Fig 1.
Surusicaris elegans gen. et sp. nov., holotype specimen ROM 62976.
A. Complete view of the part. B. Secondary electron image of anterior area showing details of eyes and frontalmost appendages. C. Close-up of anterior section showing the three pairs of anterior uniramous legs. D. Camera lucida drawing of the “head.” All images were taken under cross-polarized light except in B. Abbr. a+ds: anus+dark stain; alx: anterior limb (1–3); cl?: caudal lobe?; dis: distal inner spine; fa: frontalmost appendage; g(?): gut(?); hg: hindgut; lfa: left frontalmost appendage; le: left eye; px: podomere (1–5); pis: proximal inner spines; rfa: right frontalmost appendage; rcv: right carapacal valve; re: right eye; tlx: trunk limb (1–12); tsx: terminal spine (1–3). Scale numbers in mm.
Fig 2.
Elemental maps of Surusicaris elegans gen. et. sp. nov. holotype specimen ROM 62976.
A. Part. B. Counterpart. C. Backscatter image of the insert in A, showing the conspicuous preservation of the exopods. D. Carbon mapping of the same area. E. Backscatter image and elemental maps of various elements on a single posterior exopod (insert in B). Note the exact overlap of Fe and Mg in the area between the caeca. A, B, images taken under cross-polarized light. Scale numbers in mm.
Fig 3.
Diagrammatic reconstruction of Surusicaris elegans gen. et sp. nov in profile view.
For clarity, exopods are figured in light grey and caeca in black. A. Habitus. Only the right appendages are drawn and the distalmost segment of the frontalmost appendage is here hypothetically subdivided into three additional segments, based on the anomalocaridid morphology. Exopods are appressed onto the endopod posteriorward to show the tripartite branching of the caeca. The tailpiece is conjectural. B. Antero-posterior view of trunk limbs, with exopod opened up. Abbr. aul: anterior uniramous limbs; cv: carapacal valve; e: eye; ebp: endobasipod; es: exopodial setae; exp: exopod; fa: frontalmost appendage; fg: foregut; ic: invasive caeca; m: mouth; mg: midgut; orc: outer raptorial complex; tl: trunk limb; tp: tailpiece.
Fig 4.
Degrees of sclerotization in lobopodous and stenopodous limbs of fossil lobopodians and arthropods.
A. Surusicaris elegans gen. et sp. nov., holotype (ROM 62976), showing the three right anterior uniramous limbs with faint traces of cuticular boundaries. B. Limbs of Diania cactiformis Liu et al., counterpart of YKLP 11319 (from [79]). Note the thickness and regularity of the subdivisions. C. Aysheaia pedunculata Walcott, part of holotype (USNM 57655), posterior limbs preserved at various angles. The aspect of the annulations varies from discordant through faint to regular. D. Hadranax augustus Budd and Peel [126], trunk lobopods of the mid-section of holotype (MGUH 24.527). E. Posterior endopod of the bivalved arthropod Odaraia alata Walcott. Poor sclerotization can lead to a deformed preservation of segments’ shape, but the rectangular aspect of some segments and the general elongated habitus usually remain characteristic of arthrodization. F. Anterior limb of Canadaspis perfecta preserved twisted and dislocated. In certain cases, taphonomy can reshape an arthrodized limb to a much more compact structure, although the undulation or folding of segments’ boundaries are not necessarily associated with such deformations. G. The “poorly sclerotized” anterior endopods of the fuxianhuiid Chengjiangocaris kunmingensis Yang et al. [14] (YKLP 12024). Note their relatively advanced arthrodization in comparison to Surusicaris. Abbr. alx: anterior limb (1–3). Scale numbers in mm.
Fig 5.
Comparative nature of the biramous trunk limbs in Surusicaris elegans gen. et sp. nov. A, E-G.
Surusicaris elegans, holotype (ROM 62976). A. Close-up of trunk limb 3. B. Blind caeca (dark grey) projecting from the midgut (light grey) into the coxae in a wolf spider (redrawn from Foelix [90]). C. Possible equivalents in the trilobitomorph Misszhouia longicaudata Zhang and Hou (redrawn from Vannier and Chen [85]). D. 6th thoracopod (left side) of the leptostracan (Crustacea: Malacostraca) Dahlella caldariensis Hessler (from Shu et al. [18], original picture provided by Jean Vannier). The central, exposed, part of the figure shows the exopod (below) as an anastomosed tissue spread out in-between a trident of hemolymph channels, and the mostly muscular attachment of the limb to the body (above). The epipodite and its musculature have been attenuated, as they seem to be absent on the limbs of Surusicaris; we have also faded the endopod, as its trace is too shallow to be revealed under SEM observation (see E and F). E. Backscatter image of the 8th trunk limbs of the counterpart of the holotype (see also Fig 2). F. Superposed elemental maps of E: carbon in red, iron in magenta and magnesium in blue (Fe and Mg match exactly). Note the analogy with D. G. Surusicaris elegans, 5th trunk limb (Image flipped horizontally, with inverted grayscale). B, G, images taken under cross-polarized light. Abbr. cx: coxa; en: endopod; ep: epipod(ite); ex: exopod; fe: femur; fo: fold; mg: midgut; tr: trochanter. Scale numbers in mm.
