CHAPTERFOUR

TEETHEVERYWHERE

Thetoothgetsshortshriftinanatomyclass:wespendalloffiveminutesonit.Inthepantheonoffavoriteorgans—I’llleaveittoeachofyoutomakeyourlist—teethrarelyreachthetopfive.Yetthelittletoothcontainssomuchofourconnectiontotherestoflifethatitisvirtuallyimpossibletounderstandourbodieswithoutknowingteeth.Teethalsohavespecialsignificanceforme,becauseitwasinsearchingforthemthatIfirstlearnedhowtofindfossilsandhowtorunafossilexpedition.Thejobofteethistomakebiggercreaturesintosmaller

pieces.Whenattachedtoamovingjaw,teethslice,dice,andmacerate.Mouthsareonlysobig,andteethenablecreaturestoeatthingsthatarebiggerthantheirmouths.Thisisparticularlytrueofcreaturesthatdonothavehandsorclawsthatcanshredorcutthingsbeforetheygettothemouth.True,bigfishtendtoeatlittlerfish.Butteethcanbethegreatequalizer:smallerfishcanmunchonbiggerfishiftheyhavegoodteeth.Smallerfishcanusetheirteethtoscrapescales,feedonparticles,ortakeoutwholechunksof

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fleshfrombiggerfish.Wecanlearnalotaboutananimalbylookingatitsteeth.

Thebumps,pits,andridgesonteethoftenreflectthediet.Carnivores,suchascats,haveblade-likemolarstocutmeat,whileplanteatershaveamouthfullofflatterteeththatcanmacerateleavesandnuts.Theinformationalvalueofteethwasnotlostontheanatomistsofhistory.TheFrenchanatomistGeorgesCuvieroncefamouslyboastedthathecouldreconstructananimal’sentireskeletonfromasingletooth.Thisisalittleoverthetop,butthegeneralpointisvalid;teethareapowerfulwindowintoananimal’slifestyle.Humanmouthsrevealthatweareall-purposeeaters,for

wehaveseveralkindsofteeth.Ourfrontteeth,theincisors,areflatbladesspecializedforcutting.Therearmostteeth,themolars,areflatter,withadistinctivepatternthatcanmacerateplantoranimaltissue.Thepremolars,inbetween,areintermediateinfunctionbetweenincisorsandmolars.Themostremarkablethingaboutourmouthsisthe

precisionwithwhichwechew.Openandcloseyourmouth:yourteethalwayscometogetherinthesameposition,withupperandlowerteethfittingtogetherprecisely.Becausetheupperandlowercusps,basins,andridgesmatchclosely,weareabletobreakupfoodwithmaximalefficiency.Infact,amismatchbetweenupperandlowerteethcanshatterourteeth,andenrichourdentists.Paleontologistsfindteethwonderfullyinformative.Teeth

arethehardestpartsofourbodies,becausetheenamelincludesahighproportionofthemineralhydroxyapatite—

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highereventhanisfoundinbones.Thankstotheirhardness,teethareoftenthebest-preservedanimalpartwefindinthefossilrecordformanytimeperiods.Thisislucky;sinceteetharesuchagreatcluetoananimal’sdiet,thefossilrecordcangiveusagoodwindowonhowdifferentwaysoffeedingcameabout.Thisisparticularlytrueofmammalhistory:whereasmanyreptileshavesimilarteeth,thoseofmammalsaredistinctive.ThemammalsectionofatypicalpaleontologycoursefeelsalmostlikeDentistry101.Livingreptiles—crocodiles,lizards,snakes—lackmuch

ofwhatmakesmammalianmouthsunique.Acrocodile’steeth,forexample,allhaveasimilarblade-likeshape;theonlydifferencebetweenthemisthatsomearebigandotherssmall.Reptilesalsolackthepreciseocclusion—thefitbetweenupperandlowerteeth—thathumansandothermammalshave.Also,whereaswemammalsreplaceourteethonlyonce,reptilestypicallyreceivevisitsfromthetoothfairyfortheirentirelives,replacingtheirteethcontinuallyastheywearandbreakdown.Averybasicpieceofus—ourmammalianwayofprecise

chewing—emergesinthefossilrecordfromaroundtheworldthatrangesfrom225millionto195millionyearsago.Atthebase,intheolderrocks,wefindanumberofreptilesthatlooksuperficiallydog-like.Walkingonfourlegs,theyhavebigskulls,andmanyofthemhavesharpteeth.Theretheresemblancestops.Unlikedogs,thesereptileshaveajawmadeupofmanybones,andtheirteethdon’treallyfitwelltogether.Also,theirteetharereplacedin

