Kylinxia zhangi ZENG,ZHAO AND HUANG

Zeng, Han, Zhao, Fangchen, Niu, Kecheng, Zhu, Maoyan & Huang, Diying, 2020, An early Cambrian euarthropod with radiodont-like raptorial appendages, Nature 588, pp. 101-105 : 101-105

Kylinxia zhangi ZENG,ZHAO AND HUANG , GEN.ETSP.NOV.

Etymology. Kylin:CHIMERICCREATUREINCHINESEMYTHOLOGY; xia:CHINESE WORDFORSHRIMP-LIKEARTHROPOD; AND zhang: AFTERYEHUIZHANG, WHO CONTRIBUTED THE PARATYPE. Holotype. NANJINGINSTITUTEOFGEOLOGYANDPALAEONTOLOGY (NIGP) 171304, PARTANDCOUNTERPART (FIGS. 1A, C, D, G, 2C, IANDEXTENDEDDATA FIGS.1A, B, D, 3N, O , 4G , 5 , 6B, G, I ). Referredmaterial. PARATYPE:YINGLIANGSTONENATURALHISTORYMUSEUM (YLSNHM) 01124 (FIGS. 1B, E, F, H, 2G, HANDEXTENDEDDATAFIGS . 1C, E, 4A, F , 6A, H ). OTHERSPECIMENS:NIGP 171305 ( EXTENDEDDATAFIGS.2A, B , 4D , 6C ), NIGP 171306 ( EXTENDEDDATAFIGS.2C, D , 4B , 6D ), NIGP 171307 ( EXTENDEDDATAFIGS. 2E–G , 4C , 6E ) ANDNIGP 171308 (EXTENDEDDATA FIGS.2H, I , 4E , 6F ). Localityandhorizon. JIANSHAN,HAIKOU,KUNMING,YUNNAN,CHINA;MAO- TIANSHANSHALEMEMBER,YU’ANSHANFORMATION, WutingaSpiS – EoredliChia BIOZONE (CAMBRIANSTAGE 3). Diagnosis. AN EUARTHROPOD POSSESSINGA PAIR OF UPWARD-ORIENTATED FRONTALMOST APPENDAGES,EACHCONSISTING OF A STOUTSHAFT AND A DISTAL ARTICULATED REGION COMPOSED OF 15 PODOMERES BEARING ELONGATE TRI- ANGULARENDITESWITHTWOROWSOFUPTOSEVENSHARPAUXILIARYSPINES. FIVESTALKEDCOMPOUNDEYES. HEADSHIELDWITHROUNDEDGENALANGLES. ANTERIORMOSTFOURPOST-ORALAPPENDAGESDIFFERENTIATEDFROMPOSTERIOR ONES.TRUNKCONSISTINGOFUPTO 25 TERGITES.PYGIDIUMMERGEDFROMAT LEASTFIVESOMITES.TAILFANCOMPRISINGTHREELOBES. Descriptionandcomparisons Kylinxia ISKNOWNFROMSIXSPECIMENSWITHWELL-PRESERVEDSOFTPARTS (ADETAILEDDESCRIPTIONISPROVIDEDINTHESUPPLEMENTARYDISCUSSION). ITHASFIVESTALKEDCOMPOUNDEYESONTHEHEAD, OFWHICHTHEANTERIOR TWO ARE AT LEAST TWICE AS LARGE AS THE POSTERIOR THREE (‘AE’ AND ‘PE’ IN FIGS.1C, 2C, GANDEXTENDEDDATAFIGS . 1D, E, 2B, D, F, I , 3N, O , 4A–G , 5B ). THISCONFIGURATIONOFEYESISREMINISCENTOFTHEPECULIARFIVEEYESIN OpabiniaregaliS 10. THEEYEARRANGEMENTBETWEENTHETWOTAXAIS COMPARABLE,WITH THE ANTERIORAND POSTERIOR EYES IN Kylinxia CORRE- SPONDINGTOTHEOUTERANDINNEREYESIN Opabinia , RESPECTIVELY 10 (‘AE’, ‘PE’, ‘OE’ AND ‘IE’, IN FIG. 2A–C , EXTENDEDDATA FIG. 3G, H, N, O ). IN BOTH Kylinxia AND Opabinia , THEEYES AREBORDEREDWITHMARGINALRIMS (‘RM’ IN FIGS. 1C , 2B, C ).THETWOORTHREEMEDIANEYESINEUARTHROPODSINCLUDING CAMBRIANHELMETIIDS ANDHYMENOCARINES 11 (‘LE’ AND ‘ME’ INEXTENDED DATAFIG.3J–M ) MAYTHUSBEHOMOLOGOUSTOTHEPOSTERIORORINNERCOM- POUNDEYES IN Kylinxia OR Opabinia , RESPECTIVELY,BYREFERRING TOTHEIR COMPARABLEANATOMICAL POSITIONS AND SMALLERSIZES.

