page - Paleo Lab 06 - The Cambrian Explosion of Life labs/Lab06_Cambrian.pdfPaleo Lab 06 - The...

page - �Paleo Lab 06 - The Cambrian Explosion of Life

An Introduction to Index Fossils

CLASSIFICATION AND TAXONOMY

Geologistsfollowtheleadofbiologistsintheclassificationandnamingoforganisms(taxonomyaftertaxon=name).Sinceitsintroductionearlyinthe18thcentury,thesystemdevisedbyLinnaeushasbeenusedtocategorizeorganismsandtoaffixaformalnametoeachspecies.Majorclassificationcategoriesorgroupingsarelistedbelowontheleftwithanexampleusingthespeciesofthecommonhousecattoshowhowthesystemmaybeused:

KINGDOM Animalia PHYLUM Chordata CLASS Mammalia ORDER Carnivora FAMILY Felidae GENUS Felis SPECIES domestica

Thebasicunitofthesystemisthespecies,andeachcategoryaboveitincludesthedivisionsbelow.Atwopart(binomial)formaldesignationforeachspecies,followingtheschemeofLinnaeus,iscomposedofthecapitalizednameofthegenus(plural=genera)followedbytheuncapitalizednameofthespecies(plural=species).Notethatthebinomialisprintedinaspecialway(italics)todistinguishit.

Afteratleast5000yearsofdomestication,manhasdrawnoutofthatspeciessuchavarietyoftraitsthroughselectionandcross-breedingthatitmaybemoredifficulttorealizethatallsuchcatsbelongtothesamespeciesthanitistorecognizethattheysharethesamegenuswith the lion (Felis leo),leopard(Felis pardus),jaguar(Felis onca),tiger(Felis tigris),andthecougarorpuma(Felis concolor).AllwildcatsbelongtothesamegenusbutareclassifiedinthreeseparatespeciesfoundinEurope,Africa,andtheAmericas(theocelot),Felis sylvestris, F. ocreta, and F. pardalis,respectively.

Panthers,lynxes(bobcats),andcheetahsareassignedtoothergenerawithinthefamilyofcats(Felidae)whichalongwithdogs,hyenas,bears,raccoons,badgers,skunks,otters,seals,andwalrusesassignedtootherfamiliesconstitutetheCarnivora,theorderofflesh-eatingmammals.Alltheseanimalssharewithhumanstheproductionofmilk,hairontheskin,asinglebonelowerjaw,differentiatedandspecializedteeth,andahighlydevelopedbrain,featureswhichdistinguishmembersoftheClassMammaliafromthoseinotherclassesinthePhylumChordata.

Improvementinknowledgethroughadvancementsininstrumentsforstudyhasshown,furthermore,thelimitationsoftheoriginaltwokingdom(animalandplant)conceptofLinnaeus.Biologistsnowuseafivekingdomdivisiontabularizedbelow:

page - �Body Kingdom CellularComposition Designation Organization

unicellular Bacteria procaryoticunicellular Protista eucaryoticmulticellular Fungi eucaryoticmulticellular Plantae eucaryoticmulticellular Animalia eucaryotic

Organismswithprocaryoticcellularorganizationlacktheinternalmembraneswithintheircellswhichdefinesuchpartsasthenucleus.Thesingle-celledbacteriaandblue-greenalgaecomprisetheKingdomBacteriawhereastheotherfourkingdomscontaineithersingle-celledormany-celledorganismswitheucaryoticcellularorganization,thatis,thepresenceofinternalmembraneswithincells.

Thefivekingdomsareunequallyrepresentedinthefossilrecord.Obviously,organismswithmineralizedpartssuchasbonesandshellswouldbeexpectedtohavehigherrepresentation.However,remainsoforganic-walledmoneransandprotistanshavebeenrecoveredfromtheoldestrockscontainingfossils.