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adecidedlyreptilianway:newteethpopinandoutthroughouttheanimals’lives.Gohigherintherocksandweseesomethingutterly

different:theappearanceofmammalness.Thebonesofthejawgetsmallerandmovetotheear.Wecanseethefirstevidenceofupperandlowerteethcomingtogetherinpreciseways.Thejaw’sshapechanges,too:whatwasasimplerodinreptileslooksmorelikeaboomeranginmammals.Atthistime,too,teetharereplacedonlyonceperlifetime,asinus.Wecantraceallthesechangesinthefossilrecord,especiallyfromcertainsitesinEurope,SouthAfrica,andChina.Therocksofabout200millionyearsagocontainrodent-

likecreatures,suchasMorganucodonandEozostrodon,thathavebeguntolooklikemammals.Theseanimals,nobiggerthanamouse,holdimportantpiecesofusinside.Picturescannotconveyjusthowwonderfultheseearlymammalsare.Forme,itwasarealthrilltoseecreatureslikethemforthefirsttime.WhenIenteredgraduateschool,Iwantedtostudyearly

mammals.IchoseHarvardbecauseFarishA.Jenkins,Jr.,whomwemetinthefirstchapter,wasleadingexpeditionstotheAmericanWestthatsystematicallyscouredtherocksforsignsofhowmammalsdevelopedtheirdistinctabilitiestochew.Theworkwasrealexploration;Farishandhisteamwerelookingfornewlocalitiesandsites,notreturningtoplacesotherpeoplehaddiscovered.Farishhadassembledatalentedgroupoffossilfinderscomprising

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stafffromHarvard’sMuseumofComparativeZoologyandafewfree-lancemercenaries.ChiefamongthemwereBillAmaral,ChuckSchaff,andthelateWillDowns.Thesepeopleweremyintroductiontotheworldofpaleontology.Farishandtheteamhadstudiedgeologicalmapsand

aerialphotostochoosepromisingareaswheretheymightfindearlymammals.Then,eachsummer,theygotintheirtrucksandheadedoffintothedesertsofWyoming,Arizona,andUtah.BythetimeIjoinedthem,in1983,theyhadalreadyfoundanumberofimportantnewmammalsandfossilsites.Iwasstruckbythepowerofpredictions:simplybyreadingscientificarticlesandbooks,Farish’steamcouldidentifylikelyandunlikelyplacestofindearlymammals.Mybaptisminfieldpaleontologycamefromwalkingout

intheArizonadesertwithChuckandBill.Atfirst,thewholeenterpriseseemedutterlyrandom.Iexpectedsomethingakintoamilitarycampaign,anorganizedandcoordinatedreconnaissanceofthearea.WhatIsawlookedliketheextremeopposite.Theteamwouldplunkdownonaparticularpatchofrock,andpeoplewouldscatterineveryconceivabledirectiontolookforfragmentsofboneonthesurface.Forthefirstfewweeksoftheexpedition,theyleftmealone.I’dsetofflookingforfossils,systematicallyinspectingeveryrockIsawforascrapofboneatthesurface.Attheendofeachdaywewouldcomehometoshowoffthegoodieswefound.Chuckwouldhaveseveralbagsofbones.Billwouldhavehiscomplement,usuallywithsomesortoflittleskullorotherprize.AndIhadnothing,my

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emptybagasadreminderofhowmuchIhadtolearn.Afterafewweeksofthis,Idecideditwouldbeagood

ideatowalkwithChuck.Heseemedtohavethefullestbagseachday,sowhynottakesomecuesfromtheexpert?Chuckwashappytowalkwithmeandexpoundonhislongcareerinfieldpaleontology.ChuckisallWestTexaswithaBrooklynflourish:cowboybootsandwesternvalueswithaNewYorkaccent.Whileheregaledmewithtalesofhispastexpeditions,Ifoundthewholeexperienceutterlyhumbling.First,Chuckdidnotlookateveryrock,andwhenhechoseonetolookat,forthelifeofmeIcouldn’tfigureoutwhy.Thentherewasthereallyembarrassingaspectofallthis:ChuckandIwouldlookatthesamepatchofground.Isawnothingbutrock—barrendesertfloor.Chucksawfossilteeth,jaws,andevenchunksofskull.Anaerialviewwouldhaveshowntwopeoplewalking

aloneinthemiddleofaseeminglylimitlessplain,wherethevistaofdustyredandgreensandstonemesas,buttes,andbadlandsextendedformiles.ButChuckandIwerestaringonlyattheground,attherubbleandtalusofthedesertfloor.Thefossilswesoughtweretiny,nomorethanafewincheslong,andourswasaverysmallworld.Thisintimateenvironmentstoodinextremecontrasttothevastnessofthedesertpanoramathatsurroundedus.Ifeltasifmywalkingpartnerwastheonlypersonontheentireplanet,andmywholeexistencewasfocusedonpiecesofrubble.ChuckwasextraordinarilypatientwithmeasIpestered

