofbioturbatedstrata550millionyearsagoisevidenceof whenthismajorstepwithinevolutiontookplace.
Thistrulywasamajorstep,perhapstakeninresponsetotheinvention ofpredation.Forthoughjellyfishandseaanemonesmaypossessmuscle tissues,andnerves,andasimplegut,attheembryonicstagetheyonly possesstwogermlayersoftissueandthislimitstherangeofstructures theycanproduce.Theseanimalsarecalleddiploblasts,andsomeofthem, likecorals,neverthelesswentontobuildthegreatestskeletalstructures thathaveeverfunctioned.Mostanimals,fromwormstohumans,are triploblasts,inthattheembryopossessesthreelayersofgermcells.This evolutionarystepwasofcrucialimportanceforskeletons,becauseit allowedtriploblaststomakeamuchwiderrangeofstructures,including organs.Butforourstoryitenabledthedevelopmentoftheinternalbony skeletonofhumans,whichisderivedfromthemesodermgermlayer, andthemineralizedexoskeletonofatrilobite,whichisderivedfromthe outerlayer,theectoderm.Andcrucially,itproducestheunmineralized butneverthelesstoughoutercuticleofaworm,ofthekindoforganism that firstbegantunnellingintothePrecambriansea floor.
Thisrevolutionmarksoureon,foritheraldedthetransformationof thebiosphereshortlytofollow,and,asoneresult,madehardskeletonsa crucialpartofthebiologicaltoolkitforsurvival.Thebodyofbilaterians wasamajorfactor perhaps the majorfactor inkick-startingthis biosphericrevolution,andasregardsournarrativeitprovidedthestrongestofselectivepressurestoencouragedevelopmentofthesolidskeletons thatsoonfollowed.Itwasaninnovation fittomarkaneon.
TheformalboundaryforthebeginningofthePhanerozoicEon theone westilllivein waschosenforaparticularlydistinctivekindofearlyburrow, whichrepresentedakindofthree-dimensionalcorkscrewingmovement,
andnamed Treptichnuspedum, firstseentoappearwithinsea floorsediments thatnowcomprisestratainNewfoundland,Canada.Theseburrowsare similartoburrowsmadetodaybya priapulidworm,asitcorkscrews throughsea floorsedimentinsearchofprey.Thisnewtrickofmuscular movementwasthoughttorepresentthemostconsistentlytraceable levelfortheeon’sbeginningand,althoughitishasprovedalittleproblematic inpractice,3 thatistheboundarywhich,fornow,remainsinplace.
Anyanimalthenpossessingtheabilitytocrawl,slither,orburrow couldexploitthefoodreservesinorganic-richmud,simplybyeatingthat mud,whilemicrobialmatscouldbetreatedasfoodtoo.Atthatdelicate point,thestagewassetforfurtherdevelopments.Thatmuscularity,for manyofthoseearlyforms,begantobedirectedtowardsthehuntingand eatingofotherorganisms,particularlyiftheywerejustalittlesmaller andalittlelessmobile.Theevolutionaryarmsrace,forsolongslowand quiescent asfaraswecandecipher hadbeguninearnest. Cloudina was the firstdefensiveripostethatweknowofinthisarmsrace.
Theevidenceisclearinthefossilsthemselves.Inwell-preserved assemblagesoftheseskeletons,uptoone-fifthof Cloudina tubesmay showevidenceofattack,intheformofneatcircularpunctureholesin theshells.4 Someoftheholesareincomplete theydidnotpenetrateall theshell,andotherswereunsuccessful,forthe Cloudina individuallivedto bepuncturedagainlaterinitslife sometimesseveraltimes.Theshell thereforeundoubtedlyformedaprotectivearmourthatincreasedthe chancesofsurvivaltotheskeleton-maker.
Evenatthisearlystageinthearmsrace,though,theecologywasnot simple.Thereseemstohavebeenjustonepredatorintheexamplesstudied, forthereappearstobejustonetypeofpuncturemark.Buttherewas anotherpotentialtarget. Cloudina assemblagesmaybeaccompaniedin placesbyanother,similartubularshell, Sinotubulites (whichdiffersbyhaving atwo-layeredtubethatisopenatbothends5).Wherethesecoexistandthere isevidenceofpredation,itis Cloudina thatisthevictim,while Sinotubulites shellshave inthespecimensexaminedtodate,atleast shownnosigns ofattack.Theearlypredatorseemstohavebeenselective.