Consideringthemodesoffossilizationoutlinedinthepreviouslaboratoryexercise,theremains.oforganismswhichlivedinshallowmarineenvironmentsnearthemarginsofcontinentsarebestrepresentedinthefossilrecord.Forthisreason,majorcontributorsamongthephylalistedbelowwillbeemphasizedinthisexercise:

HowLIFEhasbeenclassifiedinthepast.

page - �ClassificationOfMajorBiologicalGroupsCommonlyRepresentedByFossils

KINGDOM BACTERIA bacteria blue-greenalgae(Stromatolites)

KINGDOM PROTISTA PhylumProtozoa ClassSarcodina-“amoebas” OrderForaminiferida-marineamoebasmostlywithcalciteskeletons OrderRadiolaria-marineamoebaswithopalskeletons

KINGDOMFUNGI-thefungi

KINGDOMPLANTAE-theplantkingdom “algae”—severalgroupsofaquaticplants,someofwhichsecretecalciteor aragoniteskeletons *PhylumBryophyta-themossesandliverworts PhylumTracheophyta-thevascularorprincipallandplants

*botanistsusetheterm“division”forthisrank

KINGDOMANIMALIA-theanimalkingdom PhylumPorifera—thesponges PhylumCnidaria ClassScyphozoa—thejellyfish ClassAnthozoa—thecorals Order Rugosa Order Tabulata Order Scleractinia PhylumBryozoa—thebryozoansor“moss”animals PhylumBrachiopoda—the“lampshells” ClassInarticulata—brachiopodswithunhingedvalves ClassArticulata—brachiopodswithhingedvalves PhylumMollusca—themollusks ClassGastropoda—thesnails ClassPelecypoda(Bivalvia)—theclams Class Cephalopoda Subclass Nautiloidea SubclassAmmonoidea SubclassColeoidea—thesquidsandoctopuses PhylumArthropoda ClassTrilobita—trilobites ClassOstracoda—ostracodes ClassInsecta—theinsects

page - � PhylumEchinodermata ClassBlastoidea—blastoids ClassCrinodea—sealiliesandfeatherstars ClassAsteroidea—starfish ClassEchinoidea—seaurchins,hearturchins,sanddollars PhylumProtochordata(Hemichordata)—includesextinctgraptolites PhylumChordata—chieflyvertebrateanimals ClassPisces—fish ClassAmphibia—frogs,toads,salamanders ClassReptilia—extinctdinosaurs,turtles,snakes,lizards ClassAves—birds ClassMammalia—mammals

Theclassificationschemeelaboratedaboveservesnotonlytogrouporganismsaccordingtorelationshipsbuttodemonstrate,bythenumberofheadingsrequired,thediversityofthebiosphere,aswell.However,therearemanymorephylaandothermajorgroupingsthatareimportanttobiologiststoday,butareinsignificantorunknownasfossils.

SELECTEDFOSSIL-PRODUCINGANIMALANDPLANTGROUPS

PHYLUMPORIFERA—THESPONGES Thespongesaremostlymarinemulticellularanimalsintermsofbasicorganizationbutlacktissuesororgansystems.Thespongebodymaybeglobular,cylindrical,disc-,orvaseshapedbutisgenerallyasymmetrical.Spongescharacteristicallyhaveperforatedwalls(sourceofthephylumnamewhichmeanspore-bearing)thataremodifiedbyfoldsandcanalswhichgreatlyincreasethefood-gatheringsurfaceaswaterispumpedthroughthebody.Thebodyissupportedbyspiculescomposedofopalinesilica,calciumcarbonate(usuallycalcite),oranorganicsubstancecalledspongin.Spiculesaresingleormulti-rayedinvariousdesignsresemblingtinypitchforks,tripods,shepherd’scrooks,tuningforks,barbells,andjacks.Thespiculesaremoreorlessconnectedtoformlattices.Morecommonly,however,onlyscatteredspiculesremainasthesoftbodydecays.Thearchaeocyathanswerecup-orcone-shapedindividualswithperforateddoublewallsandradialpartitions,andwereproblematicalsponge-likeorganismswithoutsurvivingrelatives.Ontheotherhand,anotherimportantfossilgroup,thestromatoporoids,whichappearinmanyearlyreefs,secretedamoremassive,layeredskeletonandarenowregardedasextinctsponges.

