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

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.