An early Cambrian euarthropod with radiodont-like raptorial appendages Author Zeng, Han StateKey Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China; Department of Paleobiology, National Museum of Natural History, SmithsonianInstitution, Washington, DC, USA Author Zhao, Fangchen StateKey Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China; College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China. fczhao@nigpas.ac.cn Author Niu, Kecheng Yingliang Stone Natural History Museum, Nan’an, China. Author Zhu, Maoyan StateKey Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China; College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China. Author Huang, Diying StateKey Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China. dyhuang@nigpas.ac.cn text Nature 2020 2020-11-04 588 101 105 journal article https://DOi.ORg/10.1038/s41586-020-2883-7 c6316cf7-316b-4958-b2be-f25d4f8021dc 4717877 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. 1State Key Laboratory of Palaeobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences,Nanjing,China.2Center for Excellence in Life and Palaeoenvironment,Chinese Academy of Sciences,Nanjing,China.3Department of Paleobiology,National Museum of Natural History, Smithsonian Institution,Washington,DC,USA.4College of Earth and Planetary Sciences, Universityof Chinese Academyof Sciences,Beijing, China.5 Yingliang Stone Natural History Museum,Nan’an,China. ✉e-mail: fczhao@nigpas.ac.cn ; dyhuang@nigpas.ac.cn Fig.1 |Anatomyof Kylinxiazhangi from theearly Cambrian Chengjiang biota. a, NIGP 171304A.BOXEDREGIONSARESHOWNIN c, d, g. b, YLSNHM 01124. BOXEDREGIONSARESHOWNIN e, f, h. THEREGIONWITHADASHEDOUTLINEISSHOWNIN FIG.2G.c,HEADREGIONIN a. THEREGIONWITHADASHEDOUTLINEISSHOWNINFIG.2I. d, e,TRUNK APPENDAGESFROM a, b, RESPECTIVELY.f, DIFFERENTIATED APPENDAGES FROM b. g, h, POSTERIORBODYREGIONFROM a, b, RESPECTIVELY.ARROWHEADSINDICATE ARTHRODIALMEMBRANESBETWEENENDOPODITEPODOMERES.SCALEBARS,10 MM (a, b), 2 MM (d, e, g,h), 1 MM (c, f). AC,ALIMENTARY CANAL;AE,ANTERIOR EYE; DA,DIFFERENTIATED APPENDAGE;DG,DIGESTIVEGLAND;EC,ELIMINATEDGUTCONTENT; EN,ENDOPODITE;ES,EYESTALK;EX,EXOPODITE;FA,FRONTALMOST APPENDAGE; FG,FOREGUT;HG,HINDGUT;HS,HEADSHIELD;NB,NERVOUSTISSUEBETWEEN OESOPHAGEALANDOCULARREGIONS;ND,NERVEINTODIFFERENTIATED APPENDAGE;NF, NERVEINTOFRONTALMOSTAPPENDAGE;NT,NERVEINTOTRUNKAPPENDAGE; OS,OESOPHAGUS;P, PODOMEREOFFRONTALMOST APPENDAGE;PE,POSTERIOREYE; PT,PROTOPODITE;PY,PYGIDIUM;RM,RIMOFEYE;SA,OBLIQUEARTHRODIALMEMBRANE OFSHAFT;SE,SHAFTENDITE;SH,SHAFTOFFRONTALMOST APPENDAGE;T,TRUNKTERGITE; TF,TAILFLAP;VN,VENTRALNERVECORD.NUMBERSREFERTOTHETOPOLOGICALPOSITIONOF THECORRESPONDINGSTRUCTURE. 