THE FRONTALMOST APPENDAGESOF Kylinxia ARERADIODONT-LIKE (‘FA’ IN FIGS.1C , 2GANDEXTENDEDDATAFIGS .2B, D, F,I, 4A–G ).THESEAPPENDAGES SHARE KEY MORPHOLOGICALFEATURESOF THE RADIODONTFAMILIES ANOMALO- CARIDIDAEAND AMPLECTOBELUIDAEREPRESENTEDBY AnomaloCariS 12 AND RamSkoeldia 13, RESPECTIVELY(‘FA’ INFIG.2D, GANDEXTENDEDDATAFIG .4F–I; DETAILEDCOMPARISONS ARE PROVIDEDIN THESUPPLEMENTARYDISCUSSION). THEIRSHAREDSIMILARITIESINCLUDE12–15DISTALARTICULATEDPODOMERES(‘P1’– ‘P15’ INFIGS.1C , 2GANDEXTENDEDDATAFIGS .2D,F, 4A–C,F–I ),ASHAFTREGION WITHAN OBLIQUEARTHRODIAL MEMBRANEAND A SINGLEPAIROF ENDITES (‘SH’, ‘SA’ AND ‘SE’ INFIGS.1C , 2 GANDEXTENDEDDATAFIGS .1D, E, 2B,F,I , 4A–K , 5B ), ENDITESOFALTERNATINGLENGTHS (‘SE’AND‘ED’INFIGS.1C,2D–HANDEXTENDED DATAFIGS.1D, E , 2B, I , 4A, D–I ) ANDROUGHLY SYMMETRICALLY ARRANGED AUX- ILIARYSPINESONEACHENDITE ( ARROWHEADSINFIG.2E,F,H,I );WEALSOINFERA

SIMILARFUNCTIONALMORPHOLOGY 12, 13 ( FIG.2D ,GANDEXTENDEDDATAFIG.4F–I). HOWEVER,IN CONTRAST TO RADIODONTS,THE UPWARD ORIENTATION ANDTHE ABSENCEOF OUTERSPINESIN THEFRONTALMOST APPENDAGES OF Kylinxia (‘FA’ INFIGS.1C , 2 GANDEXTENDEDDATAFIGS .1D, E, 2B,D,F,I , 4A–G )AREFEATURESOF MEGACHEIRANS 14 ( FIG.2 JANDEXTENDEDDATAFIG .7A–F,H),GREAT-APPENDAGE BIVALVEDEUARTHROPODS 15 ANDISOXYIDS 16, 17 ( EXTENDEDDATAFIG.7J–O ).THE PRESENCE OF AUXILIARY ENDITIC SPINESONTHE FRONTALMOST APPENDAGES OF RADIODONTS ( ARROWHEADSINFIG.2E,F ), Kylinxia ( ARROWHEADSINFIG.2H,I ) ANDMEGACHEIRANS( FIG.2K– MANDEXTENDEDDATAFIG .7D–I)STRENGTHENS THE MORPHOLOGICAL SIMILARITIESOF THEFRONTALMOST APPENDAGES AMONG THESETAXA,BECAUSESUCHAUXILIARYENDITICSPINESAREABSENTINISOXYIDS 16, 17 ANDOTHERCAMBRIANEUARTHROPODS 18 ( EXTENDEDDATAFIG.7J–O ).

THE BODY PLANOF Kylinxia —WHICH CONSISTS OF A FUSED HEAD SHIELD, A MULTI-SEGMENTED TRUNK, A PYGIDIUM AND ARTHRODIZED POST-ORAL BIRAMOUS APPENDAGES—IS TYPICAL OF DEUTEROPODS ANDIS PARTICULARLY MEGACHEIRAN-LIKE ( FIG. 1A, B AND EXTENDED DATAFIGS. 1 , 2A, C, E, H ). THE SEMI-CIRCULARHEADSHIELDOF Kylinxia WITHROUNDEDGENALANGLESISVERY SIMILARTOTHEHEADOFTHEMEGACHEIRAN HaikouCariS 14 (‘HS’ INFIG.1C , 2G, J AND EXTENDEDDATA FIGS. 1D, E , 2D, F, I , 4A–E , 7A ). Kylinxia HASUP TO 25 TRUNK TERGITES (‘T1’–‘T25’IN FIG. 1A, B AND EXTENDEDDATA FIGS. 1 , 2A, C, E, H ), WHICHFALLSWITHINTHE RANGEOF 20–33 TERGITESIN ‘MULTI-SEGMENTED’ MEGACHEIRANSINCLUDING Sklerolibyon, Jianfengia AND FortiforCepS 19

