Received 15January 2002 Accepted 13February 2002 Publishedonline 30 May 2002

Thearthropod Offacolus kingi (Chelicerata)from the Silurianof Herefordshire, England:computer basedmorpholog icalreconstruct ionsand phylogeneticafŽ nities Mark D.Sutton 1,DerekE. G.Briggs 2,David J.Siveter 3, DerekJ. Siveter 1,4* and Patrick J.Orr 5 1Departmentof Earth Sciences, University ofOxford, Parks Road, Oxford OX1 3PR, UK 2Departmentof Earth Sciences, University ofBristol, Queen’ s Road,Bristol BS8 1RJ, UK 3Departmentof Geology, University ofLeicester, University Road,Leicester LE1 7RH, UK 4GeologicalCollections, University Museum ofNatural History, Oxford OX1 3PW, UK 5Departmentof Geology, National University ofIreland,Galway, University Road,Galway, Republic of Ireland The small, non-biomineralized, three-dimensionally preservedarthropod Offacoluskingi Orr et al. from theWenlock Series (Silurian) ofHerefordshire, England, is re-evaluated,and the new family Offacolidae erected.This newstudy is basedon specimens which have beenserially ground,reconstructed by com- puterand rendered in theround as coloured models. Offacolus possessesa prosomal appendage array similar tothat of Limulus,butalso bears robustand setose exopods on appendages II– V whichare unlike thosefound in any other arthropods.Opisthosomal appendagesare similar in numberand morphology tothe book-gills of Limulus.Cladistic analysis places Offacolus basally within theChelicerata, asa sister taxon tothe eurypterids and extant chelicerates,but more derivedthan theDevonian Weinbergina . Keywords: Chelicerata; Offacolus;Offacolidae; Silurian; England

1. INTRODUCTION andhelp toresolve phylogenetic patterns(e.g. Wills et al. 1995). The HerefordshireLagersta ¨tte (Briggs et al. 1996) pre- servesa diversefauna of small invertebrates from the WenlockSeries ( ca.425 Myr ago) ofHerefordshire, 2. MATERIALAND METHODS England.The fossilspreserve soft-tissue surfaces in spec- The fossilsof the HerefordshireLagersta ¨tte occurin con- tacular three-dimensional(3D) detail, butthis morpho- cretions,preserved in 3Dinsparry calcite,which precipitatedin logical information isvery difŽcult to extract. Initial the voidleft after the decayof the animal(Orr et al. 2000a). descriptive workwas based on specimensexposed by frac- Splitting the concretionsyielded randomly oriented planar or turing theconcretions in which they occur,supplemented sub-planar sections(e.g. Ž gure2 g),which werethen photo- by partial excavation. In this manner,Orr et al. (2000b) graphed underwater using incidentlight and adigitalcamera produceda descriptionof the most abundant species in mountedon a binocularmicroscope. Both part and counterpart theHerefordshire macrofauna, the arthropod Offacolus of selectedspecimens were serially ground at 30 m mintervals, kingi.That studydetermined the gross morphology of and photographed digitallyto producesets of‘sliceimages’ (e.g. Offacolus,butmany important details werenot resolved. Ž gure 2h,i).These datasets wereused to generate3D com- For this reasonno formal investigation ofrelationships puterizedreconstructions of the fossils(Sutton et al. 2001b), in wasattempted, although achelicerate afŽnity waspro- which part and counterpart werereunited into a singlemodel posed.Subsequent investigations ofHerefordshire fossils (thetransverse cracksmost noticeablein Ž gures 1 d, 2c mark (Sutton et al. 2001a,c)have employed a3D technique joinsbetween part and counterpart).Datasets underlyingthese basedon serial grinding andcomputerized reconstruction. modelswere edited to removemost extraneous material,and In this paper, weapply anenhanced variant ofthis tech- wherepossible to resolvefossil/ matrix ambiguities.A reŽnement nique to Offacolus in order toillustrate andelucidate its introducedwith the presentstudy generatesfalse-coloured morphology fully, andto allow ananalysis ofits afŽnities. reconstructions(e.g. Ž gure1 a),which greatly aidthe illustration There is currently noconsensus on the phylogeny ofthe ofmorphology, particularlywhen renderedas stereo-pairs.Dur- cheliceratesand their relatives (seeDunlop & Selden ing the editingstage, individualstructures (e.g. appendages, ter- (1997) andWeygoldt (1998) for discussion).An under- gites,etc.) were manually coded on eachslice, enabling the standingof the morphology of Offacolus isimportant, as computerto reconstructeach separately. The resultingsurfaces theaddition ofwell preservedfossil forms can provide werethen placedtogether intheir correct positions, separately critical data that clarify relationships betweenother taxa, colouredand rendered.The point at which the colourchanges, whereone structure meets another (e.g.an appendage and the body) isarbitrary, and ithas not always beenpossible to main- *Authorand address for correspondence: Geological Collections, University Museum of Natural History,Oxford OX13PW, UK tain consistency(compare appendage VIa, Žgures 1 a, 2a—the ([email protected]). extensionof body colourinto the endopodin the latteris an

Proc.R. Soc.Lond. B (2002) 269, 1195–1203 1195 Ó 2002 TheRoyal Society DOI10.1098/ rspb.2002.1986 1196M. D.Suttonand others 3Dfossil chelicerate

Figure 1. (Caption on p. 1198.)