himwithquestionsforthebetterpartofeachday’swalk.I

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wantedhimtodescribeexactlyhowtofindbones.Overandover,hetoldmetolookfor“somethingdifferent,”somethingthathadthetextureofbonenotrock,somethingthatglistenedliketeeth,somethingthatlookedlikeanarmbone,notapieceofsandstone.Itsoundedeasy,butIcouldn’tgraspwhathewastellingme.TryasImight,Istillreturnedhomeeachdayempty-handed.Nowitwasevenmoreembarrassing,asChuck,whowaslookingatthesamerocks,camehomewithbagafterbag.Finally,oneday,Isawmyfirstpieceoftoothglisteningin

thedesertsun.Itwassittinginsomesandstonerubble,butthereitwas,asplainasday.Theenamelhadasheenthatnootherrockhad;itwaslikenothingIhadseenbefore.Well,notexactly—Iwaslookingatthingslikeiteveryday.ThedifferencewasthistimeIfinallysawit,sawthedistinctionbetweenrockandbone.Thetoothglistened,andwhenIsawitglistenIspotteditscusps.Thewholeisolatedtoothwasaboutthesizeofadime,notincludingtherootsthatprojectedfromitsbase.Tome,itwasasgloriousasthebiggestdinosaurinthehallsofanymuseum.Allofasudden,thedesertfloorexplodedwithbone;

whereonceIhadseenonlyrock,nowIwasseeinglittlebitsandpiecesoffossileverywhere,asifIwerewearingaspecialnewpairofglassesandaspotlightwasshiningonallthedifferentpiecesofbone.Nexttothetoothweresmallfragmentsofotherbones,thenmoreteeth.Iwaslookingatajawthathadweatheredoutonthesurfaceandfragmented.Istartedtoreturnhomewithmyownlittle

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bagseachnight.NowthatIcouldfinallyseebonesformyself,whatonce

seemedahaphazardgroupeffortstartedtolookdecidedlyordered.Peopleweren’tjustscatteringrandomlyacrossthedesert;therewererealthoughunspokenrules.Rulenumberone:gotothemostproductive-lookingrocks,judgingbywhateversearchimageorvisualcuesyou’vegainedfrompreviousexperience.Rulenumbertwo:don’tfollowinanybody’sfootsteps;covernewground(Chuckhadgraciouslyletmebreakthisone).Rulethree:ifyourplumareaalreadyhassomebodyonit,findanewplum,orsearchalesspromisingsite.Firstcome,firstserved.Overtime,Ibegantolearnthevisualcuesforotherkinds

ofbones:longbones,jawbones,andskullparts.Onceyouseethesethingsyouneverlosetheabilitytofindthem.Justasagreatfishermancanreadthewaterandseethefishwithin,soafossilfinderusesacatalogueofsearchimagesthatmakefossilsseemtojumpoutfromtherocks.Iwasbeginningtogainmyownvisualimpressionsofwhatfossilboneslooklikeindifferentrocksandindifferentlightingconditions.Findingfossilsinthemorningsunisverydifferentfromfindingthemintheafternoon,becauseofthewaythelightplaysalongtheground.Twentyyearslater,IknowthatImustgothrougha

similarexperienceeverytimeIlookforfossilssomeplacenew,fromtheTriassicofMoroccototheDevonianofEllesmereIsland.I’llstruggleforthefirstfewdays,almostasIdidthosedayswithChuckinArizonatwentyyearsago.

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ThedifferenceisthatnowIhavesomeconfidencethatasearchimagewillkickineventually.ThewholegoaloftheprospectingIdidwithChuckwasto

findasitewithenoughbonestomarkafossil-richlayerthatwecouldexpose.BythetimeIjoinedthecrew,Farish’steamhadalreadydiscoveredsuchazone,apatchofrockaboutahundredfeetlongthatcontainedskeletonafterskeletonofsmallanimals.Farish’sfossilquarrywasinsomeveryfine-grained

mudstone.Thetricktoworkingonitwastorealizethatthefossilswerecomingfromonethinlayer,nomorethanamillimeterthick.Onceyouexposedthatsurface,youhadaverygoodchanceofseeingbones.Theyweretiny,nomorethananinchortwolong,andblack,sotheylookedalmostlikeblacksmudgesagainstthebrownishrock.Thelittleanimalswefoundincludedfrogs(someoftheearliest),leglessamphibians,lizardsandotherreptiles,and,importantly,someoftheearliestmammals.Thekeypointisthattheearlymammalsweresmall.Very

small.Theirteethwerenotmuchmorethan2millimeterslong.Tospotthem,youhadtobeverycarefuland,moreoften,verylucky.Ifthetoothwascoveredbyacrumbofrockorevenbyafewgrainsofsand,youmightneverseeit.Itwasthesightoftheseearlymammalsthatreally

hookedme.I’dexposethefossillayer,thenscantheentiresurfacethroughmy10-powerhandlens.I’dscrutinizethewholethingonmyhandsandknees,withmyeyeandhandlensonlyabouttwoinchesfromthesurfaceoftheground.