PHYLUM CNIDARIATheaquatic,predominantlymarine,cnidariansexhibitadegreeofspecializationgreaterthanthespongesinhavingwell-developedinnerandoutertissuesbutlackorgansseeninmoreadvancedinvertebrateanimals.Twocontrastingbodyforms,eachrimmedbytentacles,characterizethephylum,oneopeningdownward(medusa),theotherupward(polyp).Thetissuesareorganizedaroundaradiallydivideddigestivecavity,andtheentirebodyofeach

page - �individualcnidariantendstoberadiallysymmetrical.Onthesurfaceofthetentaclesarespecialcellshousingstingingcapsulescontainingprey-paralyzingtoxin.Theexclusivelymarineandpolypoid,bottom-dwellingmembersofthecoralclass,theAnthozoa,possessthemostextensivefossilrecordowingtothesecretionofacalciumcarbonateexoskeletonbuiltupwardsfromabasalplatformthatencasesthebodyinacylindricalorconicalcup. Divisionoftheindividualanthozoanexoskeleton,calledthecorallite,byradiallydisposedandvertical(septa)orhorizontal(tabulae)platesservestodifferentiatetheordersoftheclass.Thus,theextinctRugosa(individualsandcolonies)andthecolonialTabulatawithtinycor-allitescrossedbyhorizontalplatesdistinguishPaleozoicrocksfromtheOrdovicianPeriodonward.ThefossilsoftheScleractinia,withseptaldivisionsinmultiplesofsix,arefoundonlyinMesozoicandCenozoic“rocks. Asolitaryhabit,asillustratedbythePaleozoichorncorals,islesscommonthanthecolonialstructureassociatedwithreefandothercarbonatebuildupsdatingfromPaleozoictimes.Bymeansofasexualcloning,coloniesofcoralsexpandlaterallyandvertically.Intoday’sseas,speciesofboththesessilecoralsandthemobilesoft-bodiedscyphozoanjellyfisharecommon.

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page - �PHYLUM BRYOZOA

Theexclusivelycolonialfossilsofbryozoanssuperficiallyresemblesomecolonialcoralsbuttheindividualunitsaremuchsmaller.Furthermore,unliketheradialcoelenterates,thebodyplanoftheindividualbryozoananimalandthemajorityoftheothermembersoftheanimalkingdomisfundamentallybilaterallysymmetrical. Typically,hundredsoftinyindividuals,lessthan1mmlong,arejoinedtogetherincolonieswitheachanimalsecretingacalcite,chitinous,ormembranousexoskeleton(zoecium)toformvariouslyshapedcolonialstructures(zoaria)rangingfromsimpleseriesoftubesthatcommonlyencrustshells,tomorecomplexzoaria.Thesezoariamaybeupward-branching,roundedcylinders,orhorizontally-encrusting,leaf-likeexpansions,orfrond-likewithverticalbranchesjoinedbycrossbarssoastoleavewindow-likeopenings(lacy),orflat-bottomedhemisphericalmasses.Insomespecies,thelacyfrondsareattachedtoaspiralbase. Inadditiontothetinyzoecialopeningscrowdingthezoariumsurface,lownodularmoundsandflatareasmayberegularilydistributedoverthecolonysurface.However,duetotheminutesizeofthezoecia,highmagnificationisrequiredtoobserveinternalfeatures.Fromtheopenendofeachzoecialtube,ahorse-shoe-shaped,tentacle-bearing,respiratoryandfeedingorgan,thelophophore,isextrudedandretractedbymuscles.Thisdeviceiscomparabletooneusedbyanotherfilter-feedingorganism,thebrachiopod.