Proc.R. Soc.Lond. B (2002) 3Dfossil chelicerate M.D.Suttonand others 1197

Figure 2. (Caption on p. 1198.)

Proc.R. Soc.Lond. B (2002) 1198M. D.Suttonand others 3Dfossil chelicerate

Figure 1. (See p. 1196.) Offacolus kingi Orr et al. (2000b); (a) IIen IIex stereo-pairs of computer reconstructions of OUMC.29557. IIen–VIen, IIex–VIex, VII, prosomal appendages(ex, IIIex exopod, en, endopod); TI –III, FI–III, mesosomal IVex tergites/appendages; C,carapace; Mt, metasoma. Colours (except c):body/exoskeleton, orange and brown; chelicerae, white; IIex–Vex, green; VIex, white; IIen –VIen, blue; VII, Vex green; FI–III, red. (a)Ventral view, ´ 15. (b)Antero-ventral view, ´ 15. (c)Oblique antero-ventral view, body/III outer (reconstructed asone object) and chelicerae, ´ 15. sunken (d)Dorsal view, ´ 15 (see also Ž gure 3a). (e)Oblique dorso- region VIen lateral viewof IIen –VIen and VII, ´ 23 (see also Ž gure 4a). (chela) ( f )Oblique lateral viewof carapaceand body anterior, carapace appendagesremoved, ´ 21 (see also Ž gure 3b). ( g) Oblique median antero-ventral view, appendagesremoved (attachment points median ridge sunken labelled), tail spine truncated, ´ 15. region

Figure 2. (Seep. 1197.) Offacolus kingi Orr et al. (2000b); labels mesosoma shoulder as in Ž gure 1. (a–f )Stereo-pairs of computer reconstructions. tergite I (a–c)OUMC.29558, colours asin Ž gure 1. (a) Ventral mesosoma view, ´ 17 (lackof basalcontinuity of appendage colour is tergite II an artefact (see § 2). (b)Oblique ventro-lateral view(lacking mesosoma anterior), appendagesexcept VIremoved, some lateral tergite III exoskeleton removed, VIcoxa/VIen/body reconstructed as metasoma tergite I one object, ´ 17. (c)Oblique lateral view, ´ 17. (d ) OUM lateral C.29559, ventro-lateral viewof single-object reconstruction, metasoma facets ´ 17. (e)OUMC.29557, dorsal viewof IIIex, ´ 18 (see also tergite II nodes Ž gure 4a), ( f )OUMC.29559, oblique postero-lateral view metasoma (anterior left, dorsal up) of VII, FI(orange) withsubsidiary tergite III metasoma

´ metasoma  aps(white, blue, red and bronze), 20. ( g–i)Photographs tergite V ( bulb’’ ) of 2Dsections. ( g)OUMC.29560, sub-sagittalsection, tergite IV tail spine ´ 15. (h–i)OUMC.29559. ( h)Sub-exsagittalsection (near midline), ´ 9. (i)Sub-exsagittalsection (more abaxialthan ´ (h)), 9. ( j )OUMC.29558, transverse section through (b) lateral spines mesosoma (VIIs, trailing setaeof appendage VII), ´ 18. axial spine

anterior artefact).This method alsointroduces a slight gap wheretwo body objects join(e.g. appendage bases, Ž gure 2a).Wherenecessary these gaps have beeneliminated by reconstructingboth objects lateral together, and renderingthem inone colour ( Ž gures 1c, 2b). shoulder Specimensof Offacolus arehoused inthe UniversityMuseum of Natural History, Oxford (OUM)and the Department of median ridge Geology,University of Leicester(LEIUG).

3. SYSTEMATICPALAEONTOLOGY median Subphylum: Chelicerata Heymons1901. Family: sunken postero- Offacolidae fam. nov. region shoulder outer ventral Diagnosis. Asfor genusand species. sunken spine Offacoluskingi Orr et al. 2000b. region 1996 newarthropod; Briggs, Siveter &Siveter, pp.248, 249, Ž g. 1a–c,e. Figure 3. Offacolus kingi Orr et al. (2000b);interpretations of 2000a,unnamedarthropod; Orr, Briggs, Siveter & external features, basedon computer reconstructions of Siveter, pp.173 –186, Ž gs 1a,b, 2–9. OUMC.29556. ( a)Dorsal viewbased on Ž gure 1d, ´ 17. (b)Oblique lateral viewof carapaceand anterior of body ¤ 2000bOffacoluskingi ;Orr, Siveter, Briggs &Siveter, withall appendagesremoved, basedon Ž gure 1f, ´ 20. pp. 1497–1504, Ž gs 1–4. Amended diagnosis. Chelicerate ca.2mm in widthwith parallel sidedcarapace; opisthosoma dividedinto meso-  ap-like, posteriorly directed.Opisthosomal appendages soma with threefree segments, metasoma of Ž ve fused recurvedand  ap-like: eachsegment of mesosoma with segments,basally articulated terminal spinewith distal single pair giving rise tothree or foursubsidiary nested articulation andbifurcation. Sevenpairs ofprosomal  aps;three pairs lacking subsidiary  apsarising from appendages:pair I(chelicerae) uniramous;II –VI biramous anterior ofmetasoma. with endopodgracile andchelate, exopod robust and Material. Holotype,OUM C.29501a,b (Orr et al. setoseexcept in VI, whereit istendril-like; VII uniramous, 2000b, Ž g. 2a,b).285 other specimens,four of whichhave

