CHAPTERFOUR
TEETHEVERYWHERE
Thetoothgetsshortshriftinanatomyclass:wespendall
offiveminutesonit.Inthepantheonoffavoriteorgans—I’ll
leaveittoeachofyoutomakeyourlist—teethrarelyreach
thetopfive.Yetthelittletoothcontainssomuchofour
connectiontotherestoflifethatitisvirtuallyimpossibleto
understandourbodieswithoutknowingteeth.Teethalso
havespecialsignificanceforme,becauseitwasin
searchingforthemthatIfirstlearnedhowtofindfossils
andhowtorunafossilexpedition.
Thejobofteethistomakebiggercreaturesintosmaller
pieces.Whenattachedtoamovingjaw,teethslice,dice,and
macerate.Mouthsareonlysobig,andteethenable
creaturestoeatthingsthatarebiggerthantheirmouths.
Thisisparticularlytrueofcreaturesthatdonothavehands
orclawsthatcanshredorcutthingsbeforetheygettothe
mouth.True,bigfishtendtoeatlittlerfish.Butteethcanbe
thegreatequalizer:smallerfishcanmunchonbiggerfishif
theyhavegoodteeth.Smallerfishcanusetheirteethto
scrapescales,feedonparticles,ortakeoutwholechunksof
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fleshfrombiggerfish.
Wecanlearnalotaboutananimalbylookingatitsteeth.
Thebumps,pits,andridgesonteethoftenreflectthediet.
Carnivores,suchascats,haveblade-likemolarstocutmeat,
whileplanteatershaveamouthfullofflatterteeththatcan
macerateleavesandnuts.Theinformationalvalueofteeth
wasnotlostontheanatomistsofhistory.TheFrench
anatomistGeorgesCuvieroncefamouslyboastedthathe
couldreconstructananimal’sentireskeletonfromasingle
tooth.Thisisalittleoverthetop,butthegeneralpointis
valid;teethareapowerfulwindowintoananimal’slifestyle.
Humanmouthsrevealthatweareall-purposeeaters,for
wehaveseveralkindsofteeth.Ourfrontteeth,theincisors,
areflatbladesspecializedforcutting.Therearmostteeth,
themolars,areflatter,withadistinctivepatternthatcan
macerateplantoranimaltissue.Thepremolars,inbetween,
areintermediateinfunctionbetweenincisorsandmolars.
Themostremarkablethingaboutourmouthsisthe
precisionwithwhichwechew.Openandcloseyourmouth:
yourteethalwayscometogetherinthesameposition,with
upperandlowerteethfittingtogetherprecisely.Because
theupperandlowercusps,basins,andridgesmatchclosely,
weareabletobreakupfoodwithmaximalefficiency.In
fact,amismatchbetweenupperandlowerteethcanshatter
ourteeth,andenrichourdentists.
Paleontologistsfindteethwonderfullyinformative.Teeth
arethehardestpartsofourbodies,becausetheenamel
includesahighproportionofthemineralhydroxyapatite—
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highereventhanisfoundinbones.Thankstotheir
hardness,teethareoftenthebest-preservedanimalpartwe
findinthefossilrecordformanytimeperiods.Thisislucky;
sinceteetharesuchagreatcluetoananimal’sdiet,the
fossilrecordcangiveusagoodwindowonhowdifferent
waysoffeedingcameabout.Thisisparticularlytrueof
mammalhistory:whereasmanyreptileshavesimilarteeth,
thoseofmammalsaredistinctive.Themammalsectionofa
typicalpaleontologycoursefeelsalmostlikeDentistry101.
Livingreptiles—crocodiles,lizards,snakes—lackmuch
ofwhatmakesmammalianmouthsunique.Acrocodile’s
teeth,forexample,allhaveasimilarblade-likeshape;the
onlydifferencebetweenthemisthatsomearebigand
otherssmall.Reptilesalsolackthepreciseocclusion—the
fitbetweenupperandlowerteeth—thathumansandother
mammalshave.Also,whereaswemammalsreplaceour
teethonlyonce,reptilestypicallyreceivevisitsfromthe
toothfairyfortheirentirelives,replacingtheirteeth
continuallyastheywearandbreakdown.
Averybasicpieceofus—ourmammalianwayofprecise
chewing—emergesinthefossilrecordfromaroundthe
worldthatrangesfrom225millionto195millionyears
ago.Atthebase,intheolderrocks,wefindanumberof
reptilesthatlooksuperficiallydog-like.Walkingonfour
legs,theyhavebigskulls,andmanyofthemhavesharp
teeth.Theretheresemblancestops.Unlikedogs,these
reptileshaveajawmadeupofmanybones,andtheirteeth
don’treallyfitwelltogether.Also,theirteetharereplacedin
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adecidedlyreptilianway:newteethpopinandout
throughouttheanimals’lives.