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Thusengrossed,I’doftenforgetwhereIwasandaccidentallytrespassonmyneighbor’sspotonlytohaveabagofdirtdumpedonmyheadasasharpremindertokeeptomyspace.Occasionally,though,I’dhitthejackpotandseeadeepconnectionforthefirsttime.Theteethwouldlooklikelittleblades,withcuspsandroots.Thecuspsonthoselittleteethrevealedsomethingveryspecial.Eachtoothhadacharacteristicpatternofwearatthefacewhereupperandlowerteethfittogether.Iwasseeingsomeofthefirstevidenceofourpatternofprecisechewing,onlyinatinymammal190millionyearsold.ThepowerofthosemomentswassomethingI’llnever

forget.Here,crackingrocksinthedirt,Iwasdiscoveringobjectsthatcouldchangethewaypeoplethink.Thatjuxtapositionbetweenthemostchild-like,evenhumbling,activitiesandoneofthegreathumanintellectualaspirationshasneverbeenlostonme.ItrytoremindmyselfofiteachtimeIdigsomewherenew.Returningtoschoolthatfall,Idevelopedtheexpedition

bugbig-time.Iwantedtoleadmyownexpeditionbutlackedtheresourcestodoanythingbig,soIsetofftoexplorerocksinConnecticutthatwereabout200millionyearsold.Wellstudiedduringthenineteenthcentury,theyhadbeenthesettingforanumberofimportantfossildiscoveries.IfiguredthatifIhitthosesamerockswithmyhandlensandmywonderfullysuccessfulearlymammalsearchimage,I’dfindlotsofgoodies.Irentedaminivan,grabbedacaseofcollectingbags,andsetoff.

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Yetanotherlessonlearned:Ifoundnothing.Backtothedrawingboard,ormoreprecisely,thegeologylibraryatschool.Ineededaplacewhere200-million-year-oldrockswere

wellexposed:inConnecticuttherewereonlyroadcuts.Theidealplacewouldbealongthecoast,wherewaveactionwouldprovidelotsoffreshlybrokenrocksurfacetolookat.Lookingatamapmademychoiceclear:upinNovaScotia,TriassicandJurassicrocks(roughly200millionyearsold)layalongthesurface.Totopitoff,thetouristliteratureabouttheareaadvertisedtheworld’shighesttides,occasionallyoverfiftyfeet.Icouldn’tbelievemyluck.Icalledtheexpertontheserocks,PaulOlsen,whohad

juststartedteachingatColumbiaUniversity.IfIwasexcitedaboutfossil-findingprospectsbeforeItalkedtoPaul,Iwasfrothingafterward.Hedescribedtheperfectgeologyforfindingsmallmammalsorreptiles:ancientstreamsanddunesthathadjusttherightpropertiestopreservetinybones.Evenbetter,hehadalreadyfoundsomedinosaurbonesandfootprintsalongastretchofbeachnearthetownofParrsboro,NovaScotia.PaulandIhatchedaplantovisitParrsborotogetherandscanthebeachforlittlefossils.ThiswaswonderfullygenerousonPaul’spartbecausehehaddibsontheareaandwasundernoresponsibilitytohelpmeout,letalonecollaborate.IconsultedwithFarishonmyemergingplans,andhenot

onlyofferedmoneybutsuggestedthatItakethefossil-findingexperts,BillandChuck.Money,Bill,Chuck,Paul

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Olsen,excellentrocks,anddecentexposures—whatmorecouldyouwant?Thefollowingsummer,Iledmyveryfirstfossilexpedition.OffIwentinarentedstationwagontothebeachesof

NovaScotiawithmyfieldcrew,BillandChuck.Thejoke,ofcourse,wasonme.WithBillandChuckalong,whobetweenthemhadmoreyearsoffieldexperiencethanIhadbirthdays,Iwastheleaderinnameonly.Theycalledthefossil-findingshots,whileIpaidthedinnerbills.TherocksinNovaScotiawereexposedinabsolutely

gorgeousorangesandstonecliffsalongtheBayofFundy.Thetideswouldgoinandoutabouthalfamileeachday,exposingenormousflatsoforangebedrock.Itwasn’tlongbeforewestartedtofindbonesinmanydifferentareas.Smallwhiteflecksofbonewerecomingoutalongthecliffs.Paulwasfindingfootprintseverywhere,evenintheflatsopenedbythemovingtideseachday.

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PaulOlsenfindingfootprintsinthetidalflatsofNovaScotia.Athightide,thewaterwouldcomeallthewaytothecliffsatleft.Thearrowheadpointstoaspotwhere,ifwetimedourtripwrong,wewouldbestuckonthecliffsforhoursatatime.Photographbytheauthor.