Fig 6.
Morphospace defined by the first three axes of a PCO analysis of the frontalmost appendage.
Analysis based on a matrix of 36 taxa and 12 characters (see Methods and S1 Dataset). Percentage of total variance explained by each axis displayed next to axis names. The pie diagrams describe the relative influence of the characters on each axis (see distribution pie, bottom left to the character list). The outer ring displays the proportional value of the Cramér index represented by all characters having a significant impact on the axis (p-value ≤ 0.05). The inner pie displays the proportional p-value for those significant characters, quantifying their impact on the ordination of the axes. The identity of characters used is shown on a hypothetical synthetic appendage in the bottom right corner. Abbr. Anomalocaris: Anomalocaris canadensis; As: Amplectobelua stephenensis; Ap: Aysheaia pedunculata; Bb: Branchia brevis; Bp: Branchiocaris pretiosa; Cf: Cupiennius foliatus; Ci: Cassubia infercambriensis; Cs: Caryosyntrips serratus; Fp: Fuxianhuia protensa; He: Haikoucaris ercaensis; Hurdia: Hurdia victoria; Ia: Isoxys acutangulus; Jd: Jianshanpodia decora; Kk: Kerygmachela kierkegaardi; Kunmingella: Kunmingella maotianshanensis; Ks: Kiisortoqia soperi; Leanchoiliids = Actaeus armatus, Alalcomenaeus cambricus, Leanchoilia superlata; Li: Lithobius forficatus, Lm: Lightiella monniotae; Mh: Megadictyon haikouensis; Nebalia: Nebalia bipes (Nb1 refers to coding of three rami, Nb2 to a single one); Oo: Occacaris oviformis; Or: Opabinia regalis; Pl: Pycnogonum litorale (extant); Pn: Peytoia nathorsti; Phalangium: Phalangium opilio (extant); Sb: Schinderhannes bartelsi; Surusicaris: Surusicaris elegans; ?Su: Sanctacaris uncata; Tb: Tamisiocaris borealis; Wf: Waptia fieldensis; Yohoiids = Fortiforceps foliosa, Yohoia tenuis.
Fig 7.
Decomposition of the tridimensional morphospace of the frontalmost appendages.
Optimal clusterings found by the k-means analysis of the morphospace constrained by the Calinski criterium. A. LOP cluster (lobopodians, opabiniids and Cassubia). B. ANO, “Anomalocaris-type” cluster. C. HUR, “Hurdia-type” cluster (and tentatively, Sanctacaris). D. MPS, intermediate morphologies of Kiisortoqia, Kunmingella and Occacaris. E. MEC, megacheirans and chelicerates. F. ANT, antennulate morphologies.
Fig 8.
Disparity, measured as the variance of PCoA axes variances, of a priori and tested morphospace groups.
A. Disparity between body plan-based clusters. B. Disparity between k-means-based clusters. See text and Fig 7 for description of acronyms.
Table 1.
Top: Values of Cramér's V calculated on the first four PCoA axes; Bottom: P-values of chi-squared tests calculated on the first four PCoA axes.
Fig 9.
Internal sub-triangular structures at the base of trunk limbs in several Cambrian panarthropods.
Note that they are part of a broader axial region (white arrows point to selected examples). A. Part (above) and counterpart (below) of Opabinia regalis Walcott USNM 155059b. B. Leanchoilia persephone Simonetta ROM 57666. C. Alalcomenaeus cambricus Simonetta ROM 45613. Here, the triangular bases seem to be somewhat prolonged onto the anterior margin of the exopods, similarly to what is often observed along the lobes of Opabinia. D. Alalcomenaeus cambricus ROM 53352. E. Alalcomenaeus sp. (ROM 62968, Marble Canyon deposit). F. Molaria spinifera Walcott USNM 57688 (lectotype). A, B (bottom), C, D (top), E (top), F (right) images taken under cross-polarized light, all other images taken under water using direct light. Scale numbers in mm.
Fig 10.
Possible examples of larger disparity in the frontalmost appendage of Isoxys types from the Chengjiang Lagerstätte.
A. Unpublished specimen (CAL04). The appendage shows similarities with Surusicaris elegans gen. et sp. nov. in being upward-directed, few-segmented and “raptorial,” although the distal claw is possibly made of parallel spines. B. HSX08 (as in Vannier et al. [13]). The appendage is here multi-segmented and antenna-like. Top images taken under cross-polarized light, bottom images with enhanced contrasts. (original photographs courtesy Jean Vannier). Abbr. dc: distal claws; e: eye(s); ic: inner claw; is: inner spine. Scale numbers in mm.
Fig 11.
Distribution of important traits of the frontalmost appendage among the three main stem body plans considered herein (from top to bottom): dinocaridid, stem ‘bivalved’ and megacheiran.
The question mark (3rd row, column c) refers to the problematic condition of Sanctacaris Briggs and Collins. Characters: a: secondary processes on inner spines; b: double row of inner spines; c: elongate, slender inner outgrowths; d: rounded undifferentiated tip; e: composition of distal “hand” formed by the differentiation of inner spines; f: composition of distal “hand” formed by the differentiation of outer spines.