Gohigherintherocksandweseesomethingutterly
different:theappearanceofmammalness.Thebonesofthe
jawgetsmallerandmovetotheear.Wecanseethefirst
evidenceofupperandlowerteethcomingtogetherin
preciseways.Thejaw’sshapechanges,too:whatwasa
simplerodinreptileslooksmorelikeaboomerangin
mammals.Atthistime,too,teetharereplacedonlyonceper
lifetime,asinus.Wecantraceallthesechangesinthefossil
record,especiallyfromcertainsitesinEurope,SouthAfrica,
andChina.
Therocksofabout200millionyearsagocontainrodentlikecreatures,suchasMorganucodonandEozostrodon,that
havebeguntolooklikemammals.Theseanimals,nobigger
thanamouse,holdimportantpiecesofusinside.Pictures
cannotconveyjusthowwonderfultheseearlymammals
are.Forme,itwasarealthrilltoseecreatureslikethemfor
thefirsttime.
WhenIenteredgraduateschool,Iwantedtostudyearly
mammals.IchoseHarvardbecauseFarishA.Jenkins,Jr.,
whomwemetinthefirstchapter,wasleadingexpeditions
totheAmericanWestthatsystematicallyscouredtherocks
forsignsofhowmammalsdevelopedtheirdistinctabilities
tochew.Theworkwasrealexploration;Farishandhis
teamwerelookingfornewlocalitiesandsites,not
returningtoplacesotherpeoplehaddiscovered.Farishhad
assembledatalentedgroupoffossilfinderscomprising
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stafffromHarvard’sMuseumofComparativeZoologyanda
fewfree-lancemercenaries.ChiefamongthemwereBill
Amaral,ChuckSchaff,andthelateWillDowns.Thesepeople
weremyintroductiontotheworldofpaleontology.
Farishandtheteamhadstudiedgeologicalmapsand
aerialphotostochoosepromisingareaswheretheymight
findearlymammals.Then,eachsummer,theygotintheir
trucksandheadedoffintothedesertsofWyoming,Arizona,
andUtah.BythetimeIjoinedthem,in1983,theyhad
alreadyfoundanumberofimportantnewmammalsand
fossilsites.Iwasstruckbythepowerofpredictions:simply
byreadingscientificarticlesandbooks,Farish’steamcould
identifylikelyandunlikelyplacestofindearlymammals.
Mybaptisminfieldpaleontologycamefromwalkingout
intheArizonadesertwithChuckandBill.Atfirst,thewhole
enterpriseseemedutterlyrandom.Iexpectedsomething
akintoamilitarycampaign,anorganizedandcoordinated
reconnaissanceofthearea.WhatIsawlookedlikethe
extremeopposite.Theteamwouldplunkdownona
particularpatchofrock,andpeoplewouldscatterinevery
conceivabledirectiontolookforfragmentsofboneonthe
surface.Forthefirstfewweeksoftheexpedition,theyleft
mealone.I’dsetofflookingforfossils,systematically
inspectingeveryrockIsawforascrapofboneatthe
surface.Attheendofeachdaywewouldcomehometo
showoffthegoodieswefound.Chuckwouldhaveseveral
bagsofbones.Billwouldhavehiscomplement,usuallywith
somesortoflittleskullorotherprize.AndIhadnothing,my
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emptybagasadreminderofhowmuchIhadtolearn.
Afterafewweeksofthis,Idecideditwouldbeagood
ideatowalkwithChuck.Heseemedtohavethefullestbags
eachday,sowhynottakesomecuesfromtheexpert?
Chuckwashappytowalkwithmeandexpoundonhislong
careerinfieldpaleontology.ChuckisallWestTexaswitha
Brooklynflourish:cowboybootsandwesternvalueswitha
NewYorkaccent.Whileheregaledmewithtalesofhispast
expeditions,Ifoundthewholeexperienceutterlyhumbling.
First,Chuckdidnotlookateveryrock,andwhenhechose
onetolookat,forthelifeofmeIcouldn’tfigureoutwhy.
Thentherewasthereallyembarrassingaspectofallthis:
ChuckandIwouldlookatthesamepatchofground.Isaw
nothingbutrock—barrendesertfloor.Chucksawfossil
teeth,jaws,andevenchunksofskull.
Anaerialviewwouldhaveshowntwopeoplewalking
aloneinthemiddleofaseeminglylimitlessplain,wherethe
vistaofdustyredandgreensandstonemesas,buttes,and
badlandsextendedformiles.ButChuckandIwerestaring
onlyattheground,attherubbleandtalusofthedesert
floor.Thefossilswesoughtweretiny,nomorethanafew
incheslong,andourswasaverysmallworld.Thisintimate
environmentstoodinextremecontrasttothevastnessof
thedesertpanoramathatsurroundedus.Ifeltasifmy
walkingpartnerwastheonlypersonontheentireplanet,
andmywholeexistencewasfocusedonpiecesofrubble.