Chuck,Bill,Paul,andIspenttwoweeksdigginginNova

Scotia,findingbits,flakes,andfragmentsofbonesstickingoutoftherocks.Bill,beingthefossilpreparatorofthegroup,continuallywarnedmenottoexposemuchofthebonesinthefieldbutrathertowrapthemupstillcoveredinsandstonesothathecouldtracethebonesinthelaboratoryunderamicroscopeinmorecontrolledconditions.Wedidthis,butI’lladmittobeingdisappointedwithwhatwebroughthome:justafewshoeboxesofrocks,withsmallchipsandflakesofbonesshowing.Aswedrovehome,Irecallthinkingthateventhoughwehadn’tfoundmuch,ithadbeenagreatexperience.ThenItookaweek’svacation;ChuckandBillreturnedtothelab.WhenIreturnedtoBoston,ChuckandBillwereoutto

lunch.Somecolleagueswerevisitingthemuseumand,havingcaughtsightofme,cameuptoshakemyhand,offercongratulations,andslapmeontheback.Iwasbeingtreatedlikeaconqueringhero,butIhadnoideawhy;itseemedlikeabizarrejoke,asiftheyweresettingmeupforsomebigcon.TheytoldmetogotoBill’slabtoseemytrophy.Notknowingwhattothink,Iran.UnderBill’smicroscopewasatinyjaw,notmorethan

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halfaninchlong.Initwereafewminuteteeth.Thejaw’sownerwasclearlyareptile,becausetheteethhadonlyasinglerootatthebase,whereasmammalteethhavemany.ButontheteethweretinybumpsandridgesthatIcouldseeevenwiththenakedeye.Lookingattheteethunderthemicroscopegavemethebiggestsurprise:thecuspshadlittlepatchesofwear.Thiswasareptilewithtooth-to-toothocclusion.Myfossilwaspartmammal,partreptile.Unbeknownsttome,Billhadunwrappedoneofour

blocksofrock,seenafleckofbone,andprepareditwithaneedleunderthemicroscope.Noneofushadknownitinthefield,butourexpeditionwasahugesuccess.AllbecauseofBill.WhatdidIlearnthatsummer?First,Ilearnedtolistento

ChuckandBill.Second,Ilearnedthatmanyofthebiggestdiscoverieshappeninthehandsoffossilpreparators,notinthefield.Asitturnedout,mybiggestlessonsaboutfieldworkwereyettocome.ThereptileBillhadfoundwasatritheledont,acreature

knownfromSouthAfricaaswellasnowfromNovaScotia.Thesewereveryrare,sowewantedtoreturntoNovaScotiathenextsummertofindmore.Ispentthewholewintertensewithanticipation.IfIcouldhavechippedthroughthewintericetofindfossils,Iwouldhavedoneit.Inthesummerof1985,wereturnedtothesitewherewe

hadfoundthetritheledont.Thefossilbedwasjustatbeachlevel,wherealittlepieceofthecliffhadfallenoffseveralyearsbefore.Wehadtotimeourdailyvisitjustso:thesite

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wasinaccessibleathightidebecausethewatercameuptoohigharoundapointwehadtonavigate.I’llneverforgetthatfirstdayofexcitementwhenweroundedthepointtofindourlittlepatchofbrightorangerock.Theexperiencewasmemorableforwhatwasmissing:mostoftheareawehadworkedtheyearbefore.Ithadweatheredawaythepreviouswinter.Ourlovelyfossilsite,containingbeautifultritheledonts,wasgonewiththetides.Thegoodnews,ifyoucouldcallitthat,wasthatthere

wasalittlemoreorangesandstonetoscanalongthebeach.Mostofthebeach,inparticularthepointwehadtogoaroundeachmorning,wasmadeupofbasaltfroma200-million-year-oldlavaflow.Wewerepositivenofossilscouldbefoundthere,foritisvirtuallyaxiomaticthattheserocks,whichwereoncesuperhot,wouldneverpreservefossilbone.Wespentfiveormoredaystimingourvisitstothesitesbythetides,pawingawayattheorangesandstonesbeyondit,andfindingabsolutelynothing.Ourbreakthroughcamewhenthepresidentofthelocal

LionsClubcamebyourcabinonenightlookingforjudgesforthelocalbeautycontest,tocrownParrsboro’sMissOldHomeWeek.Thetownalwaysreliedonvisitorsforthisoneroustask,becauseinternecinepassionstypicallyrunhighduringtheevent.Theusualjudges,anelderlycouplefromQuebec,werenotvisitingthisyear,andthecrewandIwereinvitedtosubstitute.Butinjudgingthebeautycontestandarguingoverits

conclusion,westayedupwaytoolate,forgotaboutthenext

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morning’stides,andendeduptrappedaroundabendinthebasaltcliffs.Forabouttwohours,wewerestuckonalittlepromontoryaboutfiftyfeetwide.Therockwasvolcanicandnotthetypeonewouldeverchoosetosearchforfossils.Weskippedstonesuntilwegotbored,thenwelookedattherocks:maybewe’dfindinterestingcrystalsorminerals.Billdisappearedaroundacorner,andIlookedatsomeofthebasaltbehindus.AfteraboutfifteenminutesIheardmyname.I’llneverforgetBill’sunderstatedtone:“Uh,Neil,youmightwanttocomeoverhere.”AsIroundedthecorner,IsawtheexcitementinBill’seyes.ThenIsawtherocksathisfeet.Stickingoutoftherocksweresmallwhitefragments.Fossilbones,thousandsofthem.Thiswasexactlywhatwewerelookingfor,asitewith

