As we implement this information into the phylogeny of megacheirans, we also explore related methodological issues. It is important to note the great discrepancy in the choice of outgroup among phylogenetic studies of arthropods over the last decade. Regardless of whether megacheirans were found to be monophyletic, they have not been retrieved as sister‐group to chelicerates in most studies using very basal outgroups such as lobopodians or dinocaridids. It therefore seems likely that divergences in character polarization have a traceable effect on the phylogenetic placement of these taxa. If so, another methodology altering the polarization of characters, such as the coding strategy of inapplicable states, would also be able to retrieve some of the conflicting topologies – at the very least, Arachnomorpha and Antennulata. The coding of inapplicables is itself a rarely tackled issue, yet it is important to explore its effects on the placement of megacheirans and the arthropod phylogeny as a whole (Edgecombe and Ramsköld 1999 ), in particular with respect to modern methods of parsimony, such as implied weighting (Goloboff 1993 ).

We contribute to these ongoing efforts (García‐Bellido and Collins 2007 ; Liu et al . 2007 ; Edgecombe et al . 2011 ; Haug et al . 2012a , b ; Tanaka et al . 2013 ; Siveter et al . 2014 ) by introducing a large leanchoiliid megacheiran from the newly discovered Marble Canyon locality of the Burgess Shale (Caron et al . 2014 ), in Kootenay National Park (British Columbia, Canada), based on analysis of 42 specimens, many of which exhibit unusually fine preservation of morphological detail, even by comparison with other Burgess Shale material (Caron et al . 2014 ).

As they retain many plesiomorphies – bipartite tagmatization of the body into cephalon and trunk; a head without masticatory specialization; simple biramous limbs; and pleurae (Walcott 1912a ; Raymond 1935 ; Størmer 1944 ; Simonetta 1970 ; Hou and Bergström 1997 ; Briggs and Collins 1999 ; Chen et al . 2004 ; García‐Bellido and Collins 2007 ; Liu et al . 2007 ; Edgecombe et al . 2011 ; Babcock et al . 2012 ; Haug et al . 2012a , b ; Siveter et al . 2014 ) – the phylogenetic position of megacheirans is sensitive to a few derived characters, and notably to the frontal pair of appendages (the ‘great appendages’) whose homology has been controversial (Budd 2002 ; Cotton and Braddy 2004 ; Scholtz and Edgecombe 2005 , 2006 ), though now arguably viewed as deutocerebral (Edgecombe 2010 ; Ou et al . 2012 ; Edgecombe and Legg 2014 ; Smith and Ortega‐Hernández 2014 ). The questioned homology of the ‘great appendages’ with other deutocerebral appendages such as chelicerae (Chen et al . 2004 ; Haug et al . 2012b ), or even, controversially (Cong et al . 2014 ), dinocaridid anterior claws, has greatly impacted the possibility of gathering megacheirans as a clade under some clear synapomorphy. Other features such as eyes have been disputed as well (Bruton and Whittington 1983 ; Briggs and Collins 1999 ; García‐Bellido and Collins 2007 ; Haug et al . 2012a ), although some (Tanaka et al . 2013 ) have recently argued for the presence of median eyes and used possible optic neural anatomy to support an affinity with chelicerates. This makes the continued investigation of the megacheiran anatomy and the description of new evidence paramount to comprehending the emergence of crown‐group arthropods.

The realm of fossil taxa called ‘megacheirans’ is central in this context. Formalized as a class by Hou and Bergström ( 1997 ), the ‘Megacheira’ are variably considered to form either a paraphyletic clade or a combination of some paraphyletic genera and a monophyletic clade (Budd 2002 ; Cotton and Braddy 2004 ; Legg et al . 2012 ; Siveter et al . 2014 ). Megacheirans have been retrieved in almost all possible phylogenetic positions in the ‘vicinity’ of the basal arthropod crown‐group, under both ‘Arachnomorpha’ and ‘Antennulata’ hypotheses: among earliest euarthropods (Budd 2002 ), as the sister‐group to all crown arthropods and artiopods (Legg et al . 2012 ), as the sister‐group to chelicerates (Cotton and Braddy 2004 ; Kühl et al . 2009 ) and as allied with Antennulata (Briggs et al . 2012 ) or mandibulates (Bergström 1992 ).