ChuckwasextraordinarilypatientwithmeasIpestered
himwithquestionsforthebetterpartofeachday’swalk.I
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wantedhimtodescribeexactlyhowtofindbones.Overand
over,hetoldmetolookfor“somethingdifferent,”
somethingthathadthetextureofbonenotrock,something
thatglistenedliketeeth,somethingthatlookedlikeanarm
bone,notapieceofsandstone.Itsoundedeasy,butI
couldn’tgraspwhathewastellingme.TryasImight,Istill
returnedhomeeachdayempty-handed.Nowitwaseven
moreembarrassing,asChuck,whowaslookingatthesame
rocks,camehomewithbagafterbag.
Finally,oneday,Isawmyfirstpieceoftoothglisteningin
thedesertsun.Itwassittinginsomesandstonerubble,but
thereitwas,asplainasday.Theenamelhadasheenthatno
otherrockhad;itwaslikenothingIhadseenbefore.Well,
notexactly—Iwaslookingatthingslikeiteveryday.The
differencewasthistimeIfinallysawit,sawthedistinction
betweenrockandbone.Thetoothglistened,andwhenI
sawitglistenIspotteditscusps.Thewholeisolatedtooth
wasaboutthesizeofadime,notincludingtherootsthat
projectedfromitsbase.Tome,itwasasgloriousasthe
biggestdinosaurinthehallsofanymuseum.
Allofasudden,thedesertfloorexplodedwithbone;
whereonceIhadseenonlyrock,nowIwasseeinglittlebits
andpiecesoffossileverywhere,asifIwerewearinga
specialnewpairofglassesandaspotlightwasshiningon
allthedifferentpiecesofbone.Nexttothetoothweresmall
fragmentsofotherbones,thenmoreteeth.Iwaslookingat
ajawthathadweatheredoutonthesurfaceand
fragmented.Istartedtoreturnhomewithmyownlittle
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bagseachnight.
NowthatIcouldfinallyseebonesformyself,whatonce
seemedahaphazardgroupeffortstartedtolookdecidedly
ordered.Peopleweren’tjustscatteringrandomlyacrossthe
desert;therewererealthoughunspokenrules.Rulenumber
one:gotothemostproductive-lookingrocks,judgingby
whateversearchimageorvisualcuesyou’vegainedfrom
previousexperience.Rulenumbertwo:don’tfollowin
anybody’sfootsteps;covernewground(Chuckhad
graciouslyletmebreakthisone).Rulethree:ifyourplum
areaalreadyhassomebodyonit,findanewplum,orsearch
alesspromisingsite.Firstcome,firstserved.
Overtime,Ibegantolearnthevisualcuesforotherkinds
ofbones:longbones,jawbones,andskullparts.Onceyou
seethesethingsyouneverlosetheabilitytofindthem.Just
asagreatfishermancanreadthewaterandseethefish
within,soafossilfinderusesacatalogueofsearchimages
thatmakefossilsseemtojumpoutfromtherocks.Iwas
beginningtogainmyownvisualimpressionsofwhatfossil
boneslooklikeindifferentrocksandindifferentlighting
conditions.Findingfossilsinthemorningsunisvery
differentfromfindingthemintheafternoon,becauseofthe
waythelightplaysalongtheground.
Twentyyearslater,IknowthatImustgothrougha
similarexperienceeverytimeIlookforfossilssomeplace
new,fromtheTriassicofMoroccototheDevonianof
EllesmereIsland.I’llstruggleforthefirstfewdays,almost
asIdidthosedayswithChuckinArizonatwentyyearsago.
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ThedifferenceisthatnowIhavesomeconfidencethata
searchimagewillkickineventually.
ThewholegoaloftheprospectingIdidwithChuckwasto
findasitewithenoughbonestomarkafossil-richlayer
thatwecouldexpose.BythetimeIjoinedthecrew,Farish’s
teamhadalreadydiscoveredsuchazone,apatchofrock
aboutahundredfeetlongthatcontainedskeletonafter
skeletonofsmallanimals.
Farish’sfossilquarrywasinsomeveryfine-grained
mudstone.Thetricktoworkingonitwastorealizethatthe
fossilswerecomingfromonethinlayer,nomorethana
millimeterthick.Onceyouexposedthatsurface,youhada
verygoodchanceofseeingbones.Theyweretiny,nomore
thananinchortwolong,andblack,sotheylookedalmost
likeblacksmudgesagainstthebrownishrock.Thelittle
animalswefoundincludedfrogs(someoftheearliest),
leglessamphibians,lizardsandotherreptiles,and,
importantly,someoftheearliestmammals.
Thekeypointisthattheearlymammalsweresmall.Very
small.Theirteethwerenotmuchmorethan2millimeters
long.Tospotthem,youhadtobeverycarefuland,more
often,verylucky.Ifthetoothwascoveredbyacrumbof
rockorevenbyafewgrainsofsand,youmightneverseeit.
Itwasthesightoftheseearlymammalsthatreally
hookedme.I’dexposethefossillayer,thenscantheentire
surfacethroughmy10-powerhandlens.I’dscrutinizethe
wholethingonmyhandsandknees,withmyeyeandhand
lensonlyabouttwoinchesfromthesurfaceoftheground.