( EXTENDEDDATAFIG. 7B ). ALTHOUGHAPYGIDIUMTHATCOVERSMULTIPLE APPENDAGES IN Kylinxia IS CHARACTERISTIC OF ARTIOPODANS 18 AND THE THREE-LOBED TAIL FANCOMPRISING A MIDDLE AND A PAIR OF LATERAL FLAPS OF Kylinxia HASFOUND COUNTERPARTSIN MEGACHEIRANS 19, FUXIANHUIIDS 18 ANDHYMENOCARINES 17, THEFUSEDPYGIDIUMARTICULATEDWITHATAILFANOF Kylinxia ISUNIQUEAMONGCAMBRIAN EUARTHROPODS (‘PY’ AND ‘TF’ INFIG.1G, HANDEXTENDEDDATAFIGS .1D,E, 6E–H ).THETRUNKAPPENDAGESOF Kylinxia AREBIRAMOUS,COMPRISINGANENDOPODITEOF ATLEASTSEVENPODOMERES (‘EN’ AND ARROWHEADSIN FIG. 1D, EANDEXTENDEDDATA FIGS.1D , 2C, D , 4A ) AND ANOVALEXOPODITEFLAPFRINGEDWITHLONGLAMELLAE (‘EX’ INFIG.1D, G, H AND EXTENDED DATA FIGS. 1D, E , 2F , 4C ). THE POST-ORAL APPENDAGES IN

Kylinxia AREHOMONOMOUS,EXCEPTTHATTHEANTERIORMOSTFOURAPPEND- AGE PAIRS (TWO IN THEHEAD ANDTWOIN THE TRUNK) AREDIFFERENTIATED AND SMALLER (‘DA’ INFIGS.1C,F , 2GANDEXTENDEDDATAFIGS .2B,D, F,I, 3N , 4A–E , 5B ). THEARRANGEMENT ANDMORPHOLOGYOFTHE APPENDAGESOF Kylinxia AREMOSTSIMILARTO THOSEOFMEGACHEIRANS AMONGCAMBRIANEUARTHRO- PODS 19 (‘DA’, ‘EN’ AND ‘EX’ INEXTENDEDDATA FIG. 7B ).

THE SPECIMENS OF Kylinxia EXHIBIT TWO TOPOLOGICALLY CONSISTENT STRANDS OF DARK MATTERTHROUGH THE BODY, ONE CENTRAL (‘AC’ IN FIG.1A–C AND EXTENDEDDATA FIGS. 1 , 2A, C , 6A–D ) AND THE OTHER VENTRAL (‘VN’ IN FIG. 1A ANDEXTENDEDDATAFIGS. 1A, B, D , 2A, C , 6B–D ). THE TWOSTRANDS CORRESPOND WELL IN POSITION AND MORPHOLOGY TO THE EUARTHROPOD

ALIMENTARYCANALANDVENTRALNERVECORD,RESPECTIVELY 20, 21. THEALIMEN- TARYCANALISASSOCIATEDWITHPAIREDDIGESTIVEGLANDS (‘DG’ INFIG.1D AND EXTENDEDDATAFIGS.1D, E , 2D, G , 4C ).INEACHTRUNKSEGMENTOF Kylinxia , BIFURCATING STRUCTURESINNERVATE APPENDAGES FROM THE VENTRAL NERVE CORD (‘NT’ INFIG. 1D ANDEXTENDEDDATAFIG. 6B–D ) AND ARECOMPARABLE TOTHEPAIREDLEGNERVESINOTHERCAMBRIAN EUARTHROPODS 20, 22.

INALATERAL VIEW,THEALIMENTARYCANALANDTHE VENTRALNERVECORDMEET ATTHE HEAD REGION (‘AC’,‘FG’,‘ND’ AND ‘NT’ IN FIGS. 1A–C , 2CAND EXTENDED DATAFIGS.1A,B,D , 2B, D , 3N, O , 4A,B,D , 5B ).MORPHOLOGICALINTERPRETATION AIDEDBYELEMENTALANALYSIS ( EXTENDEDDATAFIG.5 ANDSUPPLEMENTARY DISCUSSION) OF THIS REGION SHOWSOESOPHAGUS,FOREGUT (‘OS’ AND ‘FG’, RESPECTIVELY, INFIGS.1C , 2 CANDEXTENDEDDATAFIGS .1D, 2B , 3N, O , 4D , 5B, D )ANDPOSSIBLEASSOCIATEDNERVOUSTISSUES (‘NB’,‘ND’ AND ‘NF’ INFIGS. 1C , 2C, GANDEXTENDEDDATAFIGS .1A, 2B , 3N,O , 4A,D , 5B,D ). THEANTERIORMOST OFTHESEISPUTATIVENERVOUSTISSUEBETWEENTHEOESOPHAGEALANDOCULAR REGIONS (‘NB’ INFIGS. 1C , 2 CANDEXTENDEDDATAFIGS .1D, 2B , 3N, O , 4D , 5B, D ),ANDSITUATEDPOSTERIORLY ARENERVESINTOTHEFRONTALMOST APPENDAGES ANDDIFFERENTIATED POST-ORALAPPENDAGES (‘NF’ AND ‘ND’ INFIGS. 1C , 2C,G ANDEXTENDEDDATAFIGS. 1D , 2B , 3N, O , 4D , 5B, D ). TAKEN TOGETHER,THE POST-OCULARFRONTALMOST APPENDAGESOF Kylinxia ANDTHE OESOPHAGEAL POSITION OF THEIR NERVES ARE MOST CONSISTENTWITH ADEUTEROCEREBRAL IDENTITY,THEDEFININGFEATUREOF DEUTEROPODA 7, 22.