Bryozoans(1-7)

page - �PHYLUM BRACHIOPODA

Brachiopodshaveadouble-valvedshellwhichissecretedinavarietyofshapesandsizesattheedgesofthinlobesofspecialectodermaltissuecalledthemantle.Theentirebrachiopodshellisbilaterallysymmetrical,buttheplaneofsymmetrypassesthroughthemiddleofeachvalve,ratherthanbetweenthevalvesasisthecaseinmanyotheranimalswithtwo-valvedshells.Astheshellgrowsperiodically,concentricgrowthlinesmarkthesuccessivestagesofgrowth.Inthelargestdivision,theClassArticulata,thecalcitevalvesarehingedneartheposteriormarginbyprojections(teeth)inthepediclevalve,whichhasanovalortriangularopeningforpassageofafleshystalk(pedicle).Thesehingeteethfitintodepressions(sockets)inthebrachialvalve.Thelophophoreisattachedtotheinteriorofthelatter.InthecaseInarticulata,thevalveslackhingementandthetypicallysmall,thin,smoothshellsarecommonlycomposedoflayeredormixedorganicsubstances(chitinandprotein)andcalciumphosphate(apatite).LargerinarticulatesresemblingthelivinggenusLingulauseoneendofacontinuouslygrowingstalk-likepedicletoanchorthebodytothebottomofburrowsontheseafloor.Thepedicleisusedtoattachtheanimaltotheseafloorduringallorpartofthepostlarvalstageoflife.Shellshapeisrelatedtothenatureofthebottomonwhichthebrachiopodresidesandtotheneedtomaintainthegapebetweenthevalvesfreeofsediment.

Brachiopoda(8-12)

page - �PHYLUM MOLLUSCA

Partofthebodywall,themantle,secretesanexternalorinternalshell,ifpresent,composedofcalciumcarbonate(aragoniteand/orcalcite).Theshellmayconsistofseveralunconnectedparts,twoparts,withorwithoutarticulation(clams),oronepart,mostlyintheformofastraight(somecephalopods)orcoiled(mostsnails,othercephalods)conewith(cephalods)orwithout(snails)internaldivisions{septa).Theinternalshellofsomecephalopodsmaybereducedtoabodysupportfunction(cuttlefish,squids). Anopenspaceenclosedbythemantle,locatedforwardinsnailsandtotherearinclamsandothermolluscs,housespairedgills,usedprimarilyinrespirationbutservingalsoasafood-filteringscreeninclams.Thelowerpartofthebodywall,thefoot,ismodifiedforlocomotion(crawling,digging,swimming)orintotentaclesusedinpredation.Theformalnamesappliedtosomemolluscanclasses(e.g.,Gastropoda=“stomach-foot”)reflect.earlyrecognitionofthisportionofthebodybutsomeconfusionastofunction.Thethreemostimportantmolluscanclasses,intermsofquantitativerepresentationinthefossilrecord,arediscussedfurtherbelow.

CLASSPELECYPODA(BIVALVIA)

Pelecypods(fromthehatchetshapeofthefoot)arecommonlycalledclams,andincludemussels,scallopsandoysters,thevastmajorityofwhichinhabitshallowmarinewaters.Insomeclams,alternatingteethandsocketsinonevalve,matchingtheiroppositesintheother,formahingingmechanism.Otherclamslackhingeteeth,andelasticligamentsandpadsholdthevalvesinalignmentandfunctiontoopentheshell.Oneortwomusclesclosetheshellincontrasttothetwosetsofopeningandclosingmusclesinarticulatebrachiopods.Onthesmoothshellinterior,elongategrooves,andcirculardepressionswereonceoccupiedbyligamentsandtheendsofadductormuscles.Thesefeatures,alongwithavarietyoftoothpatterns,aidinidentification,classification,andinterpretationoflifehabit.