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Thusengrossed,I’doftenforgetwhereIwasand
accidentallytrespassonmyneighbor’sspotonlytohavea
bagofdirtdumpedonmyheadasasharpremindertokeep
tomyspace.Occasionally,though,I’dhitthejackpotandsee
adeepconnectionforthefirsttime.Theteethwouldlook
likelittleblades,withcuspsandroots.Thecuspsonthose
littleteethrevealedsomethingveryspecial.Eachtoothhad
acharacteristicpatternofwearatthefacewhereupperand
lowerteethfittogether.Iwasseeingsomeofthefirst
evidenceofourpatternofprecisechewing,onlyinatiny
mammal190millionyearsold.
ThepowerofthosemomentswassomethingI’llnever
forget.Here,crackingrocksinthedirt,Iwasdiscovering
objectsthatcouldchangethewaypeoplethink.That
juxtapositionbetweenthemostchild-like,evenhumbling,
activitiesandoneofthegreathumanintellectual
aspirationshasneverbeenlostonme.Itrytoremind
myselfofiteachtimeIdigsomewherenew.
Returningtoschoolthatfall,Idevelopedtheexpedition
bugbig-time.Iwantedtoleadmyownexpeditionbut
lackedtheresourcestodoanythingbig,soIsetoffto
explorerocksinConnecticutthatwereabout200million
yearsold.Wellstudiedduringthenineteenthcentury,they
hadbeenthesettingforanumberofimportantfossil
discoveries.IfiguredthatifIhitthosesamerockswithmy
handlensandmywonderfullysuccessfulearlymammal
searchimage,I’dfindlotsofgoodies.Irentedaminivan,
grabbedacaseofcollectingbags,andsetoff.
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Yetanotherlessonlearned:Ifoundnothing.Backtothe
drawingboard,ormoreprecisely,thegeologylibraryat
school.
Ineededaplacewhere200-million-year-oldrockswere
wellexposed:inConnecticuttherewereonlyroadcuts.The
idealplacewouldbealongthecoast,wherewaveaction
wouldprovidelotsoffreshlybrokenrocksurfacetolookat.
Lookingatamapmademychoiceclear:upinNovaScotia,
TriassicandJurassicrocks(roughly200millionyearsold)
layalongthesurface.Totopitoff,thetouristliterature
abouttheareaadvertisedtheworld’shighesttides,
occasionallyoverfiftyfeet.Icouldn’tbelievemyluck.
Icalledtheexpertontheserocks,PaulOlsen,whohad
juststartedteachingatColumbiaUniversity.IfIwasexcited
aboutfossil-findingprospectsbeforeItalkedtoPaul,Iwas
frothingafterward.Hedescribedtheperfectgeologyfor
findingsmallmammalsorreptiles:ancientstreamsand
dunesthathadjusttherightpropertiestopreservetiny
bones.Evenbetter,hehadalreadyfoundsomedinosaur
bonesandfootprintsalongastretchofbeachnearthetown
ofParrsboro,NovaScotia.PaulandIhatchedaplantovisit
Parrsborotogetherandscanthebeachforlittlefossils.This
waswonderfullygenerousonPaul’spartbecausehehad
dibsontheareaandwasundernoresponsibilitytohelpme
out,letalonecollaborate.
IconsultedwithFarishonmyemergingplans,andhenot
onlyofferedmoneybutsuggestedthatItakethefossilfindingexperts,BillandChuck.Money,Bill,Chuck,Paul
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Olsen,excellentrocks,anddecentexposures—whatmore
couldyouwant?Thefollowingsummer,Iledmyveryfirst
fossilexpedition.
OffIwentinarentedstationwagontothebeachesof
NovaScotiawithmyfieldcrew,BillandChuck.Thejoke,of
course,wasonme.WithBillandChuckalong,whobetween
themhadmoreyearsoffieldexperiencethanIhad
birthdays,Iwastheleaderinnameonly.Theycalledthe
fossil-findingshots,whileIpaidthedinnerbills.
TherocksinNovaScotiawereexposedinabsolutely
gorgeousorangesandstonecliffsalongtheBayofFundy.
Thetideswouldgoinandoutabouthalfamileeachday,
exposingenormousflatsoforangebedrock.Itwasn’tlong
beforewestartedtofindbonesinmanydifferentareas.
Smallwhiteflecksofbonewerecomingoutalongthecliffs.
Paulwasfindingfootprintseverywhere,evenintheflats
openedbythemovingtideseachday.
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PaulOlsenfindingfootprintsinthetidalflatsofNova
Scotia.Athightide,thewaterwouldcomealltheway
tothecliffsatleft.Thearrowheadpointstoaspot
where,ifwetimedourtripwrong,wewouldbestuck
onthecliffsforhoursatatime.Photographbythe
author.
