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Microstructure and functional morphology of the Early Cambrian problematical fossil Rhombocorniculum

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Microstructure and functional morphology of the Early Cambrian problematical fossil Rhombocorniculum

    Microstructure and functional morphology of the Early Cambrian problematical fossil

    Rhombocorniculum

PR0GRESSlNNATURALSCIENCEV01.13.No.11.November2003

    ;MicrostructureandfunctionalmorphologyoftheEarlyCambrian

    ;problematicalfossilRhombocorniculum

    ;LIGuoxiang”,ZHUMaoyanandMichaelSTEINER2

    ;(1.YanjingInstituteofGeologyandPalaeontology,ChineseAcademyofSciences,Nanjing210008,China;2.TechniseheUniversit~t

    ;Berlin,Sckr.ACK14,Ackerstrasse7176,13355Berlin,Germany)

    ;ReceivedApril4,2003;revisedMay14,2003

    ;Al~WaetRJlDmbocorniculumcancellaturnfromtheLowerCambrianofsouthernShaanxiProvince(SouthChina)isrepresented

    ;bythedextralandsinistraltoothlikesclerites(bothinnearlyequalnumbers),indicatingthatthebodyplanoftheorganismwiththese

    ;sderitesprobablywasbilaterian.Themainfeatureofthemicrostrnctureofthescleritesisthepresenceoflongitudinallyarranged,mutually

    ;eontil~loushoUowtubulas,thesharedwallsconsistingofmicrocrystallineapatite.Intransversecro88sections,thescleritewallmainlyex

    ;hibitsaspongystructure.withporesizedecreasingadapically.Functionalmorphologicalanalysisindicatesthatthescleritesserveasade-

    ;fensiveratherthanagraspingfunction.

    ;,microstructure,functionalmorphology?

    ;Theabruptappearanceofdiverseskeletalfossils

    ;nearthebaseoftheCambrianSystemisamajorpiece

    ;ofevidencefortheEarlyCambrianbioradiation

    ;eventL

    ;.Sincetheseearlyskeletalfossilsaregeneral

    ;lYsmallinsize.theyarecommonlyreferredtoas

    ;smallshellyfossils,orabbreviatedasSSFs.TheSS

    ;Fsincludeanumberofextantphylaaswellasextinct

    ;problematicalgroupsofunknownphylogeneticaffini

    ;ties.Someoftheproblematicalgroups,includingthe

    ;phosphatictoothlikefossilsdiscussedinthepresent

    ;paper,arerepresentedexclusivelybyisolatedscle

    ;rites[2,3],andtheirscleritomesarepoorlyknown.

    ;Thetoothlikefossilsareacharacteristiccomponent

    ;oftheearliestskeletalfaunas,andareoftenpresentin ;aceticacid.etchedresiduesfromLowerCambrian ;limestones.Becauseoftheirsimilaritytosimplecone

    ;likeeonodonts.toothlikeSSFsarealsoreferredtoas ;

    ;conodont.1ikefossils’’3,41.Fossilsassignedtothis

    ;parataxonomlcgroupincludeprotoconodonts,para

    ;conodonts,andseveraltaxa,includingFomitchella ;andRhombocorniculum,whichhavenoaffinitywith ;conodontsandsoarecalledpseudoconodontsL. ;RhombocorniculumWalliser,1973,atypical ;representativeofthetoothlikeSSFs,consistsof

    ;phosphatichollowconeshavingarhomboidsculpture ;andasinglespineornodeatthecornersoftherhom

    ;boids.IthasbeenreportedmainlyfromtheEarly ;Cambrian(1ateQiongzhusiantoearlyCanglangpuian ;Age),andhasawidegeographicdistribution,occur

    ;ringinsuchlocalitiesasChina71,Mongoliaand ;Siberia[8l,

    ;Kazakhstan[9.

    ;,

    ;England[.,,andNorth

    ;America[5].

    ;Forthesereasons.Rh07,l60c0rcZ7,l

    ;hasspecialsignificanceforglobalbiostratigraphiccor? ;relation[3?2],especiallyforthecorrelationofthe ;QiongzhusianandCanglangpuianstagesonthe ;YangtzeplatformwiththeAtdabanianandBotomian ;inSiberia.However,thefunctionofRhombocor

    ;niculumsclerites(defensiveorgrasping?)isstillun. ;certain,andlittleisknownaboutthephylogenetic ;affinitiesofthistaxon.