Phylogeneticimplicationsof Kylinxia

IN SUMMARY, Kylinxia HAS A CHIMERIC BODY PLAN THAT COMBINES KEY MORPHOLOGICALFEATURES OF Opabinia , RADIODONTA ANDDEUTEROPODA ( ESPECIALLYMEGACHEIRA) ( FIG. 3B ANDEXTENDEDDATAFIG. 8 ). TORESOLVE ITS PHYLOGENETIC POSITION AMONG EUARTHROPODS, WEBUILT A MORPHO- LOGICAL MATRIXBY ASSEMBLING CHARACTERSFROM PUBLISHED DATASETS OF PANARTHROPODPHYLOGENY 6, 8 (THECHARACTERLISTANDCOMPLETEREFERENCE LIST AREPROVIDEDINTHESUPPLEMENTARYDISCUSSION).OURPHYLOGENETIC RECONSTRUCTIONRESOLVES Kylinxia ASTHEMOSTBASALDEUTEROPOD AND AS A TRANSITIONALTAXONTHATBRIDGESBETWEENRADIODONTAANDDEUTEROPODA ( FIG.3A , EXTENDEDDATAFIG.9 ).THISPHYLOGENETICPLACEMENTOF Kylinxia IS STRONGLYSUPPORTEDBY ACONFIGURATIONOFEYESSIMILARTOTHATOF Opabinia, RADIODONT-LIKEFRONTALMOST APPENDAGES AND ADEUTEROPOD BODY THAT FEATURES AFUSEDHEAD SHIELD,AN ARTHRODIZED TRUNK,AFUSED PYGIDIUM ANDJOINTEDENDOPODITES.PLACEDNEARTHEROOTOFDEUTEROPODA, Kylinxia OFFERS A KEY REFERENCENODE FOR EXPLORINGTHE ORIGINS OFSEVERAL CRITICAL EVOLUTIONARYNOVELTIES DURING THE EARLY EVOLUTION OF EUARTHROPODS, INCLUDING THECOMPLETE ARTHRODIZATION OFTHE BODY,ARTHROPODIZATION OFTHETRUNKAPPENDAGES ANDCEPHALIZATIONOF AMULTI-SEGMENTEDHEAD, WHICHAREABSENTIN RADIODONTA 7 BUTPRESENTIN Kylinxia .

Kylinxia ISHELPFULINRESOLVINGTHE INTERRELATIONSHIPS AMONGMAJOR DEUTEROPODASTEM GROUPS.OUR PHYLOGENY RECOVERS A VERY BASALPAR- APHYLETIC LINEAGE OF DEUTEROPODA FEATURING TYPICAL RAPTORIAL FRON- TALMOST APPENDAGES AND CONSISTINGOF MEGACHEIRA, PANCHELICERATA, GREAT-APPENDAGEBIVALVEDFORMS ANDISOXYIDA ( FIG. 3AANDEXTENDED DATA FIG. 9 ). SUCH ABASAL POSITION OF MEGACHEIRA IS LARGELY A CONSE- QUENCE OF INCLUDING Kylinxia , WHICH COMBINES Opabinia , RADIODONT ANDMEGACHEIRANCHARACTERISTICS,INOUR ANALYSIS.A CLOSEPHYLOGENETIC LINKBETWEEN RADIODONTAAND MEGACHEIRAHASBEEN SUGGESTEDONTHE BASISOFTHEIRMORPHOLOGICALSIMILARITIESINFRONTALMOST APPENDAGES 14, 23, BUT IN THAT CONTEXT, RADIODONTA ISCONSIDERED A GROUP THATLEADS TO THE ORIGINOFONLYTHE CHELICERATA 14, 23 RATHERTHANTOTHE ORIGINOF ALL DEUTEROPODS 6, 8, 9. ACOMPARABLEBASALPLACEMENTOFPARAPHYLETICMEG- ACHEIRANS AND A TIGHT PARAPHYLETIC LINK BETWEEN MEGACHEIRANS AND PANCHELICERATESHAVEALSOBEENREPORTEDINARECENT STUDY 9. HOWEVER,IN THAT ANDOTHERCONTRIBUTIONS,ISOXYIDS 8, 9 ANDEVEN HYMENOCARINES 6 ARE RECONSTRUCTED ASMOREBASALTHANMEGACHEIRANS,ALTHOUGHTHISPLACE- MENTREMAINSUNDER DEBATE 4, 24. OURPHYLOGENETICINFERENCEINSTEAD FAVOURS A POSITIONOFISOXYIDA ABOVEMEGACHEIRA ANDPANCHELICERATA, WITHISOXYIDAANDGREAT-APPENDAGE BIVALVEDFORMSEITHERPARAPHYLETIC OR MONOPHYLETIC ( FIG. 3A , EXTENDED DATA FIG. 9 AND SUPPLEMENTARY

DISCUSSION).OUR EXPERIMENTALANALYSISWITH Kylinxia OMITTEDRECOVERS THEBASALPLACEMENTOFISOXYIDSFOUNDINPREVIOUSSTUDIES 8, 9 (EXTENDED DATA FIG. 10 , SUPPLEMENTARYDISCUSSIONAND SUPPLEMENTARY DATA 2), EMPHASIZINGTHEINFLUENCEOF Kylinxia ONTHEEUARTHROPODPHYLOGENY.