Chuck,Bill,Paul,andIspenttwoweeksdigginginNova
Scotia,findingbits,flakes,andfragmentsofbonessticking
outoftherocks.Bill,beingthefossilpreparatorofthe
group,continuallywarnedmenottoexposemuchofthe
bonesinthefieldbutrathertowrapthemupstillcovered
insandstonesothathecouldtracethebonesinthe
laboratoryunderamicroscopeinmorecontrolled
conditions.Wedidthis,butI’lladmittobeingdisappointed
withwhatwebroughthome:justafewshoeboxesofrocks,
withsmallchipsandflakesofbonesshowing.Aswedrove
home,Irecallthinkingthateventhoughwehadn’tfound
much,ithadbeenagreatexperience.ThenItookaweek’s
vacation;ChuckandBillreturnedtothelab.
WhenIreturnedtoBoston,ChuckandBillwereoutto
lunch.Somecolleagueswerevisitingthemuseumand,
havingcaughtsightofme,cameuptoshakemyhand,offer
congratulations,andslapmeontheback.Iwasbeing
treatedlikeaconqueringhero,butIhadnoideawhy;it
seemedlikeabizarrejoke,asiftheyweresettingmeupfor
somebigcon.TheytoldmetogotoBill’slabtoseemy
trophy.Notknowingwhattothink,Iran.
UnderBill’smicroscopewasatinyjaw,notmorethan
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halfaninchlong.Initwereafewminuteteeth.Thejaw’s
ownerwasclearlyareptile,becausetheteethhadonlya
singlerootatthebase,whereasmammalteethhavemany.
ButontheteethweretinybumpsandridgesthatIcouldsee
evenwiththenakedeye.Lookingattheteethunderthe
microscopegavemethebiggestsurprise:thecuspshad
littlepatchesofwear.Thiswasareptilewithtooth-to-tooth
occlusion.Myfossilwaspartmammal,partreptile.
Unbeknownsttome,Billhadunwrappedoneofour
blocksofrock,seenafleckofbone,andprepareditwitha
needleunderthemicroscope.Noneofushadknownitin
thefield,butourexpeditionwasahugesuccess.Allbecause
ofBill.
WhatdidIlearnthatsummer?First,Ilearnedtolistento
ChuckandBill.Second,Ilearnedthatmanyofthebiggest
discoverieshappeninthehandsoffossilpreparators,notin
thefield.Asitturnedout,mybiggestlessonsabout
fieldworkwereyettocome.
ThereptileBillhadfoundwasatritheledont,acreature
knownfromSouthAfricaaswellasnowfromNovaScotia.
Thesewereveryrare,sowewantedtoreturntoNova
Scotiathenextsummertofindmore.Ispentthewhole
wintertensewithanticipation.IfIcouldhavechipped
throughthewintericetofindfossils,Iwouldhavedoneit.
Inthesummerof1985,wereturnedtothesitewherewe
hadfoundthetritheledont.Thefossilbedwasjustatbeach
level,wherealittlepieceofthecliffhadfallenoffseveral
yearsbefore.Wehadtotimeourdailyvisitjustso:thesite
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wasinaccessibleathightidebecausethewatercameup
toohigharoundapointwehadtonavigate.I’llneverforget
thatfirstdayofexcitementwhenweroundedthepointto
findourlittlepatchofbrightorangerock.Theexperience
wasmemorableforwhatwasmissing:mostoftheareawe
hadworkedtheyearbefore.Ithadweatheredawaythe
previouswinter.Ourlovelyfossilsite,containingbeautiful
tritheledonts,wasgonewiththetides.
Thegoodnews,ifyoucouldcallitthat,wasthatthere
wasalittlemoreorangesandstonetoscanalongthebeach.
Mostofthebeach,inparticularthepointwehadtogo
aroundeachmorning,wasmadeupofbasaltfroma200million-year-oldlavaflow.Wewerepositivenofossils
couldbefoundthere,foritisvirtuallyaxiomaticthatthese
rocks,whichwereoncesuperhot,wouldneverpreserve
fossilbone.Wespentfiveormoredaystimingourvisitsto
thesitesbythetides,pawingawayattheorange
sandstonesbeyondit,andfindingabsolutelynothing.
Ourbreakthroughcamewhenthepresidentofthelocal
LionsClubcamebyourcabinonenightlookingforjudges
forthelocalbeautycontest,tocrownParrsboro’sMissOld
HomeWeek.Thetownalwaysreliedonvisitorsforthis
oneroustask,becauseinternecinepassionstypicallyrun
highduringtheevent.Theusualjudges,anelderlycouple
fromQuebec,werenotvisitingthisyear,andthecrewandI
wereinvitedtosubstitute.
Butinjudgingthebeautycontestandarguingoverits
conclusion,westayedupwaytoolate,forgotaboutthenext
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morning’stides,andendeduptrappedaroundabendinthe
basaltcliffs.Forabouttwohours,wewerestuckonalittle
promontoryaboutfiftyfeetwide.Therockwasvolcanic
andnotthetypeonewouldeverchoosetosearchfor
fossils.Weskippedstonesuntilwegotbored,thenwe
lookedattherocks:maybewe’dfindinterestingcrystalsor
minerals.Billdisappearedaroundacorner,andIlookedat
someofthebasaltbehindus.AfteraboutfifteenminutesI
heardmyname.I’llneverforgetBill’sunderstatedtone:“Uh,
Neil,youmightwanttocomeoverhere.”AsIroundedthe
corner,IsawtheexcitementinBill’seyes.ThenIsawthe
rocksathisfeet.Stickingoutoftherocksweresmallwhite
fragments.Fossilbones,thousandsofthem.