    ;TheRhombocorniculumscleritesherestudied ;wererecoveredthroughaceticacidtreatmentofnodu

    ;larlimestonesfromthelowerShuijingtuoFormation ;atasectionneartheXiaoyangTown,ZhenbaCoun

    ;ty,ShaanxiProvince.Biostratigraphicdataofboth ;trilobitesandbradoriidsindicatethatthescleritesare ;fromtheLowerCambrianQiongzhusianstage[.,. ;Allspecimenswereexaminedandphotographedunder ;ascanningelectronicmicroscope,andconstitutea ;majorsampleimportantforanalyzingthemicrostruc

    ;ture,functionalmorphology,andbiologicalaffinities ;ofthisorganism.

;*SupportedbytheNationalNaturalScienceFoundationofChina(GrantNos.40172002and40232020).

    theMajorStateBasicResearchDevelop-

    ;mentProgramofChina(GrantNo.G2000077700),andtheChineseAcademyofSciences(GrantNo.KZ

    CX2,116)

    ;**Towholncorrespondenceshouldbeaddressed.E-mail:gxli@nigpas.ac.cn

    ;

    ;ProgressinNaturalScienceVo1.13No.II2003 ;1Descriptionofthesclerites

    ;Followingtheconventionfortheorientationof ;simpleconelikeconodonts,thecurvedconvexmargin ;offlRhombocorniculumscleriteisdesignatedthean

    ;teriormargin,whilethecurvedconcavemarginis ;calledtheposteriormargin.Orientedwiththeanteri

    ;ormarginfacingtheviewerandthebaseofthescle

    ;ritefacingdownward,theleftsideofthescleriteis ;designatedtheleftone,whiletheoppositesideis ;calledtherightside.Twoscleritemorphotypes,one ;havingflsulcusontheleftsideandtheotherhavingfl ;sulcusontherightside,aretermedthesinistraland ;thedextralmorphotypes,respectively.

    ;Thescleritesheredescribedaresmal1.withfl ;maximumlengthof1.4mmandflmaximumwidthof ;0.12mm.Thescleritesarelaterallycultrate,curve ;posteriorly,andshowflroundedanteriorandflkeeled ;posterior.Theyhaveflthickwallandflnarrowinter

    ;nalcavitythatextendsfromthebasetothetip.One ;ofthesidesisslightlyroundedorconvex,whilethe ;otherisconcaveandhasflbroadlongitudinalsulcus. ;TIlebasalcross-sectionisasymmetricallytearshaped

    ;(Fig.1,A2)andbecomescircularadapically(Fig.1, ;F).Thesurfaceexhibitsflregularrhomboidsculpture ;withthelongestdiagonalofeachrhomboidmeasuring ;about20pmlongandextendinglongitudinally,and ;theshortdiagonalmeasuringabout15pmlong.The ;cornersoftherhomboidsbearflsinglespine(upto ;20tanlong)ornode.Thesculptureisvariableonfl ;singlespecimen,beingdistinctontheconvexside, ;faintontheconcaveside,andsmootharoundthetip ;(Fig.1,A1).

    ;2Dextralandsinistralsclerites

    ;Basedonthepositionofthesulcus,R.cancel

    ;latumscleritesfromtheShuijingtuoFormationcanbe ;subdividedintotwomorphotypes,herecalledthe ;dextralandsinistralforms.Thepresentmaterialin.

    ;eludes44sclerites,ofwhich43areslenderandoneis ;broad.Theslenderscleritesinclude23sinistral ;(F.1,E)and20dextralspecimens(Fig.1,A1 ;andB),whilethesinglebroadscleriteissinistraland ;hasaflattenedtransversecross-section(Fig.1,D1 ;andD2).

    ;Thefactthattherearefarmoreslendersclerites ;thanbroadonesisconsistentwiththeresultsofprevi

    ;Oilsresults.MambetovrecoveredscleritesofR.can

    ;cellatumfromKazakhstanwithflslender/broadratio ;of175:8114].

    ;Landing[s]subdividedscleritesfrom

    ;Canadaintoslender,broadlyasymmetrical,and ;broadlysymmetricalmorphotypes,andreportedflrfl

    ;tof16:3:1.Hinz10]obtairledaratioof34(slen.

    ;der):1(intermediate):3.2(broadlysymmetrica1):1 ;(broadlyasymmetrica1);however,Hinz’sintermedi—

    ;atescleritesareindistinguishablefromherbroad ;ones.Ourexaminationofpreviouslypublishedfigures ;revealedmanyexamplesofthetwosclerite..morpho-

    ;typeshererecognized.