Evolutionof arthropodfirstappendages

THE EVOLUTION OF THE FRONTALMOST APPENDAGES HAS BEEN A KEY ISSUE IN RESOLVING THE ORIGIN AND EARLY EVOLUTION OF EUARTHROPODS 3, 24, 25.

THE RADIODONT-LIKEFRONTALMOST APPENDAGES ON AMEGACHEIRAN-LIKE BODYIN Kylinxia PROVIDES STRONGEVIDENCEFORTHE HOMOLOGYBETWEEN RADIODONT ANDMEGACHEIRAN FRONTALMOST APPENDAGES.UNDER THEPAR- SIMONYCRITERION,THESHAREDCHARACTERSINFRONTALMOST APPENDAGESOF RADIODONTAAND Kylinxia ARE VERY UNLIKELYTO BE CONVERGENT.DESPITE THEMISMATCHEDSEGMENTALAFFINITIESOFTHEFRONTALMOST APPENDAGESIN RADIODONTA 26, AND Kylinxia ANDTHEMEGACHEIRANS 22,THEIRHOMOLOGYIS HERECONSIDEREDMOSTLIKELY(SEECONTROVERSIES AND ALTERNATIVEHYPOTH- ESES 14, 19, 23 INTHESUPPLEMENTARYDISCUSSION).NOTABLY,OURPHYLOGENETIC TESTS SHOW THAT THEUNCERTAINTIES IN CODING THE SEGMENTAL AFFINITY OF RADIODONTFRONTALMOST APPENDAGESDONOTINFLUENCETHEPHYLOGENETIC POSITION OF Kylinxia NORTHE OVERALLTREETOPOLOGY.

THEVERY BASALPHYLOGENETIC PLACEMENT OF Kylinxia AND OTHER DEU- TEROPODS THATHAVE TYPICALRAPTORIALFRONTALMOST APPENDAGESFAVOURS THEIDEATHATRAPTORIALFRONTALMOST APPENDAGES WEREAPLESIOMORPHIC FEATUREOF ANCESTRALDEUTEROPODS 8, 9, 14, 23 (‘A’–‘D’ INFIG. 3A ), INCONTRAST TO THE ALTERNATIVE ANTENNIFORM HYPOTHESIS 6, 21. SUCH ABASAL GROUP- ING ALSOCASTS NEW LIGHTON THE ORIGINS OF FRONTALMOST APPENDAGES IN MAJOR EUARTHROPOD CROWN GROUPS:THE CHELICERAE INCHELICERATA AND THE(FIRST) ANTENNAEINMANDIBULATA.THESISTERGROUPINGOFMEGACHEIRA AND PANCHELICERATA IN OUR PHYLOGENY HAS GAINED PALAEONEUROLOGICAL EVIDENCEFROMACHELICERATEGROUNDPATTERNINTHEMEGACHEIRANBRAIN 22, ALTHOUGHALTERNATIVE SCENARIOS SUCH AS A CLOSE PARAPHYLETIC GROUPING OFMEGACHEIRANS ANDPANCHELICERATES 9 AND ATRADITIONALARTIOPODAN AFFINITYOFCHELICERATA EXIST 6. THUS, OURPHYLOGENETICTOPOLOGYSUP- PORTSTHEORIGINOF CHELICERATAFROM BASALDEUTEROPODSWITH RAPTORIAL FRONTALMOST APPENDAGES AND, IN THISSENSE,CHELICERAEWEREMODIFIED FROMMEGACHEIRANGREAT APPENDAGES 14, 22, 23 (‘C1’ AND ‘C2’ INFIG. 3A ), IN CONTRAST TO THE MODEL INWHICH CHELICERAE EVOLVED FROM SMALL SENSO- RIALANTENNAE 9.