CLASS GASTROPODA

Thepredominantlymarinesnailsevolvedeventuallytooutnumberallothermolluskscombinedinbothfossilandlivingspecies,adiversitythatparallelsthewideenvironmentaladaptationwithintheclass.Withthedevelopmentofalung-likestructureforbreathingair,snailsinvadedlandhabitatsfirstinthePennsylvanianPeriod.Mostlycrawlingslowlyovertheshallowseafloororsoilsurfaceonthemuscularfoot,snailscarryacoiledshellontheback.Coilingingastropodshellsmaybeloose,withcompleterevolutions(whorls)justtouching,ortightlycoiledwithaninternalspiralstructureatthecoilingaxis.Thefossilrecordofsnailsincludesmanyinternalmoldswhichlimitourabilitytoidentifymanyspecies.

CLASS CEPHALOPODA

ByOrdoviciantime,thenautiloids,theearliestmembersofthisexclusivelymarineclasshadbecomethechiefpredatorsoftheseas,equippedlikethepresent-daycephalopods(squids,cuttlefish,octopusandpearlynautilus)withhighlydevelopedeyes,prehensiletentaclesandrapidlocomotionbymeansofjetsofwater.Afleshytube,thesiphuncle,passesthroughthe

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page - ��interiorchambersandthewalls(septa)separatingthem.Theunoccupiedearlierchambersbehindthelivingchamberwerefilledwithgasesandliquidsorwithmineraldepositsasthenautiloidgrew.InmodernNautilus,thegasesinthechamberslendbuoyancytotheanimalwithinthewatercolumn. Themostusefulfeatureforidentificationoffossilcephalopodshellsisthepatternformedbytheintersectionoftheedgesoftheseptawiththeinnershellwall.Thisintersection,calledthesuture,isstraightorgentlyundulousinthestraight,curvedorcoiledshellsofnautiloids.Ammonoidshellshavecomplexlyflutedseptaethatgivesimilarlycomplexsuturepatterns.Threeammonoidsuturetypes,goniatitic,ceratiticandammonitic,successivelyevolvedintheDevonian,MississippianandPermianperiods.GoniatiticammonoidsweremostcommoninPaleozoicfaunas,whereastheceratiticandammoniticformswerethedominantMesozoicforms.

PHYLUM ARTHROPODA

Morethanthree-quartersofalllivingspeciesofanimalsarearthropods,thenamederivedfromthesegmentedorganizationofthelegsandfeet.Insects,spiders,centipedes,lobsters,crabs,barnaclesandseveralextinctgroups,mostnotablythetrilobites,belongtothisphylum.Despitethevisibleabundanceofmembersofthephylumandthelonggeologichistoriesofthemaindivisions,thefossilrecordiscomparativelypoor.Insectfossils,forexample,arerareoutsideofexceptionalvolcanicashdepositsandresindroppings(amber)fromtrees.Onlytheostracodeswithabivalvedcarapaceofcalciteandthetrilobiteswithacalcite-impregnatedchitinousexoskeletonarerepresentedbyanextensivefossilrecord. Basedonaninvariableassociationwithfossilsofmarineorganisms,thetrilobitesappeartohavebeenexclusivelymarineduringtheirtimerange,thePaleozoicEra.Trilobitesowetheirnametothelobatecurvatureofthreelongitudinaldivisionsofthebody,theaxiallobeinthemiddleandthepleurallobesonthesides.Fromheadtotail,therearealsothreemainsubdivisions,thecephalon,thoraxandpygidium.Inthesegmentedthoraxregion,thepleurallobeseachboreajointedappendagethatwasysedforwalkingorswimmingandsupportedagillforbreathing.Distinctivetracefossilsresultedfromthemovementoftheselegs,bothwhiletheanimalcrawledandrestedontheseafloor.Apairofantennaerosefromtheundersurfaceofthecephalon,andpairedcompoundeyeswerelocatedontheuppersurface. Asistypicalofmanyarthropods,completeorpartialmolting(shedding)oftheexternalskeleton(exoskeleton)occurredbeforegrowthperiodsinbothtrilobitesandthemuchsmallerostracodes.Intrilobites,thisresultedinbreakageoftheexoskeletonintoseveralpieces,whereasthe“bean-shaped”andbivalvedcarapaceoftheostracodesusuallybecamedisarticulatedintothetwoseparatevalves.Asaresult,thefossilrecordincludesmostlymoltstagesoftheseanimals.