Proc.R. Soc.Lond. B (2002) 3Dfossil chelicerate M.D.Suttonand others 1199 beenground away andreconstructed in 3D (three Ž gured stoutspine ( Ž gures 1d, 2c).The ventral margin is weakly here,OUM C.29557 –9). Other Ž guredspecimens: OUM convexin lateral view,curving strongly anteriorly into the C.29500–1, LEIUG116110 (Briggs et al. 1996); OUM dorsal margin ( Ž gures 1g, 2b).Eachtergite is overlapped C.29507–10 (Orr et al. 2000a);OUMC.29506, 29515 – by thepreceding one (tergite Iis overlapped by the 24 (Orr et al. 2000b); OUMC.29560 (this paper). carapace), andthe length oftergite exposeddepends on the  exureof the opisthosoma. In OUMC.29558, tergite (a) Dorsalexoskeleton IIisangled ventrally with respectto tergite I( Ž gure 2c). (i) Carapace The antero-medianregion oftergite IIthusexposed is an High andparallel sidedwith wideanterior andventral articulating shelf( ‘half-ring’),dividedfrom therest of the gapes (Ž gures 1a,b,d,f,g, 2a,c),anda meanmaximum tergite by atransversefurrow steeper anteriorly than pos- widthof 2.21 mm (range 2.0 –2.4 mm, s.d. = 0.1 mm, teriorly. This morphology is assumedto be present in all n = 24). Maximum height averages 1.51 mm tergites.No external sculptureis present. (s.d. = 0.9 mm, n = 3) andmedian length along thedorsal surfaceaverages 2.08 mm (s.d. = 0.18 mm, n = 3). In (iii) Metasoma transversesection the carapace issub-semicircular with its This tagma consistsof probably Ž ve segmentsfused into widestpoint slightly dorsal oftheventral gape, andis simi- atapering unitaveraging 1.2 mm long (s.d. = 0.14 mm, lar in sizeand shape along its length, exceptnear the n = 3) in theplane ofthe ‘ ange’ (seebelow). It formsa anterior. The ventral margin isslightly convexin lateral sub-triangular structurein dorsal view,converging pos- view,curving dorsally intothe anterior margin ( Ž gure 1f ), teriorly at anangle of ca. 80° (Ž gures 1d, 2a, 3a). The at whichpoint themargin is turnedinwards to very slightly anteriormost tergite is similar tothose of the mesosoma, constrictthe gape ( Ž gure 1b).The posterior margin is andis overlapped by mesosomatergite III ( Ž gures 1d, 3a). transversein dorsal view ( Ž gure 1d)andlaterally extends Itsposterior margin is indistinctmedially, butis normally intoa shortand stout postero-ventral spine( Ž gures 1d,f, deŽ nedlaterally by afurrow( Ž gure 1d).The tergite is 2c, 3b).The anterior margin is weakly convexin dorsal slightly swollenmedially, particularly posteriorly, butis view andis sculpted into one axial andthree paired lateral notdeveloped into a discretenode. The posterior margins spines (Ž gures 1c,d,f,g, 2c, 3a,b),which diminish in size ofthesucceeding tergites are indistinct.In transversesec- laterally andproject between the exopods of appendages tionthe tergites are sub-semicircular,with alateral con- II–V(seebelow). The spinesare sub-triangular in form cave curvature forming aweakmarginal ‘ ange’. The andare initially directedadaxially butcurve abaxially dis-  ange correspondsin positionand form tothe ‘shoulder’ tally. The more median ofthese, at least,possess small in precedingtergites. In lateral view,the dorsal surfaceof subsidiary sub-triangular spinesnear eachbase (see thetagma curvesevenly tothe posterior margin ( Ž gure 2c) especially Ž gures 1d, 3a).The carapace thinsslightly andbears threepostero-dorsally directednodes that arise betweenthe marginal spines,forming internal shallow from theposterior oftergites II –IV ofthis tagma. The grooves which accommodatethe prosomal exopods nodesare inconsistentlypreserved, but that ontergite III (Ž gure 1f,g).The correspondingexternal thinning noted ofOUM C.29558 ( Ž gure 2c)is relatively spine-like.The by Orr et al. (2000b,p.1498) occursonly at theextreme posterior termination ofthe metasoma is marked by an anterior, although theanterior extensionof the ‘outer expanded ‘bulb’,from which thetail spineemerges sunkenregion ’ (seebelow) lies approximately above the (Ž gures 1a,d,g, 2a–d, 3a),andwhich probably represents groove ofthe exopod of appendage II. a Ž fth segment. The external surfaceof thecarapace bears several struc- turesof weak relief ( Ž gures 1d, f, 2c,3). Awidelow ridge (b) Bodyand ventral exoskeleton extendsalong mostof the axis ofthecarapace, dying out (i) Prosoma posteriorly in ashallow sub-semicircular depressionwhich Distinctionbetween the body and bases of appendages occupiesthe posterior third ofthe carapace. This median underthe carapace isdif Ž cult(much of what appears to sunkenregion isdelineated laterally by abroad break in be anterior ‘body’ structurein Ž gure 1g is almost certainly slopetermed the shoulder. The median ridge is  anked appendage),but body margins are more clearly recognized by aweakerdepression, the outer sunken region, which is medially. Ananterior bulge with parallel sub-planar lateral narrow anteriorly butexpands abruptly posteriorly. A margins protrudesbetween the endopods of appendage third weakly expressedbreak in slope,the lateral shoulder, II. This bulge extendsto approximately theplane ofthe runssub-parallel tothe postero-ventral margin. Noexter- prosomal ventral margin asa slightly elongate dome nal sculptureis visible. (Ž gures 1a,c,g, 2d).Immediately posterior tothis structure thereis adepression,forming anoral cavity surrounded (ii) Mesosoma bythecoxae ofappendages II –VII. Three similar tergites are present( Ž gures 1d,g, 2c, 3a), decreasingin sizefrom anterior toposterior andalso (ii) Opisthosoma decreasingslightly in height-to-width ratio. In transverse The mesosomabody region is notas deep (does not sectioneach tergite is sub-semicircular.The external sur- extendas far ventrally) asthat beneaththe carapace; these faceis divided by ashoulderinto a shallow median sunken regions are connectedby asmoothslope ( Ž gure 2b). The region apparently continuouswith that ofthe carapace, structuresvisible immediately posterior tothe base of anda near-vertical sub-planar lateral facet( Ž gures 1d, 3a). appendage VII in Ž gure 1a,g are notpart ofthebody (see In dorsal view,the lateral margins are straight, andthe below).The sternitesof the mesosoma are, in transverse posterior margin is slightly concavewithin themedian section,weakly convexmedially andweakly concavelat- sunkenregion, andcurves postero-ventrally into ashort erally; thebody between tergite andsternite occupies