Thiswasexactlywhatwewerelookingfor,asitewith
smallbones.Itturnedoutthatthevolcanicrockswerenot
entirelyvolcanic:sliversofsandstonecutthroughthecliff.
Therockshadbeenproducedbyanancientmudflow
associatedwithavolcaniceruption.Thefossilswerestuck
intheancientmuds.
Webroughttonsoftheserockshome.Insideweremore
tritheledonts,someprimitivecrocodiles,andotherlizardlikereptiles.Thetritheledontswerethegems,ofcourse,
becausetheyshowedthatsomekindsofreptilesalready
displayedourmammaliankindofchewing.
Earlymammals,suchasthoseFarish’steamuncovered
inArizona,hadveryprecisepatternsofbiting.Scrapeson
thecuspsofanuppertoothfitagainstmirrorimagesof
thesescrapesonalowertooth.Thesepatternsofwearare
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sofinethatdifferentspeciesofearlymammalscanbe
distinguishedbytheirpatternsoftoothwearandocclusion.
Farish’sArizonamammalshaveadifferentpatternofcusps
andchewingthanthoseofthesameagefromSouth
America,Europe,orChina.Ifallwehadtocomparethese
fossilstowerelivingreptiles,thentheoriginofmammalian
feedingwouldappeartobeabigmystery.AsI’ve
mentioned,crocodilesandlizardsdonothaveanykindof
matchingpatternofocclusion.Hereiswherecreatureslike
tritheledontscomein.Whenwegobackintime,torocks
about10millionyearsolder,suchasthoseinNovaScotia,
wefindtritheledontswithanincipientversionofthisway
ofchewing.Intritheledonts,individualcuspsdonot
interlockinapreciseway,astheydoinmammals;instead,
theentireinnersurfaceoftheuppertoothshearsagainst
theoutersurfaceofthelowertooth,almostlikeascissors.
Ofcourse,thesechangesinocclusiondidnothappenina
vacuum.Itshouldcomeasnosurprisethattheearliest
creaturestoshowamammaliankindofchewingalso
displaymammalianfeaturesofthelowerjaw,skull,and
skeleton.
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Atritheledontandapieceofitsupperjawdiscovered
inNovaScotia.JawfragmentillustratedbyLazlo
Meszoley.
Becauseteethpreservesowellinthefossilrecord,we
haveverydetailedinformationabouthowmajorpatterns
ofchewing—andtheabilitytousenewdiets—aroseover
time.Muchofthestoryofmammalsisthestoryofnew
waysofprocessingfood.Soonafterweencounter
tritheledontsinthefossilrecord,westartseeingallsortsof
newmammalspecieswithnewkindsofteeth,aswellas
newwaysofoccludingandusingthem.Byabout150
millionyearsago,inrocksfromaroundtheworld,wefind
smallrodent-sizemammalswithanewkindoftoothrow,
onethatpavedthewayforourownexistence.Whatmade
thesecreaturesspecialwasthecomplexityoftheirmouths:
thejawhaddifferentkindsofteethsetinit.Themouth
developedakindofdivisionoflabor.Incisorsinthefront
becamespecializedtocutfood,caninesfurtherbackto
punctureit,andmolarsintheextremebacktoshearor
mashit.Theselittlemammals,whichresemblemice,havea
fundamentalpieceofourhistoryinsideofthem.Ifyou
doubtthis,imagineeatinganapplelackingyourincisor
teethor,betteryet,alargecarrotwithnomolars.Our
diversediet,rangingfromfruittomeattoTwinkie,is
possibleonlybecauseourdistantmammalianancestors
developedamouthwithdifferentkindsofteeththatcan
occludeprecisely.Andyes,initialstagesofthisareseenin
tritheledontsandotherancientrelatives:theteethinthe
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fronthaveadifferentpatternofbladesandcuspsthan
thoseintheback.
TEETHANDBONES—THEHARDSTUFF
Italmostgoeswithoutsayingthatwhatmakesteethspecial
amongorgansistheirhardness.Teethhavetobeharder
thanthebitsoffoodtheybreakdown;imaginetryingtocut
asteakwithasponge.Inmanyways,teethareashardas
rocks,andthereasonisthattheycontainacrystalmolecule
ontheinside.Thatmolecule,knownashydroxyapatite,
impregnatesthemolecularandcellularinfrastructureof
bothteethandbones,makingthemresistanttobending,
compression,andotherstresses.Teethareextrahard
becausetheirouterlayer,enamel,isfarricherin
hydroxyapatitethananyotherstructureinthebody,
includingbone.Enamelgivesteeththeirwhitesheen.Of
course,enamelisonlyoneofthelayersthatmakeupour
teeth.Theinnerlayers,suchasthepulpanddentine,are
alsofilledwithhydroxyapatite.