OURPHYLOGENETICRECONSTRUCTIONRETRIEVES ANUPTREECLADEOFDEUTER- OPODSWITHTYPICALANTENNAEASTHEIRFIRST APPENDAGES,WHICHCOMPRISES ARTIOPODA,FUXIANHUIIDA ANDMOREMANDIBULATE-RELATEDGROUPS(‘E1’– ‘E3’ IN FIG. 3AANDEXTENDEDDATAFIG .9; SUPPLEMENTARY DISCUSSION).A SIMILARMONOPHYLYOFTHESEGROUPSWASREVEALEDRECENTLY 9, DESPITETHE FACTTHATOTHERIDEASHAVEBEENPROPOSED 6.ITISNOTABLETHATTHEANTENNAE INSOMEARTIOPODANS 18, 27 (‘E1’ INFIG.3 AANDEXTENDEDDATAFIG .7P, Q) AND HYMENOCARINES 28 (‘E3’ INFIG.3A ) POSSESSPAIREDNEEDLE-SHAPEDENDITIC SPINES AND THESEHAVE BEENINTERPRETEDTOHAVE A PREDATORYFUNCTION. HOWEVER,ALMOSTIDENTICALANTENNALFORMS AREPRESENTIN ISOXYIDSSUCH AS ISoxySVoluCriS AND ISoxySaurituS 17, 29 (‘D3’ INFIG. 3AANDEXTENDED DATA FIG. 7L–O ), WHEREAS THE FRONTALMOST APPENDAGES OF OTHER ISOXY- IDSEXHIBITLESS ANTENNIFORM FEATURES 16, 17, 30 (‘D1’ AND ‘D2’ INFIG. 3A AND EXTENDED DATAFIG. 7J, K ). GIVEN THE INTERMEDIATE POSITION OF ISOXYIDA BETWEENTHEDEUTEROPODSWITHRAPTORIALFRONTALMOST APPENDAGES AND THOSE WITH TYPICAL ANTENNAE ON OUR TREES ( FIG. 3A AND EXTENDEDDATA FIG. 9 ), THE REMARKABLE MORPHOLOGICAL VARIATIONS SHOWN BY ISOXYID FRONTALMOST APPENDAGESINDICATETHATTHEFIRST ANTENNAEOFMANDIBULATA MIGHTHAVE BEENDERIVEDFROMRAPTORIALPROTOTYPESRESEMBLINGTHOSE IN ISOXYIDA (‘D1’–‘D3’ IN FIG. 3A ).

THE Kylinxia DEMONSTRATES THE IMPORTANT ROLE OFTRANSITIONAL FOS- SILS IN RESOLVING THE EARLY HISTORY OF EUARTHROPODS. THE EMERGENCE OF STEM-GROUP EUARTHROPODS WITH MORPHOLOGICALLY AND FUNCTIONALLY DIVERSEFRONTALMOST APPENDAGES ILLUSTRATESTHE BROADEXPLORATION OF MORPHOSPACEAND ECOSPACE BY EARLYEUARTHROPODS DURING THECAM- BRIAN EXPLOSION 30, WHICHPROBABLYLAIDTHEFOUNDATIONFORTHEIR LATER EVOLUTIONARY SUCCESSES.

Figures

Fig.2 |Comparative anatomy ofhead structures inKylinxia zhangi, Opabiniaregalis, radiodonts andmegacheirans. a, b, O. regaliS. a, USNM (NATIONAL MUSEUMOF NATURAL HISTORY)155600B,HEAD REGION.b, USNM205258, EYES.c, K.Zhangi, NIGP 171305B,HEAD REGION ANDEYES.d–f, RADIODONTS. d, AnOmalOCariS SP.,ELRC (EARLYLIFE RESEARCH CENTRE) 20001A,HEAD REGION. e, f, ENDITES OF RADIODONTFRONTAL APPENDAGES.e, RamSkOeldia COnSimiliS, NIGP 162527A.f, ParanOmalOCariS multiSegmentaliS, NIGP 154564B.g–i, K.Zhangi. g, YLSNHM 01124,HEAD REGION INFIG.1B.THE BOXEDREGION IS SHOWN INh. h, i, ENDITES OF THE FRONTALMOSTAPPENDAGES.h, YLSNHM 01124,ENDITES FROM g. i, NIGP 171305A,ENDITE FROM FIG.1C. j–m, MEGACHEIRANS.j, HaikOuCariS erCaienSiS, NIGP EC44154A,HEAD REGION.k–m,ENDITES OF MEGACHEIRANGREAT APPENDAGES.NOTE THENUMEROUS AUXILIARY SPINES ON THEENDITES.k, H. erCaienSiS, NIGP 171309.l, FOrtifOrCepSfOliOSa, NIGP 169954.m, ParapeytOia yunnanenSiS, NIGP 171310.ARROWHEADS INDICATE AUXILIARY SPINES.SCALE BARS, 5 MM (d), 2 MM(a, g, j), 1MM (b, c, e, h, m), 200 ΜM(f, i, k, l). AM,ARTHRODIAL MEMBRANE;DC,DISTAL CLAW;DF,DIFFERENTIATEDFLAPS;DS,DORSAL SCLERITE;ED,ENDITE ONDISTAL ARTICULATEDPODOMERE;F,BODYFLAP;GA,GREATAPPENDAGE;HD,HEAD; HR,MARGINAL RIMOF HEAD SHIELD;IE,INNER EYE;LE,LATERAL EYE;MO,MOUTHOPENING; MS,MAIN SPINE;OE,OUTER EYE;PB,PROBOSCIS.OTHER ABBREVIATIONS AS INFIG.1. NUMBERSREFER TOTHE TOPOLOGICAL POSITION OF THE CORRESPONDINGSTRUCTURE.