smallbones.Itturnedoutthatthevolcanicrockswerenotentirelyvolcanic:sliversofsandstonecutthroughthecliff.Therockshadbeenproducedbyanancientmudflowassociatedwithavolcaniceruption.Thefossilswerestuckintheancientmuds.Webroughttonsoftheserockshome.Insideweremore

tritheledonts,someprimitivecrocodiles,andotherlizard-likereptiles.Thetritheledontswerethegems,ofcourse,becausetheyshowedthatsomekindsofreptilesalreadydisplayedourmammaliankindofchewing.Earlymammals,suchasthoseFarish’steamuncovered

inArizona,hadveryprecisepatternsofbiting.Scrapesonthecuspsofanuppertoothfitagainstmirrorimagesofthesescrapesonalowertooth.Thesepatternsofwearare

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sofinethatdifferentspeciesofearlymammalscanbedistinguishedbytheirpatternsoftoothwearandocclusion.Farish’sArizonamammalshaveadifferentpatternofcuspsandchewingthanthoseofthesameagefromSouthAmerica,Europe,orChina.Ifallwehadtocomparethesefossilstowerelivingreptiles,thentheoriginofmammalianfeedingwouldappeartobeabigmystery.AsI’vementioned,crocodilesandlizardsdonothaveanykindofmatchingpatternofocclusion.Hereiswherecreaturesliketritheledontscomein.Whenwegobackintime,torocksabout10millionyearsolder,suchasthoseinNovaScotia,wefindtritheledontswithanincipientversionofthiswayofchewing.Intritheledonts,individualcuspsdonotinterlockinapreciseway,astheydoinmammals;instead,theentireinnersurfaceoftheuppertoothshearsagainsttheoutersurfaceofthelowertooth,almostlikeascissors.Ofcourse,thesechangesinocclusiondidnothappeninavacuum.Itshouldcomeasnosurprisethattheearliestcreaturestoshowamammaliankindofchewingalsodisplaymammalianfeaturesofthelowerjaw,skull,andskeleton.

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AtritheledontandapieceofitsupperjawdiscoveredinNovaScotia.JawfragmentillustratedbyLazloMeszoley.

Becauseteethpreservesowellinthefossilrecord,we

haveverydetailedinformationabouthowmajorpatternsofchewing—andtheabilitytousenewdiets—aroseovertime.Muchofthestoryofmammalsisthestoryofnewwaysofprocessingfood.Soonafterweencountertritheledontsinthefossilrecord,westartseeingallsortsofnewmammalspecieswithnewkindsofteeth,aswellasnewwaysofoccludingandusingthem.Byabout150millionyearsago,inrocksfromaroundtheworld,wefindsmallrodent-sizemammalswithanewkindoftoothrow,onethatpavedthewayforourownexistence.Whatmadethesecreaturesspecialwasthecomplexityoftheirmouths:thejawhaddifferentkindsofteethsetinit.Themouthdevelopedakindofdivisionoflabor.Incisorsinthefrontbecamespecializedtocutfood,caninesfurtherbacktopunctureit,andmolarsintheextremebacktoshearormashit.Theselittlemammals,whichresemblemice,haveafundamentalpieceofourhistoryinsideofthem.Ifyoudoubtthis,imagineeatinganapplelackingyourincisorteethor,betteryet,alargecarrotwithnomolars.Ourdiversediet,rangingfromfruittomeattoTwinkie,ispossibleonlybecauseourdistantmammalianancestorsdevelopedamouthwithdifferentkindsofteeththatcanoccludeprecisely.Andyes,initialstagesofthisareseenintritheledontsandotherancientrelatives:theteethinthe

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fronthaveadifferentpatternofbladesandcuspsthanthoseintheback.

TEETHANDBONES—THEHARDSTUFF

Italmostgoeswithoutsayingthatwhatmakesteethspecialamongorgansistheirhardness.Teethhavetobeharderthanthebitsoffoodtheybreakdown;imaginetryingtocutasteakwithasponge.Inmanyways,teethareashardasrocks,andthereasonisthattheycontainacrystalmoleculeontheinside.Thatmolecule,knownashydroxyapatite,impregnatesthemolecularandcellularinfrastructureofbothteethandbones,makingthemresistanttobending,compression,andotherstresses.Teethareextrahardbecausetheirouterlayer,enamel,isfarricherinhydroxyapatitethananyotherstructureinthebody,includingbone.Enamelgivesteeththeirwhitesheen.Ofcourse,enamelisonlyoneofthelayersthatmakeupourteeth.Theinnerlayers,suchasthepulpanddentine,arealsofilledwithhydroxyapatite.Therearelotsofcreatureswithhardtissues—clamsand