Proc.R. Soc.Lond. B (2002) 1200M. D.Suttonand others 3Dfossil chelicerate approximately half theheight ofthe tergite ( Ž gures 1g, (a) 2b, j ).The contactbetween sternite and tergite is at or near theexternal shoulder(the  ange in themetasoma). endopod II The Ž rst sterniteof the metasoma is ofsimilar form, but P6 P5 endopod III themore posterior segmentsincreasingly losethe lateral P5 concaveregion, until theventral surfaceis essentially con- P4 P4 P6 vex, interruptedonly by theterminal bulb (seeabove). P3 P3 P2 endopod IV P4 (c) Tail spine P5 P2 P3 P6 The tail spine,known completely in only twospeci- P2 mens,is 2.5 mm long in OUMC.29557 ( Ž gures 1a,b,d, coxa P3 P4 endopod V 3a)and2.6 mm in OUMC.29560 ( Ž gure 2g)(thespines P2 P5 ofOUM C.29558, Ž gure 2a,c,andOUM C.29559, Ž gure coxa P4 P5 2d,are incomplete).The surfaceappears irregular. Asin- P3 gle articulation is presentat ca.80% along its length: the coxa IV P1 proximal sectiondoes not taper, is weakly compressed P6 coxa V P2 dorso-ventrally, andhas aweakventral furrow( Ž gure 1b); thedistal sectionconsists of two sub-parallel tapering coxa P1 endopod VI spines.The basal articulation allowed at least 180 ° of b dorso-ventralmovement (compare Ž gure 2c,d), and 60° ( ) oflateral movement( Ž gure 1a,b,d).The mobility ofthe P6 setae distal articulation is lesswell known,but OUM C.29557 P5 spines displays a50 ° ventral de ection (Ž gure 1a,b,d). P6 P6 setae (d) Prosomalappendages P4 spines P6 Sevenpairs (I –VII anterior toposterior) arise radially P5 P5 from theprosoma. Proximal details are dif Ž cultto resolve P4 P4 asthe bases of appendages II –VIare oftenpreserved as a P3 P3 single mass,undifferentiated from thebody (e.g. Ž gure ? ? P2 ? ? 1g). P2

(i) AppendageI (chelicerae) P1 P1 Uniramous, ca.1.5 mm long, with thebase situated antero-medially ( Ž gure 1b,c).In cross-sectionit issub-cir- (c) cular proximally buthighly laterally compresseddistally. It projectsanteriorly, curving ventrally andthen posteriorly, axial carapace spine terminating near theventral bulge ofthe anterior body ? (seeabove). Podomeres are dif Ž cultto discern, although P6 ? P1 ? P5 P2 athreefolddivision, into short proximal, long medial and P4 P3 ? exopod ? ? short  ared distal units,may bepresent ( Ž gure 1c). The endopod P3 coxa third ofthese units terminates in ashortchela ( Ž gures P4 ? 1a,c, 2a). P5 P2 P6 ? P1 ? (ii) AppendagesII– V prosoma Biramous, with eachlimb-stem (termeda coxa) dorso- Figure 4. Offacolus kingi Orr et al. (2000b); (a,b)interpret- ventrally elongate,and successively anteriorly (II) tolat- ations of prosomal appendages, basedon computer erally (V) positionedaround the prosoma. The attachment reconstructions of OUMC.29556. ( a)Oblique dorso-lateral pointsof the coxae totheprosoma have notbeen resolved, viewof right-hand rami of endopods of appendagesII –VI, andhence the most basal regions ofthese appendages basedon part of Ž gure 1e, ´ 32. (b)Dorsal viewof exopods have notbeen identi Ž ed.The positionof the boundaries of appendage III basedon Ž gure 2e, ´ 26; dashed line in P6 betweencoxa andrami are poorly constrained( Ž gure 4c), is breakof slope. ( c)Idealized reconstruction showing part of andthe possibility that thecoxa containsmultiple podom- dorsal carapace (sagittalsection) and prosomal appendage II erescannot be excluded.Endopods are attachedventrally (exsagittal section) exopod, endopod and ‘coxa’;anterior to andexopods dorsally ( Ž gures 1c,g, 2h, 4c). left, ca. ´ 15. Question marksindicate uncertainties in position of boundaries. Note thatthe attachment point of The endopodsof appendages II –Vare sub-equalin size, coxa/prosoma is unresolved, and thatthe ‘coxa’ has not been each . 2–2.5 mm in extendedlength, andform afan-like ca unambiguously shown to bea single unit. arrangement extendinglaterally andanteriorly from the oral cavity ( Ž gure 1a,e).Their bases(together with those ofthe endopod of VIandthe base of VII) projectventrally ramus generally appears sinuous,but six post-coxal ofthe prosoma, andnest together aroundthe oral cavity podomereshave beenidenti Ž ed (P1–P6; Ž gures 1e, 4a). (Ž gures 1a, 2d).The cross-sectionof each ramus varies P1 isnormally sub-horizontal ( Ž gure 1c showsit best), somewhatproximally todistally, butis always sub-circular anddirected sub-anteriorly in anterior appendagesto sub- or sub-elliptical (weakly compressedlaterally). Each laterally orevenpostero-laterally in posterior appendages