    ;Theexistenceofsinistralanddextralscleritesin ;nearlyequalnumbersindicatesthatthebodyplanof ;R.cancellatumprobablywasbilaterian.Although, ;unlikesomepreviouscoilections[5,10],nosymmetrical ;broadscleritesareexaminedfromSouthShaanxi,the ;absenceorpresenceofsymmetricalbroadscleritesis ;consistentwiththishypothesis.Thesymmetrical ;scleritesprobablywerelocateddorsallyinthecentral ;partofthebody,whiletheasymmetricalonesproba

    ;blywerelocatedinoronthesidesofthebody. ;3Microstructures

    ;R.cancellatumscleritesexhibitflthreelayer

    ;structure[5,n,2.consistingofthin.

    ;organicouterand

    ;innerlayersandathickmiddlelayercomposedof印一

    ;atiterodsarrangedobliquelytothewallsL5j.Thein

    ;nerlayerwasnotexaminedinthepresentstudy.The ;12gmthickouterlayerisPhosphaticandmassive ;instructure(Fig.1,F),butthephosphaticcomposi

    ;tionmaybesecondary.Thethicknessofthemiddle ;layeriSvariable:thinattwolateralsides(minimum ;thickness5gm),andthickattheposteriorend ;(maximumthickness40m).Themiddlelayercon

    ;sistsmainlyofhollowtubules,thewallsofwhich ;consistofmicrocrystallineapatite(commonlylass ;than0.2m).Thetubulewalls(1essthan0.5m ;thick)areshared.Intransversesections,thewallof ;thescleriteisporous,andtheporediameterislarger ;inthebasalcrosssection(1--2/Lm;Fig.1,A3)than

    ;inthedistalcross-section(1ess0.5btm;Fig.1,F). ;Inlongitudinalsections.thetubulesparalleltheIon

    ;gitudinalaxis(Fig.1,C2andC3),andthereareno ;septaorotherstructureswithinthetubules. ;Thepresentdescriptionofthemicrostructureof ;themiddlelayerisevidentlydifferentfromthatprevi

    ;ouslyreported’.However.thepreviousconclusion

    ;wasbasedprimarilyonexaminationofpolishedcross ;sections,whichcommonlywereetchedwithacids ;(possiblyHC1)priortoexaminationunderflSEM. ;

    ;ProgressinNaturalScienceVo1.13No.112003833 ;Acidetchingcandissolvetheoriginalphosphatic ;structure,exposingsecondaryinfillingsthatmayap

    ;pearfibrousandcanbeinterpretedastheoriginalmi

    ;crostructure.Ourworkshowsthatthescleritescon

    ;sistmainlyoflongitudinallyarrangedhollowtubules ;thatmayhavehousedorganicmatter.Followingthe ;decayofthisorganicmatter,thetubuleswerefilled ;withdiageneticcement.Forthisreason,thetubule

    ;likestructureiSdifficulttodiscern.

    ;Fig.1.SEMphotogmp~ofRhombocorniculumcancellatumfromtheLowerCambrianShingtuoForma

    tionofsouthernShaanxi.

    ;scalebaris100IxmforAI,B,C1,DI,E;20IxmforA2,D2;10IxmforC2,F;5I’mforA3;2IxmforAI,rightla

    teralview,

    ;cat.No.135540;A2,baSaltransversalsection;A3,spongystructure;B,rightlateralview,eat.No.135541

    ;cl,afragment,

    ;cat.No.135542;C2,basalpartindetail;c3,fibroustubules;Dl,leftlateralview,cat.No.135543;l:h,basalt

    ransversalsection;E,

    ;ldtlateralview,eat.No.135544,F,distaltransversalsection,cat.No.135545.

    ;4Functionalmorphology

    ;Rhombocorniculumscleriteshavebeeninter

    ;pretedashavingservedasagraspingfunction[,] ;similartothatofthegraspingapparatusesofproto

    ;conodonts(chaetognaths).However,thisinterpreta

    ;tionisbasedmainlyonmorphologicalsimilarity ;(analogy)withnocompellingindependentevidence. ;Indeed,themicrostructureandsurfacesculpturesof

    ;thescleritessuggestthattheyservedasadefensive ;

    ;ProgressinNaturalScienceVo1.13No.112003 ;ratherthanagraspingfunction

    ;Rhombocorniculumscleritesdifferhistolc’gically

    ;fromprotoconodonts.Asnotedabove,thewal1of ;Rhombocorniculumscleritesconsistsmainlyofhol

    ;1ow.mutuallycontiguoustubules.Intransversesea

    ;tions,thescleritewallisporous,andinlongitudinal ;seetionSthehollowtubulesaresomewhatcoaxia1.In ;contrast,thewallsofprotoconodontelementsconsist ;mainly0ffibrouslamellaewithabasalinternal

    ;growthmodelL,amicrostructurethatisverysimilar ;tothatofthegraspingspinesofchaetognaths[.’..