lobsters,forexample.Buttheydonotusehydroxyapatite;lobstersandclamsuseothermaterials,suchascalciumcarbonateorchitin.Also,unlikeus,theseanimalshaveanexoskeletoncoveringthebody.Ourhardnesslieswithin.Ourparticularbrandofhardness,withteethinsideour

mouthsandbonesinsideourbodies,isanessentialpartof

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whoweare.Wecaneat,moveabout,breathe,evenmetabolizecertainmineralsbecauseofourhydroxyapatite-containingtissues.Forthesecapabilities,wecanthankthecommonancestorwesharewithallfish.Everyfish,amphibian,reptile,bird,andmammalontheplanetislikeus.Allofthemhavehydroxyapatite-containingstructures.Butwheredidthisallcomefrom?Thereisanimportantintellectualissueatstakehere.By

knowingwhere,when,andhowhardbonesandteethcameabout,wewillbeinapositiontounderstandwhy.Whydidourkindofhardtissuesarise?Didtheycomeabouttoprotectanimalsfromtheirenvironment?Didtheycomeabouttohelpthemmove?Answerstothesequestionslieinthefossilrecord,inrocksapproximately500millionyearsold.Someofthemostcommonfossilsinancientoceans,500

millionto250millionyearsold,areconodonts.Conodontswerediscoveredinthe1830sbytheRussianbiologistChristianPander,whowillreappearinafewchapters.Theyaresmallshellyorganismswithaseriesofspikesprojectingoutofthem.SincePander’stime,conodontshavebeendiscoveredoneverycontinent;thereareplaceswhereyoucannotcrackarockwithoutfindingvastnumbersofthem.Hundredsofkindsofconodontsareknown.Foralongtime,conodontswereenigmas:scientists

disagreedoverwhethertheywereanimal,vegetable,ormineral.Everybodyseemedtohaveapettheory.Conodontswereclaimedtobepiecesofclams,sponges,

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vertebrates,evenworms.Thespeculationendedwhenwholeanimalsstartedtoshowupinthefossilrecord.Thefirstspecimenthatmadesenseofeverythingwas

foundbyaprofessorofpaleontologyrummagingthroughthebasementattheUniversityofEdinburgh:therewasaslabofrockwithwhatlookedlikealampreyinit.Youmightrecalllampreysfrombiologyclass—theseareveryprimitivefishthathavenojaws.Theymaketheirlivingbyattachingtootherfishandfeedingontheirbodilyfluids.Embeddedinthefrontofthelampreyimpressionweresmallfossilsthatlookedstrangelyfamiliar.Conodonts.Otherlamprey-likefossilsstartedtocomeoutofrocksinSouthAfricaandlaterthewesternUnitedStates.Thesecreaturesallhadanexceptionaltrait:theyhadwholeassemblagesofconodontsintheirmouths.Theconclusionbecameabundantlyclear:conodontswereteeth.Andnotjustanyteeth.Conodontsweretheteethofanancientjawlessfish.Wehadtheearliestteethinthefossilrecordforover150

yearsbeforewerealizedwhattheywere.Thereasoncomesdowntohowfossilsarepreserved.Thehardbits,forexampleteeth,tendtogetpreservedeasily.Softparts,suchasmuscle,skin,andguts,usuallydecaywithoutfossilizing.Wehavemuseumcabinetsfulloffossilskeletons,shells,andteeth,butpreciousfewgutsandbrains.Ontherareoccasionswhenwefindevidenceofsofttissues,theyaretypicallypreservedonlyasimpressionsorcasts.Ourfossilrecordisloadedwithconodontteeth,butittookus150

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yearstofindthebodies.Thereissomethingelseremarkableaboutthebodiestowhichconodontsbelonged.Theyhavenohardbones.Theseweresoft-bodiedanimalswithhardteeth.Foryears,paleontologistshavearguedaboutwhyhard

skeletons,thosecontaininghydroxyapatite,aroseinthefirstplace.Forthosewhobelievedthatskeletonsbeganwithjaws,backbones,orbodyarmor,conodontsprovidean“inconvenienttooth,”ifyouwill.Thefirsthardhydroxyapatite-containingbodypartswereteeth.Hardbonesarosenottoprotectanimals,buttoeatthem.Withthis,thefish-eat-fishworldreallybeganinearnest.First,bigfishatelittlefish;then,anarmsracebegan.Littlefishdevelopedarmor,bigfishobtainedbiggerjawstocrackthearmor,andsoon.Teethandbonesreallychangedthecompetitivelandscape.Thingsgetmoreinterestingstillaswelookatsomeof

thefirstanimalswithbonyheads.Aswemoveupintimefromtheearliestconodontanimals,weseewhatthefirstbony-headskeletonslookedlike.Theybelongedtofishcalledostracoderms,areabout500millionyearsold,andarefoundinrocksallovertheworld,fromtheArctictoBolivia.Thesefishlooklikehamburgerswithfleshytails.Theheadregionofanostracodermisabigdiskcovered

byashieldofbone,lookingalmostlikearmor.IfIweretoopenamuseumdrawerandshowyouone,youwouldimmediatelynoticesomethingodd:theheadskeletonisreallyshiny,muchlikeourteethorthescalesofafish.