Proc.R. Soc.Lond. B (2002) 3Dfossil chelicerate M.D.Suttonand others 1201

(Ž gures 1a, 2a).In horizontal sectionit issub-triangular CRUSTACEA with asharp medially-directed vertex (seeOrr et al. 2000b, Androctonus Ž g. 2i, j ).Itsjunction with thecoxa is obscure.P2 is 60 Baltoeurypterus directedantero-dorsally in all appendages.P3 –P4 direct 60 Tachypleus theramus anteriorly andthen ventrally, although one OFFACOLUS Weinbergina example is preserveddirected sub-dorsally (OUM Cheloniellon C.29557; Ž gures 1a,b,d,e, 4a).Divergent spinesarise dis- Aglaspis tally onP4, themost prominent onthe dorsal surface(see

Alalcomenaeus a h

Ž gure 1a,right handendopods of IIandIII). P5 termin- Yohoia p r atesin achela,one element of whichis always sub-parallel 67 Sanctacaris o m

Habelia o tothe surface of P5 proximal toit. The other elementis n h preservedat varying angles toP5, andis therefore Leanchoilia c a 83 Sarotrocercus r assumedto represent a mobile sixth podomereP6 (see A Ž gure 4a). Sidneyia Olenoides The exopodsof appendages II –Vare sub-equalin size, Triarthrus each ca.1.5 mm in extendedlength (excluding terminal Agnostus setae).The diameter ofeach is consistently greater than Naraoia 58 that ofthe corresponding endopod. These rami form a Molaria dorsal andlateral fan-setemerging from theanterior gape Emeraldella ofthe carapace. They separate dorsally andlaterally from Burgessia thebasal appendage mass,close to the carapace ( Ž gure MARRELLOMORPHA 1c,g).The rami are in similar positionsin all specimens, ATELOCERATA andshow little variation in angle betweenpodomeres. Six outgroups (non-arthropod) podomeresare identi Ž ed (P1–P6; Ž gures 2e, 4b). P1, whichappears toattach basally tothe coxa, is sub-circular Figure 5. Fifty per cent majority rule consensus tree of tosub-elliptical (compressedad/ abaxially) in cross-section 32386 equally parsimonious trees (470 steps)produced by andis directedessentially anteriorly. P2 is sub-triangular, cladistic analysis. Databaseis that of Wills et al. (1998) with with acutevertices directed adaxially, andis directed Offacolus added and Agnostus coded as ‘2’ for character 34 anteriorly andadaxially. P3 is sub-circular in cross-sec- (see table1). Detailsof clades other than the tion,and is directedanteriorly. No variation in angle has Arachnomorpha ( sensu Wills et al. (1998)) are not shown. beenobserved between P2 andP3, whichmay instead Figures below nodes indicate thepercentage of fundamentals in whichthe grouping occurs —theseare 100% where no representa single long andcurved unit. P4 andP5 are Ž gure appears. Analysis performed using P aup v. 3.1.1 shorter,increasingly sub-circular in cross-section,and (Swofford 1993), using 10 random addition replicates from a directthe ramus increasingly abaxially. The adaxial sur- seed value of 1,followed byTBR branch-swappingheuristic faceof P5 distally bears twolong divergent spinesthat in searchesusing D eltran optimization. Analysis washalted mostcases are parallel with thesurface,  anked by two when themaximum number of trees supported bythe lessclearly de Ž nedand shorter divergent spines.A similar software wasreached after three replicates. pair ofspines is presenton the adaxial surfaceof P4: these are shorter,less robust, and lack  anking spines.P6 is directedabaxially with respectto P5, andterminates in a (Ž gures 1a, 2a). The ramus  ares posteriorly andlaterally brush-like splay oflong setae.The setaeof adjacentrami from an initially ventrally directedproximal podomereto overlap orabut ( Ž gures 1b,d, 2c),andare assumedto have form a large  ap-like structure( Ž gures 1a,e, 2a) which workedas a functionalunit, although thereis aconsistent extendsventrally ofthe opisthosomal appendages.In all median gap betweenthe appendage IIexopods. specimensexamined, this  ap lies closeto theplane ofthe ventral gape ofthe opisthosoma ( Ž gure 2c, j ).It averages (iii) AppendageVI 0.6 mm long at thelongest point (near thelateral margin) Biramous, with coxa andendopod similar tothose of and0.75 mm wide,and median separation averages appendagesII –V (Ž gure 1e).The coxa is positionedpos- 0.35 mm. Itsposterior andlateral margin bears approxi- teriorly andslightly laterally tothat ofappendage V( Ž gure mately 14 stoutand radially toposteriorly directedsetae, 1g).The tendril-like exopodis attachedto the dorso-lat- thelongest of which are upto0.3 mm in length andon the eral tip ofthe coxa ( Ž gure 2b),in apositionserially hom- posterior margin. OUMC.29557 ( Ž gure 1a,g) has poorly ologousto the exopods of II –V.The structureis sub- preservedsub-symmetrical lobe-like structuresarising circular in cross-section, ca.0.1 mm in diameter and from near thebase of appendage VII anddirected pos- 2mm in length. It appears tohave been  exible, andis teriorly. Theseprojections are absentfrom other speci- preservedcoiled in avertical plane in all specimensexam- mensexamined, and are thuslikely tobe external ined (e.g. Ž gures 1a, 2b).In OUMC.29557 oneexopod diagenetic calcite rather than fossil.The possibility that ofVI protrudesinto the opisthosoma whereit is preserved they are biological structurescannot however be ruled out. between  apsof the opisthosomal appendages(not Ž gured). (e) Opisthosomalappendages Eachsternite of the mesosoma bears asimilar pair of (iv) AppendageVII appendages(FI –FIII), apparently attachedin alateral Uniramous,arising ventro-laterally from theprosoma at position,towards the anterior ofeachsternite ( Ž gure 1g). theposterior ofthe oral cavity andfrom near themidline Each ramus  aresout rapidly into awide  ap-like struc-