The revival of the ‘stem‐group’ concept (Budd 1997 ; Budd and Jensen 2000 ; Budd 2002 ) has had a strong influence on the cladistic reappraisal of many large clades, and in particular on our understanding of early arthropod evolution (Budd 2002 ; Chen et al . 2004 ; Cotton and Braddy 2004 ; Waloszek et al . 2005 ; Scholtz and Edgecombe 2006 ; Edgecombe 2010 ; Briggs et al . 2012 ; Legg et al . 2012 ; Edgecombe and Legg 2014 ; Siveter et al . 2014 ). The phylogenetic placement and systematic boundaries of a number of panarthropodan stem‐groups in this context have nourished vivid debates (Budd 2002 ; Cotton and Braddy 2004 ; Scholtz and Edgecombe 2006 ; Budd and Telford 2009 ; Edgecombe 2010 ; Edgecombe and Legg 2014 ). These disputes often surround topologies among which the Arachnomorpha (i.e. trilobitomorph, or ‘artiopod’, taxa allied with chelicerates; Briggs and Fortey 1989 ; Edgecombe and Ramsköld 1999 ; Cotton and Braddy 2004 ; Legg 2014 ) and Antennulata (i.e. trilobitomorphs allied with mandibulates, including crustaceans; Bergström 1992 ; Legg et al . 2012 ; Ortega‐Hernández et al . 2013 ) are prominently opposed.

The use of implied weighting (Goloboff 1993 ; Goloboff et al . 2008b ) allows for the differential weighting of characters according to their degree of homoplasy. This methodology helps circumvent certain conflicts of phylogenetic signal between characters and has resulted in better‐resolved topologies, notably for fossil arthropods (Edgecombe et al . 2011 ; Legg et al . 2012 ; Ortega‐Hernández et al . 2013 ). Because the method is a measure of fit based on the consistency index (CI; Goloboff 1993 ), that is the difference between the expected and realized number of steps, missing entries do not directly affect the weighting of characters a priori . However, weighting schemes depend on the treatment of missing data by tree‐building algorithms. As this approach is currently implemented in paup* and tnt, missing entries could influence re‐weighted topologies in different ways. If uncertainties are optimized as potential states, it is expected that some characters will be given a higher weight than in a more homoplastic evolutionary scenario constrained by inapplicability. On the other hand, if a greater uncertainty in the data set favours polytomous topologies, characters containing inapplicable entries may become relatively less influential. Because both coding options (‘missing’ and ‘newstate’) may imply different degrees of homoplasy for the same character when inapplicable entries are used, the use of implied weights is expected to provide different results in each case. The extent and significance of these differences are of interest here in evaluating the sensitivity of phylogenetic analyses to these methodological approaches in the context of early arthropod evolution, and in particular with respect to megacheirans.

Problems related to the ‘missing’ coding have been identified early on (Platnick et al . 1991 ; Maddison 1993 ). The addition of steps on the tree due to the inapplicability of intermediary taxa coded as uncertainty is a serious bias, but can in theory be avoided using tnt (Goloboff 1993 ). Further investigation of the impact of this method on phylogenetic outcomes has otherwise been limited to relatively simple examples. Whether states are or are not spuriously optimized (i.e. topologies whose inapplicable taxa are or are not putatively coded with a known state, with or without addition of steps, see Fig. 1 B), the amount of signal lost during the analysis due to increased uncertainty in the data set – and the increase of polytomies – has not been evaluated. At the scale of one trait taken independently, the ‘missing’ coding outperforms the ‘newstate’ coding, provided, in the case of an inapplicable outgroup, that a sovereign presence/absence character is used (Fig. 1 ).

Inapplicability is based on the principle of a dependency, that is on the presence of a sovereign character informing the coding of a state in other, related characters. In general, this character is binary and codes for the presence or absence of a homologous structure that can be decomposed into several characters describing its variations. Consequently, if the presence or absence of the character is included within the dependent character, information is incomplete, and the taxa grouped by the possession of that character may not form a clade (Fig. 1 ). Alternatively, by coding inapplicability as an additional state, part of the phylogenetic information is redundant, namely the absence of the sovereign character in uncoded taxa (Fig. 1 ).

Hypothetical examples of discrepancies between ‘missing’ and ‘newstate’ coding for a single character isolated (left) or in association with a sovereign presence/absence character (right), one outgroup (O) and six taxa (A to F). A, the outgroup chosen is inapplicable for this character. ‘Newstate’ coding (‘2’ upper row) drags inapplicable taxa (C and D) downtree, which artificially generates some signal to group other taxa (A, B, E, F) together. ‘Missing’ (‘?’) coding loses the phylogenetic signal (lower row). When a sovereign character is added (right matrix), topological results are similar, but ‘newstate’ adds an extra step (2 to 0 or 1 in addition to 0 to 1) that in fact repeats the phylogenetic signal and artificially doubles the support for the clade. B, the outgroup is coded for one of the known states. The inapplicable taxa are optimized as having experienced a reversal of the structure, although the ‘missing’ coding (by itself) does not assume this reversal to be a single event. The ‘missing’ coding option requires only one minimal step, but the possible topologies are numerous, and therefore, the uncertainty is high. ‘Missing’ coding also considers illogical options of state assignment for C and D. With the addition of a sovereign character (right matrix, middle column), the ‘newstate’ coding adds support to the reversal by counting the transition from presence to absence twice.