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page - ��PHYLUM ECHINODERMATA

Theknobby,oftenspine-studdedsurfaceofthemulti-platedinternalskeletonofthestarfishandseaurchinsisthebasisforthe“spiny-skin”namegiventothisexclusivelymarinephylum.Echinodermsalsoarerecognizedcommonlybytheirgenerallyfive-fold(pentameral)symmetry.Withthedemiseoftheorganism,theindividualskeletalpartseasilydisarticulate.Evenso,thecharacteristiccalciteskeletalpartsremainandcanbeidentified.Echinodermsarecommonlygroupedintotwoinformalsubdivisionsbasedonthedegreeofattachmenttotheseafloor.Pelmatozoa(echinodermswithastemorstalk)aremoreorlesspermanentlyattachedfollowingaswimminglarvalstage.Eleutherozoa(free-movingechinoderms)generallyarenotattachedtotheseabottom,butcrawloverit,burrowintoitorswimaboveit. AllclassesofechinodermswereestablishedbyOrdoviciantime.However,itisthecrinoidsandalliedpelmatozoansthatdominatedthePaleozoic,althoughbytheendofitmostofthemhadbecomeextinct.Incontrast,MesozoicandCenozoicechinodermfaunashavebeendominatedbyechinoidsandstarfish. Blastoidsresemblecrinoidsinhavingathecaofmainbodypartsrisingaboveasupportingstalkofmanysegments(columnals)attachedatthebasebyaroot-likesystem.Thebudshapedthecaoftheblastoidiscomposedofauniform,symmetricalpatternofthirteenplatesarrangedinthreerows.EmbaymentsineachofthefiveprincipalplateshouseoneoffiveambulacraIareasequippedwithrowsofthread-likebrachiolesthatwereusedforfoodcapture. Comparedtotheblastoidtheca,thecrinoidcalyxismuchmorecomplicatedinhavingadorsalcupoftwotomanycircletsoflessregularplatesandaventralplatedcover(tegmen)also.Fivearmswithambulacralgroovesoriginateatspecialcupplates(radials)andareusedforfoodcapture.Thestalkedsupport,rootedhabit,andbranchedcrown,gaverisetothepopularbutbiologicallyinaccuratename“sealily”appliedtothecrinoids. Theechinoids,whichcompeteinimportancewiththecrinoidsascontributorstothefossilrecordofechinoderms,includetheseaurchinsandsanddollarsalongwithanumberofextinctspeciesfirstobservedasfossilsinrocksofOrdovicianage.Theskeleton(test)isaglobularorflattenedboxcomposedofrowsofplatesofdifferentshapeswhicharearrangedintofiveambulacralareaswhichalternatewiththesamenumberofinterambulacralareas.Theformerdistinguishedbyperforatedplateswhichaccomodatethetubefeetofthewater-vascularsystem.Inlife,thehighlyattractiveplatedesignishiddenbynumerousmovablespineswhichareattachedtoknob-likebasesontheskeletonsurface.Inreferencetothedegreeofradialsymmetrydisplayedbytheskeletalarchitecture,echinoidsareclassifiedaseither“regular”or“irregular”thelattergrouphavingadistinctlybilateralappearance.Ajawapparatus,composedofanumberofsharp-edgedplatesandcalledAristotle’slanternoccupiesalargecentralopeningintheskeletonsurroundingthemouth.Echinoidsaremostlymuddybottomsedimenteatersandscavengersincontrastwiththeaggressivelypredaceousstarfish.The“irregular”echinoids,whichappearfirstintheJurassicPeriod,includethesanddollarsandhearturchins.