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 Fig.2 |Comparativeanatomyofheadstructuresin Kylinxiazhangi , Opabiniaregalis , radiodontsandmegacheirans. a, b, O . regaliS . a, USNM (NATIONALMUSEUMOFNATURALHISTORY)155600B,HEADREGION.b, USNM205258, EYES.c, K.zhangi , NIGP 171305B,HEADREGIONANDEYES.d–f, RADIODONTS. d, AnomaloCariS SP.,ELRC (EARLYLIFERESEARCHCENTRE) 20001A,HEADREGION. e, f, ENDITESOFRADIODONTFRONTALAPPENDAGES.e, RamSkoeldia ConSimiliS , NIGP 162527A.f, ParanomaloCariSmultiSegmentaliS , NIGP 154564B.g–i, K. zhangi . g, YLSNHM 01124, HEADREGIONINFIG.1B .THEBOXEDREGIONISSHOWNIN h. h, i, ENDITESOFTHEFRONTALMOSTAPPENDAGES.h, YLSNHM 01124,ENDITESFROM g. i, NIGP 171305A, ENDITEFROMFIG.1C. j–m , MEGACHEIRANS.j, HaikouCariS erCaienSiS , NIGPEC44154A,HEAD REGION.k–m,ENDITES OFMEGACHEIRANGREAT APPENDAGES.NOTETHENUMEROUS AUXILIARYSPINESONTHEENDITES.k, H . erCaienSiS , NIGP 171309.l, FortiforCepSfolioSa , NIGP 169954.m, Parapeytoia yunnanenSiS , NIGP 171310.ARROWHEADSINDICATEAUXILIARY SPINES.SCALEBARS, 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 ONDISTALARTICULATEDPODOMERE;F,BODYFLAP;GA,GREATAPPENDAGE;HD,HEAD; HR,MARGINALRIMOFHEADSHIELD;IE,INNEREYE;LE,LATERAL EYE;MO,MOUTHOPENING; MS,MAIN SPINE;OE,OUTEREYE;PB,PROBOSCIS.OTHERABBREVIATIONS ASINFIG.1 . NUMBERSREFERTOTHETOPOLOGICALPOSITIONOFTHECORRESPONDINGSTRUCTURE. ( 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 . Fig. 3|Reconstruction of Kylinxia andevolutionof thefrontalmost appendages inearlyeuarthropods. a, SIMPLIFIEDCLADOGRAMOFPANARTHROPODS BASEDONAMATRIXOF 81 TAXAAND 283 CHARACTERS (EXTENDEDDATAFIG.9 AND SUPPLEMENTARYDATA 1)SHOWING Kylinxia INRED.DOUBLELINESINDICATE PARAPHYLETICGROUPINGS.THEGROUPINGOFGREAT-APPENDAGEBIVALVEDFORMS AND ISOXYIDAISRECONSTRUCTED ASPARAPHYLETICANDMONOPHYLETICINBAYESIANAND PARSIMONY ANALYSES,RESPECTIVELY (INDICATEDBYTHEDASHEDLINES ANDQUESTION MARK).DIAGRAMSONTHELEFTDEPICTTHEFRONTALMOSTAPPENDAGESINREPRESENTATIVE TAXA.THEBOXES,COLOURS ANDLETTERSA–EINDICATEDIFFERENT APPENDAGEFORMS. A,B,RADIODONT-LIKEFORMS.A,RADIODONT (PURPLE), RamSkoeldia ConSimiliS ; B, Kylinxiazhangi (RED).C,GREAT-APPENDAGEFORMS (YELLOW).C1,MEGACHEIRAN, FortiforCepSfolioSa ; C2, PANCHELICERATE, LimuluS polyphemuS ; C3, GREAT- APPENDAGEBIVALVEDEUARTHROPOD, OCCaCariSoViformiS . D,ANTENNA-LIKE RAPTORIALAPPENDAGESINISOXYIDA,SHOWINGINTERSPECIFICVARIATIONS(GREEN). D1, ISoxySaCutanguluS ; D2, ISoxySCurViroStratuS ; D3, ISoxySaurituS . E,TYPICAL ANTENNAEINCAMBRIANEUARTHROPODS,INCLUDINGTYPICALSENSORIALFORMS AND SOMEPUTATIVERAPTORIALFORMS.E1,ARTIOPODAN, Kuamaialata AND KiiSortoQia Soperi ; E2,FUXIANHUIID, Shankouia zhenghei ANDOTHERS;E3,HYMENOCARINE, ClyperCariS Serrata . b, ARTISTICRECONSTRUCTIONBY J.SUN.MORPHOLOGICALDETAILS OFTHELATERALTAILFLAPSWEREINFERREDFROM MEGACHEIRANSSUCHAS FortiforCepS . 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.