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Aconodont(left)andanostracoderm(right).Conodontswereoriginallyfoundisolated.Then,aswholeanimalsbecameknown,welearnedthatmanyofthemfunctionedtogetherasatoothrowinthemouthsofthesesoft-bodiedjawlessfish.Ostracodermshaveheadscoveredwithabonyshield.Themicroscopiclayersofthatshieldlookliketheyarecomposedoflittletooth-likestructures.ConodonttoothrowreconstructioncourtesyofDr.MarkPurnell,UniversityofLeicester,andDr.PhilipDonoghue,UniversityofBristol.

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Oneofthejoysofbeingascientististhatthenaturalworldhasthepowertoamazeandsurprise.Here,inostracoderms,anobscuregroupofancientjawlessfish,liesaprimeexample.Ostracodermsareamongtheearliestcreatureswithbonyheads.Cuttheboneoftheskullopen,embeditinplastic,popitunderthemicroscope,andyoudonotfindjustanyoldtissuestructure;rather,youfindvirtuallythesamestructureasinourteeth.Thereisalayerofenamelandevenalayerofpulp.Thewholeshieldismadeupofthousandsofsmallteethfusedtogether.Thisbonyskull—oneoftheearliestinthefossilrecord—ismadeentirelyoflittleteeth.Teethoriginallyarosetobitecreatures;later,aversionofteethwasusedinanewwaytoprotectthem.

TEETH,GLANDS,ANDFEATHERS

Teethnotonlyheraldawholenewwayofliving,theyrevealtheoriginofawholenewwayofmakingorgans.Teethdevelopbyaninteractionoftwolayersoftissueinourdevelopingskin.Basically,twolayersapproacheachother,cellsdivide,andthelayerschangeshapeandmakeproteins.Theouterlayerspitsoutthemolecularprecursorsofenamel,theinnerlayerthedentineandpulpoftheinsideofthetooth.Overtime,thestructureofthetoothislaiddown,thentweakedtomakethepatternsofcuspsandtroughsthatdistinguisheachspecies.

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Thekeytotoothdevelopmentisthataninteractionbetweenthesetwolayersoftissue,anoutersheetofcellsandaninnerlooselayerofcells,causesthetissuetofoldandmakesbothlayerssecretethemoleculesthatbuildtheorgan.Itturnsoutthatexactlythesameprocessunderliesthedevelopmentofallthestructuresthatdevelopwithinskin:scales,hair,feathers,sweatglands,evenmammaryglands.Ineachcase,twolayerscometogether,fold,andsecreteproteins.Indeed,thebatteriesofthemajorgeneticswitchesthatareactiveinthisprocessineachkindoftissuearelargelysimilar.

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Teeth,breasts,feathers,andhairalldevelopfromtheinteractionsbetweenlayersofskin.

Thisexampleisakintomakinganewfactoryorassembly

process.Onceplasticinjectionwasinvented,itwasusedinmakingeverythingfromcarpartstoyo-yos.Teetharenodifferent.Oncetheprocessthatmakesteethcameintobeing,itwasmodifiedtomakethediversekindsoforgans

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thatliewithinskin.Wesawthistakentoaverygreatextremeintheostracoderms.Birds,reptiles,andhumansarejustasextremeinmanyways.Wewouldneverhavescales,feathers,orbreastsifwedidn’thaveteethinthefirstplace.Thedevelopmentaltoolsthatmaketeethhavebeenrepurposedtomakeotherimportantskinstructures.Inaveryrealsenseorgansasdifferentasteeth,feathers,andbreastsareinextricablylinkedbyhistory.Athemeofthesefirstfourchaptersishowwecantrace

thesameorganindifferentcreatures.InChapter1wesawthatwecanmakepredictionsandfindversionsofourorgansinancientrocks.InChapter2wesawhowwecantracesimilarbonesallthewayfromfishtohumans.Chapter3showshowtherealheritablepartofourbodies—theDNAandgeneticrecipethatbuildsorgans—canbefollowedinverydifferentcreatures.Here,inteeth,mammaryglands,andfeathers,wefindasimilartheme.Thebiologicalprocessesthatmakethesedifferentorgansareversionsofthesamething.Whenyouseethesedeepsimilaritiesamongdifferentorgansandbodies,youbegintorecognizethatthediverseinhabitantsofourworldarejustvariationsonatheme.

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