Fig. 3|Reconstruction ofKylinxia and evolutionof thefrontalmost appendages inearly euarthropods. a, SIMPLIFIED CLADOGRAMOF PANARTHROPODS BASED ON AMATRIX OF 81 TAXAAND 283 CHARACTERS (EXTENDED DATAFIG.9 AND SUPPLEMENTARY DATA 1)SHOWING KylinxiaIN RED.DOUBLE LINES INDICATE PARAPHYLETIC GROUPINGS.THEGROUPING OF GREAT-APPENDAGEBIVALVED FORMS AND ISOXYIDA IS RECONSTRUCTED AS PARAPHYLETIC AND MONOPHYLETICIN BAYESIAN AND PARSIMONY ANALYSES,RESPECTIVELY (INDICATEDBY THE DASHEDLINES AND QUESTION MARK).DIAGRAMSONTHE LEFTDEPICTTHE FRONTALMOSTAPPENDAGESIN REPRESENTATIVE TAXA.THE BOXES,COLOURS AND LETTERSA–E INDICATE DIFFERENT APPENDAGEFORMS. A,B,RADIODONT-LIKE FORMS.A,RADIODONT (PURPLE),RamSkOeldiaCOnSimiliS; B,Kylinxia Zhangi(RED).C,GREAT-APPENDAGE FORMS (YELLOW).C1,MEGACHEIRAN, FOrtifOrCepS fOliOSa; C2, PANCHELICERATE,LimuluS pOlyphemuS; C3, GREAT- APPENDAGE BIVALVED EUARTHROPOD,OCCaCariSOVifOrmiS. D,ANTENNA-LIKE RAPTORIAL APPENDAGES INISOXYIDA,SHOWINGINTERSPECIFIC VARIATIONS(GREEN). D1, ISOxySaCutanguluS; D2, ISOxySCurVirOStratuS; D3, ISOxyS aurituS. E,TYPICAL ANTENNAE INCAMBRIAN EUARTHROPODS,INCLUDING TYPICALSENSORIAL FORMS AND SOME PUTATIVERAPTORIAL FORMS.E1,ARTIOPODAN,Kuamaia lataAND KiiSOrtOQia SOperi; E2,FUXIANHUIID,ShankOuia ZhengheiAND OTHERS;E3,HYMENOCARINE, ClyperCariS Serrata. b, ARTISTIC RECONSTRUCTIONBY J.SUN.MORPHOLOGICAL DETAILS OF THE LATERAL TAIL FLAPSWERE INFERRED FROM MEGACHEIRANS SUCH AS FOrtifOrCepS.

ExtendedData Fig.4 |Drawingsof thehead regionin Kylinxia and comparison offrontalmost appendages in Kylinxia andAnomalocaris. a–g, K.Zhangi. a–e, CAMERALUCIDA DRAWINGSOF HEAD REGIONS,SEE COLOURS OF DIFFERENT ANATOMICALSTRUCTURES AT THE BOTTOM.a, YLSNHM 01124,AS IN FIG.2G. b, NIGP 171304A,AS IN EXTENDED DATAFIG.2D. c, NIGP 171307A,AS IN EXTENDED DATA FIG.2F.d, NIGP 171305A,AS IN EXTENDEDDATA FIG.2B.e, NIGP 171308,AS IN EXTENDED DATA FIG.2I. f, g, FRONTALMOST APPENDAGES OF Kylinxia. f, YLSNHM 01124.g, NIGP 171304A.h–k, FRONTAL APPENDAGESOF AnOmalOCariS CanadenSiS. h,i, FULLAPPENDAGES.h, GSC 75535.i, ROMIP 51212.j, k, SHAFT REGION.j, ROMIP 51215.k,ROMIP 59947.ABBREVIATIONS AS IN FIGS.1, 2 ANDEXTENDED DATA FIG.2.

ExtendedData Fig.5 | Anatomyand geochemical analysisof cephalic soft tissuesin Kylinxia zhangi,holotypeNIGP 171304a. a, MAGNIFICATION OF THE HEAD ANDANTERIORMOST TRUNK UNDER VISIBLE LIGHT.b, INTERPRETIVE DRAWINGOF a BY INTEGRATING OBSERVATIONS UNDER VISIBLE LIGHT ANDFROM GEOCHEMICAL ANALYSIS, SEE EXTENDED DATAFIG.4 FOR ACOLOUR KEY.c, BACKSCATTER SCANNING ELECTRON IMAGE OF a UNDER ASCANNING ELECTRONIC MICROSCOPE.d–h, ELEMENTAL MAPS OF a FROM ENERGY-DISPERSIVE X-RAY SPECTROSCOPY.d, CMAP,SHOWING ALIMENTARY CANAL WITHPOSITIVESIGNAL.e, FE MAP,SHOWINGNERVOUS TISSUEWITH POSITIVE SIGNAL.f, OVERLAY IMAGE OF CAND FE MAPS WITHTHE BLENDING MODE OF FILTERING COLOUR IN ADOBE PHOTOSHOP CS6,SHOWING THECOMPOSITIONAL DIFFERENCES BETWEEN ALIMENTARY CANAL AND NERVOUS TISSUES.g, PMAP,SHOWINGPUTATIVE DIGESTIVE GLANDS WITH POSITIVESIGNAL.h, SI MAP,SHOWING SOFTTISSUES INNEGATIVE RELIEF.PR,PROTOCEREBRAL TISSUE.OTHER ABBREVIATIONS AS INFIGS.1, 2.