Whatkept Sinotubulites safe?Perhapsitsecretedtoxinstokeepthe unknownpredatoratbay,orhadadditionalprotectivearmourmadeof organicmaterial,thatwasnotpreserved.Inthearmsracemadepossible bymobilityandarmour,complexitywasthererightfromthestart and itwasonlytoincrease,withtheextraordinaryoutburstofskeleton constructionthatwas,veryshortly,tofollow.
AnEruptionofSkeletons
‘Darwin’sdilemma’ markedthesuddenchangefromancientbarrenstrata, seeminglydevoidofsignsoflife,tomorerecentstrataaboundinginthe remainsofshellsandcarapaces,ofwhich Cloudina and Sinotubulites werejust theadvanceguard.CharlesDarwinsawthisabsenceasamajorprobleminhis attemptstoexplainhow lifeevolvedonEarth andwouldhavebeeneven morepuzzledhadhebeenawarethattheEarthnurturedskeleton-bearinglife foramereone-eighthofitshistory.Hewouldhaveexpectedtoseethe fossilizedremainsofmoreprimitive ancestors,extendingbackintothe mistsoftime.Tohimcomplex,skeletonized,fossilizablelifeseemedto comeinabruptlyatthe ‘explosion’ offossilsatthebeginningoftheCambrian. However,aftermorethanacenturyofpainstakingstudyofthestrata, wecannowaskmoredetailedquestionsaboutthisCambrianexplosion oflife.Quitehowsuddenwasit,andhowcloselytiedtotheacquisition ofskeletons?Wastheexplosionessentiallyoneofeasilyfossilizable skeletonsthatfollowedalong,cryptichistoryofcomplex,soft,multicellularorganisms?Didskeleton-makingarisejustonce,ordiditappear independentlyindifferentevolutionarylinesoforganisms?Wasthe abilitytomakeskeletonstriggeredbythedevelopmentofsufficiently sophisticatedbiology,orwasitcausedbysomekindofenvironmental changes,tomakeskeleton-makingeasier?Orbyboth?Thereisaplethora ofquestionshere,basedaroundthecomplexrelationofskeletons tobiologyandtoplanetaryconditions.Wecanunpickthem someof them,atleast onebyone.
ThefossilrecordismuchbetterknownthanitwasinDarwin’stime (Darwinhimselfwasratherdeprecatingabouttheuseoffossilsin providingevidenceforwhathecalled ‘descentwithmodification’ and whatwenowknowasbiologicalevolution;in TheOriginofSpecies,he placedmoreweightonevidencefromanimalbreeding).Crucially,the fossilrecordhasnowbeennumericallycalibrated thatis,ithasbeen datedinnumbersofmillionsofyears,usingradiometricages.Amidthe skeleton-bearingstrata,thereare,hereandthere,layersofvolcanicash. Thesecommonlycontaincrystalsofmineralssuchaszircon(zirconium silicate)andmonazite(aphosphateofrareearthelements)which,when theygrew,includedsignificantamountsoftheradioactiveelementuranium.Radioactivedecayoftheuraniumtoleadwithinthecrystalis thebasisofahighlyeffectiveatomicclock,whichingoodcircumstances canestablishtheageofastratumtothenearestmillionyearsorso even atadistanceofhalfabillionyears.
Cloudina, Sinotubulites,andothers those firsttrueshell-formers followedshortlyafterthevery firstburrows,andwerethemselves followedsoonafter,ingeologicalterms,bythedistinctivethreedimensionalfossilizedburrowsof Treptichnuspedum,chosentomarkthe officialbeginningoftheCambrianPeriodandsimultaneouslythePhanerozoicEon(andsimultaneouslytoo,oftheintermediatecategoryof timedivision,thePalaeozoicErathatendedcatastrophically252million yearsago).Stratalaiddownoverthenextfewmillionyearsofthe CambrianPeriodincludethenextstageinskeletonformation.These arethe ‘smallshellyfossils’—avarietyofminutebutton-like,ortubelike,orshell-likefossils,whichtypicallyrangefromfractionsofamillimetretoafewmillimetresinsize.Thesearedisarticulatedremains,likea broken-upsuiteofchainmailarmour.Andasaresult,itisoftendifficult toknowwhatanimaltheybelongedto.Manyoftheseanimalsremain puzzling,butsomehave,bycarefulstudy,beenrelatedtodifferenttypes oforganism.TheCambrianexplosionwasgatheringpace.
Then,alittlemorethan520millionyearsago,thereappearedthe signaturefossiloftheCambrian,thetrilobite(Figure2).Tomanypeople,