Proc.R. Soc.Lond. B (2002) 1202M. D.Suttonand others 3Dfossil chelicerate

Table1. Coding of Offacolus according tothescheme of Wills et al. (1995, 1998). Note thata new state ‘2’ representing ‘robust and multi-segmented ’ is introduced for character 34 ( ‘Form of outer rami of appendagesof second post-acronal somite ’). This character stateis also applied to thegenus Agnostus. 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 1001000111001011 000011100??070c112010000 g 1 1 1 1 0 0 0 4 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 ????100100000000 0?1000??0?01???????????????????? ? ture,strongly recurvedand ca. 1–1.5 mm in extended assumedin ourdescription: the appendage Irami ofOrr length (Ž gures 1a, 2a,d,f,h,i),which barely projectsbelow et al. (2000b)are chelicerae;the ‘cephalic shield ’ is a cara- theopisthosomal tergites.The ramus is thickenedproxi- pace(prosomal exoskeleton);and the ‘thorax’ and ‘pos- mally (Ž gure 2d,i)andbears shortclosely spacedsetae(?) terior tagma ’ are themesosoma and metasoma ofan distally (Ž gure 2j ).Anumberof nested ‘subsidiary  aps’ opisthosoma.Other possiblehomologies includethe (Ž gure 2d,f,i)are attachedto the ventral surfaceof the identiŽ cation ofthe ventral bulge ofthe antero-median thickenedproximal region ofeach ramus ( Ž gure 2i). bodyas a labrum, themedian carapace ridge asa cardiac Thesestructures lack athickenedbase, and are smaller ridge, andthe median sunkenarea ofthe carapace asa andless strongly recurvedthan themain ramus ( Ž gure cardiac area. Afurther possibility is that theouter sunken 2i).The more anterior subsidiary  aps ofeachramus are region is homologous with theinter-ophthalmic region of successivelylonger andmore laterally positioned( Ž gure theXiphosurida, and that its margin is thusan ophthalmic 2f ).FIandFII possessfour subsidiary  aps;only three ridge. Wealso suggestthat thelateral shoulderof Offacolus have beenobserved in FIII. Three rami similar toFI –III may beat least analogous tothe cheek ridge ofthe Car- occurbeneath the anterior ofthe metasoma ( Ž gures 1a, boniferousxiphosurid Xaniopyramis (Siveter &Selden 2a,d,i),anddecrease successively in sizeposteriorly (the 1987). third is vestigial). Theserami lack subsidiary  aps, and The endopodsof Offacolus are closely comparable tothe appear toattach tothe antero-lateral regions ofmetasoma walking legs ofthe extant xiphosurid Limulus in terms of sternites I–III. morphology andposition of origin, andare assumedto be homologous.Endopod podomeres PI –P6 of Offacolus can (f ) Discussion be identiŽ edwith thetrochanter, femur, patella, tibia, and Our descriptionlargely expandson rather than contra- tarsusand pretarsus of Limulus.The chelicerae,labrum dictsthat ofOrr et al. (2000b),butseveral critical differ- andanterior ofthe body of Offacolus are also similar to encesexist, especially with regard totheappendages. Orr thoseof theextant genus.Both Limulus and Offacolus pos- et al. (2000b)consideredall theprosomal appendagesto sess Ž ve post-cheliceral walking legs in theprosoma, arise ventrally, from aroundthe oral cavity. The newevi- although IIandVI are somewhatmodi Ž edin theformer. denceindicates that the ‘robust rami’ (exopodsof II –V) Both genera also possessa posteromedially placed  ap- arise dorsally rather than ventrally. Wenow consider the like seventhprosomal appendage,and hence we assume ventrally placed ‘bilobed termini ’ ofthe previous authors that thechilaria of Limulus are reducedhomologues of tobe artefacts, variously ofpreservation, ofphotography appendage VII of Offacolus.AppendageVI is biramous in (foreign particles in thewater layer), or ofsectioning near Limulus,thecoxa bearing alateral spatulate ‘ abellum’ tothe junction between the coxa andendopod podomere (e.g.Ruppert &Barnes1994, Ž g. 13-3). The tendril-like P1. Orr et al. (2000b)consideredappendage IItobe uni- exopodof VI in Offacolus may bethe homologue ofthis ramous andthe four pairs of ‘robust rami’ tocorrespond structure.The prosomal appendagesof thewell-preserved to ‘gracile rami ’ IIIa–VIa (endopodsof III –VI). Here we Devonian xiphosuran Weinbergina (Stu¨rmer &Bergstro¨m showthat they belong toII –V,thusour IIen (endopods 1981) are somewhatless like thoseof Offacolus. Weinbergina ofappendage II) andIIex –Vex (exopodsof appendages also has Limulus-like chelicerae,but possesses six pairs of II–V)are equivalent toIIandIIIb –VIb respectively ofOrr similar non-chelatewalking legs,the posteriormost of et al. (2000b).The tendril-like exopodof VI wasnot whichare presumably homologousto the chilaria of Limulus observedpreviously. The newevidence, contrary toOrr andappendage VII of Offacolus.It doesnot possess a et al. (2000b),indicatesthat only asingle appendage pair  abellum. occursbeneath each tergite, andthat eachopisthosomal Limulus, Weinbergina and Offacolus all possesssix pairs tergite correspondsto a single segment.Structures inter- oflamellate appendagesin theopisthosoma. Those of pretedby Orr et al. (2000b, p. 1503) as ‘sectionsthrough Limulus also bear subsidiary  aps (forming a ‘book-gill’ thenear-vertical lateral margins ofeachtergite ’ and ‘setae apparatus), although theseare far more numerousthan extendingpostero-ventrally from theposterior faceof each in Offacolus. The  attenedopisthosomal limbs in Limulus, ramus’ are hereinterpreted as primary rami andsubsidiary however,are sub-planar rather than recurved,and the  apsrespectively. Orr et al. (2000b)reportedthat the anteriormost is modi Ž edinto an operculum. The opistho- anterior fourtergites ofthemetasoma bore medial nodes, somal appendagesof Limulus also bear reducedendopods, butthe new material showsthe anteriormost ofthese to whichhave notbeen identi Ž ed in Offacolus.The book-gills lack adiscretenode, bearing only apoorly de Ž ned of Weinbergina are poorly known,but are sub-planar and median swelling. bear alarge numberof ‘gill lamellae ’ (Stu¨rmer & Berg- Increasedcon Ž dencein thechelicerate af Ž nities of stro¨m1981), andhence appear tobe closer to those of Offacolus (seebelow) allows ustoidentify many structures Limulus than Offacolus. with thoseof chelicerates.Several ofthesehomologies are Manycharacters of Offacolus serveto distinguish it from