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page - ��PHYLUM PROTOCHORDATA

Thereareanumberofexamplesoffossildiscoveriesandcollectionsmadeseveralyears,evenacenturyormore,beforetheirassignmenttoaspecificbiologicalgrouphasgainedgeneralacceptancebypaleontologistsandbiologists.Themajorityhavebeenfossilsoflongextinctorganismswithoutmodernrelatives.Today,aftertwocenturiesofsystematicshufflingbackandforth,graptolitesor,moreformally,Graptozoa,areunderstoodtobechitinoidcolonialstructuresthathousednumerousminuteorganismsbelongingtoanextinctgroupofhemichordates.HemichordatesareplacedinthePhylumProtochordatawhichincludesanimals“intermediate”betweenthehigherinvertebratessuchasechinodermsandthechordates. Whenuncompressedgraptolitespecimens,ratherthanthemorecommoncompressedcarbonfilms,arestudied,thecolonialskeleton(rhabdosome)isseentobecomposedoflinearseries of tubular cups connected at the base to a continuous tubular portion in one group or thread-likerodinanother.Thestyleachievedoncompletionofcolonygrowthprovidesabasisforrecognizingevolutionarytrends.

PHYLUM CHORDATA

Inadditiontoacartillaginousnotochord,chordateanimalspossess,duringoneormorelifestages,adorsalnervecord,apharynxwithgillslits,andtypically,atail.Ofthenearly45,000livingspeciesmostchordatesarevertebratesinwhichacolumnofvertebrae(backbone)developsaroundandmostlyreplacesthenotochord.Inthebonyfish,amphibians,reptiles,birds,andmammals,bone,composedofapatitereplacestheinitialskeletalcartilagecompletely,exceptbetweenvertebraeandotherparts.Insomefish(lampreys,sharks,rays)thereisnobonereplacement. Aswithfossilsofothergroupsoforganisms,theskeletalremainsofvertebrateshaveprovidedarecordoftheorderoffirstappearancebeginningwiththeearliestfishintheLateOrdovicianfollowedbytheamphibiansthenthereptilesbytheendofthePalezoicEraand,lastly,themammalsintheTriassicandbirdsintheJurassic.Morerecently,sedimentsintheLateCenozoicfaultbasinsofEastAfricahavefurnishedarecordoftheancestryofourownplacentalmammalspecies,Homosapiens.

PLANT FOSSILS

Plantsarealargeanddiversegroupingofphotosyntheticorganisms,which,forourdiscussionwillincludemulticelledandsingle-celledforms,andalsowillincludethephotosyntheticmonerans.Thedominanttypeofpreservationofplantfossilsisbycarbonization,becausethe“skeletons”ofmostplantsareformedbycomplexorganicsubstancesincludingcelluloseandlignin.Somemarineplants,however,arecapableofsecretingmineralmaterial.Theseplantsmostlybelonginthebroadgroupingofalgae,themostconspicuousmineralremainsofwhicharecalciteandaragonite.Inadditiontothepreservationofeitherorganicormineralbodyparts,somealgae,previouslymentioned,arecapableofforminglayeredsedimentaryaccumulations(stromatolites)bytrappingfinesedimentparticlesontheirstickycellandfilamentsurfaces.Fossilsoflandplantsexhibitanenormousvarietyofshapesandforms.Someofthemost

page - �6commontypesoflandplantfossilsarepollenandsporegrains,butthemostobviousareimpressionsandcarbonfilmsofleavespluspetrifiedwood.Otherparts,suchasflowersandseedsaremuchlesscommon.

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