ExtendedData Fig.6 |Morphological details oftrunk in Kylinxiazhangi. d, NIGP 171306.e–h, TAIL REGION.e, NIGP 171307A.f, NIGP 171308.g, NIGP a–d, SOFT TISSUEIN TRUNK.ARROWHEADS INDICATE PUTATIVEBIFURCATING NERVESINTO 171304A.h, YLSNHM 01124.i, MAGNIFICATIONOF THE TAIL FLAPS INg, SHOWINGSETAE. PAIREDTRUNK APPENDAGES.a, YLSNHM 01124.b, NIGP 171304A.c, NIGP 171305A. A,TRUNK APPENDAGE;ST,SETAE.OTHERABBREVIATIONS AS INFIGS.1, 2.

Fig.1 |Anatomy ofKylinxiazhangi from the earlyCambrian Chengjiang biota. a, NIGP 171304A.BOXED REGIONSARESHOWN INc, d, g. b, YLSNHM 01124. BOXED REGIONS ARESHOWN IN e, f, h. THE REGIONWITH ADASHED OUTLINEIS SHOWN IN FIG.2G.c,HEAD REGION INa. THEREGION WITHA DASHEDOUTLINE IS SHOWN INFIG.2I. d, e,TRUNK APPENDAGESFROM a, b, RESPECTIVELY.f, DIFFERENTIATED APPENDAGES FROM b. g, h, POSTERIORBODY REGION FROMa, b, RESPECTIVELY.ARROWHEADS INDICATE ARTHRODIAL MEMBRANES BETWEEN ENDOPODITEPODOMERES.SCALEBARS,10 MM

Extended Data Fig.7 | Frontalmost appendages of Cambrian megacheirans, isoxyids and artiopodans. a–i, GREAT APPENDAGESOF MEGACHEIRANS.a, d, HaikOuCariS erCaienSiS, NIGP 171309.a, WHOLE SPECIMEN. d, GREAT APPENDAGEIN a.b, e,FOrtifOrCepS fOliOSa,NIGP 169954.b, WHOLE SPECIMEN.e, GREAT APPENDAGEIN b. c, f, g, YOhOia tenuiS, USNM179053.c, WHOLE SPECIMEN.f, GREATAPPENDAGE INc. g, ENDITES IN f, ARROWHEADS INDICATE AUXILIARY SPINES.h, i,ParapeytOia yunnanenSiS, NIGP 171310.h, GREATAPPENDAGE. i, ENDITE SHOWING AUXILIARY SPINES INh. j–q, FRONTALMOST APPENDAGESSHOWING

ExtendedData Fig.8 |Ecological reconstructionof Kylinxiazhangi in the earlyCambrian Chengjiang biota. ARTISTIC RECONSTRUCTIONBY J.SUN.

ExtendedData Fig.10 | Consensus trees fromBayesianand parsimony analyses ofpanarthropod relationshipswith Kylinxia zhangiomitted. a,THE 50%MAJORITY-RULE CONSENSUS TREEFROM A BAYESIAN ANALYSIS.NODAL SUPPORTSARE POSTERIORPROBABILITIES.MAJOR TAXONOMICGROUPS ARE INDICATED BY BARS ONTHE RIGHTOF TIPS.b, STRICT CONSENSUS OF ASINGLE MOST PARSIMONIOUSTREE OF 49.60546 STEPS(CONSISTENCY INDEX =0.518,RETENTION INDEX= 0.874) FROM A PARSIMONY ANALYSIS USING IMPLIED CHARACTER WEIGHTING (CONCAVITY CONSTANT k = 3). NODAL SUPPORTS AREGROUPPRESENT ANDCONTRADICTED FREQUENCY DIFFERENCES.IN a, b, NODALSUPPORTS OF 100% ARENOT SHOWN.NOTE THATISOXYIDA AND GREAT-APPENDAGE BIVALVED FORMS(BOTHIN RED) ARE PLACEDAT A MORE BASAL POSITION THANMEGACHEIRA AND PANCHELICERATA WHEN K.Zhangi IS OMITTED,WHICH IS A MAJORDIFFERENCE FROM THE RESULTSWITH K.Zhangi INCLUDED (EXTENDED DATA FIG.9).