Proc.R. Soc.Lond. B (2002) 3Dfossil chelicerate M.D.Suttonand others 1203 all other chelicerates,and to justify the erection of a new Briggs, D.E.G., Siveter, D.J.&Siveter, D.J.1996 Soft- family. Theseinclude the form ofthe anterior margin bodied fossils from aSilurian volcaniclastic deposit. Nature (with anterior gape anddistinctively arranged spines),the 382, 248–250. presenceof an articulation in thetail spine,the low seg- Dunlop, J.A.&Selden, P.A.1997 Theearly history and phy- mentcount and pattern oftagmosis in theopisthosoma, logeny of thechelicerates. In Arthropodrelationships (ed. R. A.Fortey &R.H.Thomas),pp. 221 –235. London: Chap- andespecially thebiramous natureof the prosomal man & Hall. appendagesII –V.The exopodsof II –V in Offacolus have Giribet, G., Edgecombe, G.D.&Wheeler, W.C.2001 noclear homologue in Limulus, Weinbergina, or any Arthropod phylogeny basedon eight molecular loci and related arthropod. Biramous anterior appendagesare morphology. Nature 413, 157–161. knownfrom arachnomorph taxa, for example theCambr- Heymons, R.1901 Die Entwicklungsgeschichte der Scolop- ian Sanctacaris (Briggs &Collins1988), buttheir exopods ender. Zoologica 13, 2–5. are almost always Ž lamentousor antenniform(Wills et al. Hwang, U.W.,Friedrich, M.,Tautz, D., Park, C.J.&Kim, 1998); robustmulti-segmented exopods are unusual W.2001 Mitochondrial protein phylogeny joins myriapods (presentfor example in Agnostus, see Mu¨ller &Walossek withchelicerates. Nature 413, 154–157. (1987)), andunlikely tobe plesiomorphic. Thusalthough Mu¨ller, K.J.&Walossek, D.1987 Morphology, ontogeny and life habitof Agnostuspisiformis from theUpper Cambrian of thepresence of these exopods in Offacolus may bea primi- Sweden. Fossils and Strata 19, 1–124. tive character, their form is likely tobe derived. Orr, P.J., Briggs, D.E.G., Siveter, D.J.&Siveter, D.J.2000 a Three-dimensional preservation of anon-biomineralized arthropod in concretions in Silurian volcaniclastic rocks 4. PHYLOGENY from Herefordshire, England. J.Geol. Soc. Lond. 157, Asuiteof characters clearly identi Ž es Offacolus as a che- 173–186. licerate (sensu Dunlop& Selden1997; Wills et al. 1998), Orr, P.J., Siveter, D.J., Briggs, D.E.G., Siveter, D.J.&Sut- especially thechelicerae, the prosoma andopisthosoma ton, M.D.2000 b Anew arthropod from theSilurian Kon- (thelatter dividedinto amesosomaand metasoma), and servat–Lagersta¨tte of Herefordshire, England. Proc.R. Soc. Lond. B 267, 1497–1504. (DOI 10.1098/rspb.2000.1170.) thenumber of appendagesin theprosoma. Several phylo- Ruppert, E.E.&Barnes, R.D.1994 Invertebrate zoology , 6th geneticand taxonomic schemeshave beenused for the edn. NewYork: Saunders College Publishing. chelicerates,and a numberof cladistic studieshave been Siveter, D.J.&Selden, P.A.1987 Anew, giant xiphosurid publishedin recentyears (Dunlop& Selden1997; Wills from thelower Namurian of Weardale, County Durham. et al. 1998; Wheeler &Hayashi 1998). In order toanalyse Proc.Yorkshire Geol. Soc. 46, 153–168. theposition of Offacolus,wehave madeuse ofthedatabase Stu¨rmer, W.