    ;Moreover,whereasRhombocorniculumscleriteshave ;athickwallandanarmwintemalcavitythatdoesnot ;extendtothetip,protoconodontsarethin-walled, ;andtheinternalcavityextendstotheapex.Inshort, ;histologicalevidencerefutesthehypothesisofaclose ;phylogeneticrelationshipbetweenRhombocor

    ;niculumandprotoconodonts(chaetognathgrasping ;spines[18]).

    ;TheapinosesculptureofRhombocorniculum ;scleritesisalsoinconsistentwiththehypothesisofa ;graspingfunction.SkovstedandPeel[.]arguedthat ;thepresenceofscale-likeprocessesonMongolitubu

    ;lusspinesruledoutagraspingfunctionforthese ;structures,andwethinkthatasimilarargumentcan ;beappliedt0Rhombocorniculumsclerites.The ;spinoseornamentationcouldnothavefacilitatedthe ;seizing/graspingofprey,butratherwouldhaveham

    ;peredsuchactivity.Thegraspingspinesofchaetog

    ;nathsusuallyareenclosedbyahood[18],andretrac

    ;tionofthishoodexposesthespinesforpreycapture. ;ThespinoseornamentationofRhombocorniculum ;seleriteswouldundoubtedlyhaveobstructedthere

    ;tractionofachaetognathlikehoodandthushindered

    ;preycapture.ThefactthatsomeRhombocorniculum ;scleritesarestronglytwisted[,.0]alsoisl’nc0nsistent

    ;withagraspingfunction.Therefore,Rhombocor

    ;niculumscleritesarenotcomparabletothegrasping ;spinesofchaetognaths,andmayinsteadhaveserved ;asadefensivefunction.

    ;Rhombocorniculumhasvariouslybeenassigned

;totheCambroscleritida[ts,20],

    ;theHa1kieriidaeIs].

    ;theprotoconodonts[2l.,orthepara?nodonts[221.

    ;However,assignmentsofthisfossi1totheCam

    ;broscleritidaortheHalkieriidaearearbitrary[4,51. ;Moreover,assignmenttotheParaconodontidaby ;MiillerLhasbeenshowntobeuntenablethrough ;histologicalinvestigationsL.

    ;Histologicalanalysisal

    ;soindicatesthatRhombocorniculumcannotbeas

    ;signedtotheprotoconodonts.However,because ;Rhombocorniculumscleriteswerethoughttobesimi

    ;1arinfunctiontoprotoconodonts,thedescriptive ;term”pseudoconodonts”wasintroducedbyLanding

    ;eta1.L5JforRh0760?r,zfZ7sclerites.Later.the

    ;hypothesisofagraspingfunctionbecamewidelyac

    ;cepted[4,to].

    ;Asnotedabove,Rhombocorniculumsclerites ;differmarkedlyfromgraspingspinesandmayhave ;servedasadefensivefunction.Indeed,when ;MeshkovaassignedRhombocorniculumtotheCam

    ;brosc1eritida[201.

    ;sheinterpretedthespine-likefossils

    ;asexternal,defensivesclerites.Unfortunately,how

    ;ever,Meshkova’sCambroscleritidaisaninvalidtax—

    ;onthatincludesphylogeneticallyunrelatedfossilssuch ;asProtohertzina,tommotiids,andSachitesL.

    ;thoughtheexistenceofdextralandsinistralselerites ;indicatesthatthebodyplanofRhombocorniculum ;wasbilaterian,verylittleisknownaboutthedetails ;0fitsanatomyincludingthearrangementofthescle

    ;ritesinoronthesoftbody.Presumablytheseques

    ;tionsandtheproblemoftheaffinitiesofthisorganism ;willberesolvedbythediscoveryofsoft..bodiedspeci.. ;AdmowledgementsWearegratefultoDr.H.VanIten ;forcommentsandimprovingthemanuscript,toZhangJunming ;forassistanceinthefieldandMaoYongqiangforSEMphotog. ;raphy.

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