Therearelotsofcreatureswithhardtissues—clamsand
lobsters,forexample.Buttheydonotusehydroxyapatite;
lobstersandclamsuseothermaterials,suchascalcium
carbonateorchitin.Also,unlikeus,theseanimalshavean
exoskeletoncoveringthebody.Ourhardnesslieswithin.
Ourparticularbrandofhardness,withteethinsideour
mouthsandbonesinsideourbodies,isanessentialpartof
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whoweare.Wecaneat,moveabout,breathe,even
metabolizecertainmineralsbecauseofour
hydroxyapatite-containingtissues.Forthesecapabilities,
wecanthankthecommonancestorwesharewithallfish.
Everyfish,amphibian,reptile,bird,andmammalonthe
planetislikeus.Allofthemhavehydroxyapatite-containing
structures.Butwheredidthisallcomefrom?
Thereisanimportantintellectualissueatstakehere.By
knowingwhere,when,andhowhardbonesandteethcame
about,wewillbeinapositiontounderstandwhy.Whydid
ourkindofhardtissuesarise?Didtheycomeaboutto
protectanimalsfromtheirenvironment?Didtheycome
abouttohelpthemmove?Answerstothesequestionsliein
thefossilrecord,inrocksapproximately500millionyears
old.
Someofthemostcommonfossilsinancientoceans,500
millionto250millionyearsold,areconodonts.Conodonts
werediscoveredinthe1830sbytheRussianbiologist
ChristianPander,whowillreappearinafewchapters.They
aresmallshellyorganismswithaseriesofspikesprojecting
outofthem.SincePander’stime,conodontshavebeen
discoveredoneverycontinent;thereareplaceswhereyou
cannotcrackarockwithoutfindingvastnumbersofthem.
Hundredsofkindsofconodontsareknown.
Foralongtime,conodontswereenigmas:scientists
disagreedoverwhethertheywereanimal,vegetable,or
mineral.Everybodyseemedtohaveapettheory.
Conodontswereclaimedtobepiecesofclams,sponges,
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vertebrates,evenworms.Thespeculationendedwhen
wholeanimalsstartedtoshowupinthefossilrecord.
Thefirstspecimenthatmadesenseofeverythingwas
foundbyaprofessorofpaleontologyrummagingthrough
thebasementattheUniversityofEdinburgh:therewasa
slabofrockwithwhatlookedlikealampreyinit.Youmight
recalllampreysfrombiologyclass—thesearevery
primitivefishthathavenojaws.Theymaketheirlivingby
attachingtootherfishandfeedingontheirbodilyfluids.
Embeddedinthefrontofthelampreyimpressionwere
smallfossilsthatlookedstrangelyfamiliar.Conodonts.
Otherlamprey-likefossilsstartedtocomeoutofrocksin
SouthAfricaandlaterthewesternUnitedStates.These
creaturesallhadanexceptionaltrait:theyhadwhole
assemblagesofconodontsintheirmouths.Theconclusion
becameabundantlyclear:conodontswereteeth.Andnot
justanyteeth.Conodontsweretheteethofanancient
jawlessfish.
Wehadtheearliestteethinthefossilrecordforover150
yearsbeforewerealizedwhattheywere.Thereasoncomes
downtohowfossilsarepreserved.Thehardbits,for
exampleteeth,tendtogetpreservedeasily.Softparts,such
asmuscle,skin,andguts,usuallydecaywithoutfossilizing.
Wehavemuseumcabinetsfulloffossilskeletons,shells,
andteeth,butpreciousfewgutsandbrains.Ontherare
occasionswhenwefindevidenceofsofttissues,theyare
typicallypreservedonlyasimpressionsorcasts.Ourfossil
recordisloadedwithconodontteeth,butittookus150
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yearstofindthebodies.Thereissomethingelse
remarkableaboutthebodiestowhichconodontsbelonged.
Theyhavenohardbones.Theseweresoft-bodiedanimals
withhardteeth.
Foryears,paleontologistshavearguedaboutwhyhard
skeletons,thosecontaininghydroxyapatite,aroseinthe
firstplace.Forthosewhobelievedthatskeletonsbegan
withjaws,backbones,orbodyarmor,conodontsprovidean
“inconvenienttooth,”ifyouwill.Thefirsthard
hydroxyapatite-containingbodypartswereteeth.Hard
bonesarosenottoprotectanimals,buttoeatthem.With
this,thefish-eat-fishworldreallybeganinearnest.First,
bigfishatelittlefish;then,anarmsracebegan.Littlefish
developedarmor,bigfishobtainedbiggerjawstocrackthe
armor,andsoon.Teethandbonesreallychangedthe
competitivelandscape.