&Bergstro¨m, J. 1981 Weinbergina ,axiphosuran of Wills et al. (1995, 1998), whichis based on a large arthropod from theDevonian Hunsru¨ ck Slate. Pala¨ontologis- numberof characters andincludes both fossiland extant che Zeitschrift 55, 237–255. taxa. While theconclusions of Wills et al. (1995, 1998) Sutton, M.D., Briggs, D.E.G., Siveter, D.J.&Siveter, D.J. are at oddswith recentanalyses of molecular data (Giribet 2001a An exceptionally preserved vermiform mollusc from theSilurian of England. 410, 461–463. et al. 2001; Hwang et al. 2001), this database remains the Nature Sutton, M.D., Briggs, D.E.G., Siveter, D.J.&Siveter, D.J. bestavailable cladistic framework for theanalysis offossil 2001b Methodologies for thevisualization and reconstruc- taxa. Table 1showsour codings for Offacolus under this tion of three-dimensional fossils from theSilurian Hereford- scheme, and Ž gure 5summarizes theresults of this analy- shire Lagersta¨tte. Paleontologia Electronica 4,art. 2: 17 pp., sis,which are essentially identical tothose of Wills et al. 1MB.Seehttp:/ /palaeo-electronica.org/2001 _1/s2/issue1 _ (1998, Ž g. 2.1). Offacolus groups with thechelicerates, 01.htm. emerging asmore derivedthan Weinbergina (and hence Sutton, M.D., Briggs, D.E.G., Siveter, D.J.&Siveter, D.J. within theChelicerata, albeit notwithin thecrown-group), 2001c Athree-dimensionally preserved fossil polychaete andrepresenting a sistertaxon tothe eurypterids andthe worm from theSilurian of Herefordshire, England. Proc. R. extant chelicerates.This phylogeny suggeststhat all higher Soc. Lond. B 268, 2355–2363. (DOI 10.1098/rspb.2001. taxa within which Offacolus couldbe accommodated (e.g. 1788.) Swofford, D.L.1993 Paup—Phylogenetic analysis usingparsi- theXiphosura, Synziphosurida or theunnamed taxon of mony,v. 3.1.1. Champaign, IL:Illinois Natural History Sur- Anderson& Selden(1997)) are paraphyletic, andhence vey. wedonot assign thefamily Offacolidae toany taxa below Weygoldt, P.1998 Evolution and systematicsof theChe- therank ofChelicerata. licerata. Exp. Appl. Acarol. 22, 63–79. Wheeler, W.C.&Hayashi,C. Y.1998 Thephylogeny of the ANERCgrant GR3/12053 funded thisresearch. We thankJ. extant chelicerate orders. Cladistics 14, 173–192. Hay(University Museum, Oxford), P.Jackson and J.Hyde Wills, M.A., Briggs, D.E.G., Fortey, R.A.&Wilkinson, M. (Department of EarthSciences, University of Oxford), for 1995 Thesigni Ž cance of fossils in understanding arthropod technical assistance, and T.Hall and J.Sinclair of alocal quar- evolution. Verhandlungen derDeutschen Zoologischen Gesell- rying company for general assistance. schaft 57, 13–33. Wills, M.A., Briggs, D.E.G., Fortey, R.A., Wilkinson, M.& Sneath, P.H.A.1998 An arthropod phylogeny basedon REFERENCES fossil and Recent taxa. In Arthropodfossils and phylogeny (ed. Anderson, L.I.&Selden, P.A.1997 Opisthosomal fusion and G.F.Edgecombe), pp. 33 –105. NewYork: Columbia Uni- phylogeny of Palaeozoic Xiphosura. Lethaia 30, 19–31. versity Press. Briggs, D.E.G.&Collins, D.1988 AMiddle Cambrian che- licerate from Mount Stephen, British Columbia. Palaeon- As this paper exceedsthe maximum lengthnormally permitted, the tology 31, 779–798. authors have agreedto contributeto production costs.

Proc.R. Soc.Lond. B (2002)