Thingsgetmoreinterestingstillaswelookatsomeof
thefirstanimalswithbonyheads.Aswemoveupintime
fromtheearliestconodontanimals,weseewhatthefirst
bony-headskeletonslookedlike.Theybelongedtofish
calledostracoderms,areabout500millionyearsold,and
arefoundinrocksallovertheworld,fromtheArcticto
Bolivia.Thesefishlooklikehamburgerswithfleshytails.
Theheadregionofanostracodermisabigdiskcovered
byashieldofbone,lookingalmostlikearmor.IfIwereto
openamuseumdrawerandshowyouone,youwould
immediatelynoticesomethingodd:theheadskeletonis
reallyshiny,muchlikeourteethorthescalesofafish.
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Aconodont(left)andanostracoderm(right).
Conodontswereoriginallyfoundisolated.Then,as
wholeanimalsbecameknown,welearnedthatmanyof
themfunctionedtogetherasatoothrowinthemouths
ofthesesoft-bodiedjawlessfish.Ostracodermshave
headscoveredwithabonyshield.Themicroscopic
layersofthatshieldlookliketheyarecomposedof
littletooth-likestructures.Conodonttoothrow
reconstructioncourtesyofDr.MarkPurnell,University
ofLeicester,andDr.PhilipDonoghue,Universityof
Bristol.
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Oneofthejoysofbeingascientististhatthenatural
worldhasthepowertoamazeandsurprise.Here,in
ostracoderms,anobscuregroupofancientjawlessfish,lies
aprimeexample.Ostracodermsareamongtheearliest
creatureswithbonyheads.Cuttheboneoftheskullopen,
embeditinplastic,popitunderthemicroscope,andyoudo
notfindjustanyoldtissuestructure;rather,youfind
virtuallythesamestructureasinourteeth.Thereisalayer
ofenamelandevenalayerofpulp.Thewholeshieldis
madeupofthousandsofsmallteethfusedtogether.This
bonyskull—oneoftheearliestinthefossilrecord—ismade
entirelyoflittleteeth.Teethoriginallyarosetobite
creatures;later,aversionofteethwasusedinanewwayto
protectthem.
TEETH,GLANDS,ANDFEATHERS
Teethnotonlyheraldawholenewwayofliving,theyreveal
theoriginofawholenewwayofmakingorgans.Teeth
developbyaninteractionoftwolayersoftissueinour
developingskin.Basically,twolayersapproacheachother,
cellsdivide,andthelayerschangeshapeandmakeproteins.
Theouterlayerspitsoutthemolecularprecursorsof
enamel,theinnerlayerthedentineandpulpoftheinsideof
thetooth.Overtime,thestructureofthetoothislaiddown,
thentweakedtomakethepatternsofcuspsandtroughs
thatdistinguisheachspecies.
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Thekeytotoothdevelopmentisthataninteraction
betweenthesetwolayersoftissue,anoutersheetofcells
andaninnerlooselayerofcells,causesthetissuetofold
andmakesbothlayerssecretethemoleculesthatbuildthe
organ.Itturnsoutthatexactlythesameprocessunderlies
thedevelopmentofallthestructuresthatdevelopwithin
skin:scales,hair,feathers,sweatglands,evenmammary
glands.Ineachcase,twolayerscometogether,fold,and
secreteproteins.Indeed,thebatteriesofthemajorgenetic
switchesthatareactiveinthisprocessineachkindof
tissuearelargelysimilar.
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Teeth,breasts,feathers,andhairalldevelopfromthe
interactionsbetweenlayersofskin.
Thisexampleisakintomakinganewfactoryorassembly
process.Onceplasticinjectionwasinvented,itwasusedin
makingeverythingfromcarpartstoyo-yos.Teethareno
different.Oncetheprocessthatmakesteethcameinto
being,itwasmodifiedtomakethediversekindsoforgans
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thatliewithinskin.Wesawthistakentoaverygreat
extremeintheostracoderms.Birds,reptiles,andhumans
arejustasextremeinmanyways.Wewouldneverhave
scales,feathers,orbreastsifwedidn’thaveteethinthefirst
place.Thedevelopmentaltoolsthatmaketeethhavebeen
repurposedtomakeotherimportantskinstructures.Ina
veryrealsenseorgansasdifferentasteeth,feathers,and
breastsareinextricablylinkedbyhistory.
Athemeofthesefirstfourchaptersishowwecantrace
thesameorganindifferentcreatures.InChapter1wesaw
thatwecanmakepredictionsandfindversionsofour
organsinancientrocks.InChapter2wesawhowwecan
tracesimilarbonesallthewayfromfishtohumans.
Chapter3showshowtherealheritablepartofourbodies—
theDNAandgeneticrecipethatbuildsorgans—canbe
followedinverydifferentcreatures.Here,inteeth,
mammaryglands,andfeathers,wefindasimilartheme.
Thebiologicalprocessesthatmakethesedifferentorgans
areversionsofthesamething.Whenyouseethesedeep
similaritiesamongdifferentorgansandbodies,youbegin
torecognizethatthediverseinhabitantsofourworldare
justvariationsonatheme.
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