‘Newstate’ coding can indeed generate problematic outcomes. For example, taxa may be dragged downtree because inapplicability is converted to a plesiomorphic state, in the case where inapplicability is coded in the outgroup and is thus polarized as plesiomorphic (Fig. 1 A). Conversely, independent losses of a trait will cause taxa coded with inapplicable states to be grouped together when the outgroup is polarizing for one of the observed states (Fig. 1 B; Strong and Lipscomb 1999 ; Seitz et al . 2000 ). Such artefacts result from redundancies of information, since the signal for absence is repeated through the inapplicability of the outgroup (Fig. 1 ).

Existing cladistic software (e.g. paup* (Swofford 2002 ) or tnt (Goloboff et al . 2008a )) can treat inapplicable states in one of two ways: as ‘missing’ (or ‘?’) or as ‘additional states’ (i.e. ‘newstate’ coding). Neither of these options offers a theoretically satisfying approach to the concept of inapplicability (which is phylogenetic information, but not another state), and the possible coding strategies have been discussed accordingly (Platnick et al . 1991 ; Maddison 1993 ; Lee and Bryant 1999 ; Strong and Lipscomb 1999 ; Seitz et al . 2000 ; De Laet 2005 ). Although no clear‐cut solution has been found, the ‘missing’ coding has generally been assumed to be a more reliable or even analytically correct approach.

When dealing with the phylogenetic origin of major body plans, such as that of arthropods, outgroup taxa (when neutrally selected within the sister‐group to the whole clade of interest) can be substantially different morphologically than the main portion of ingroup studied (in this case, Cycloneuralia from Euarthropoda (Rota‐Stabelli et al . 2011 ; Edgecombe and Legg 2014 )). ‘Inapplicable’ states will be particularly prominent in these matrices. Indeed, many homologous traits coded for the ingroup will simply be absent among members of the outgroup, and any variation based on these structures is impossible to code as an existing state (i.e. inapplicable). As a result, these characters will lack polarization.

The implied‐weight‐based cladograms were generated using a constant of k = 4 described by Goloboff ( 1993 ) as a degree of concavity weighting against homoplasies but allowing characters with a few extra steps to remain significant. However, Goloboff et al . ( 2008b ) stressed that no degree of concavity may be confidently determined a priori and that, when using implied weights, the validity of clades requires their congruency across a range of k values. Accordingly, we also ran analyses with implied weights using k = 2 and k = 10. Results were identical or nearly identical to those obtained with k = 4, and in this paper, only analyses using k = 4 are discussed below. It is worth noting that when k = 2 was used and inapplicables were coded as missing, Kiisortoqia and chelicerates were retrieved in a polytomy with Cheiromorpha and Artiopoda + Crustacea on the strict consensus topology, although the grouping of chelicerates with crustaceans and artiopods was still present in the majority‐rule consensus. This means that a strong weighting against homoplasies weakens the association of chelicerates with the crustacean/artiopod clade.

Characters were optimized with mesquite v.2.75 (Maddison and Maddison 2011 ) and cladograms were rendered graphically with figtree v.1.3.1 (Rambaut 2014 ). Tree construction was based on parsimony and consisted of a heuristic search using the tree bisection and reconnection branch‐swapping method on 100 additional sequence replicates in paup*. Jackknife and bootstrap resamplings were run on 100 replicates using ‘the new technology’ method (tnt) to search topologies. tnt yielded similar topologies to those obtained with paup*.

We generated the consensus topologies for unweighted and weighted analyses, each with inapplicable entries coded both as uncertainties (‘missing’ coding) and as additional states (‘newstate’ coding). We first used the ‘missing’ treatment of inapplicable states as the default option in paup* (command options gapmode=missing), and in a subsequent analysis, gaps (‘–’) were manually replaced with an additional state (‘9’). The latter method should be equivalent to using options gapmode=newstate, but allowed us to test the influence of a differential coding of characters: one type of coding for characters which were inapplicable for outgroup taxa, another for those for which outgroup taxa were coded as known states but were inapplicable for some ingroup taxa. Accordingly, characters for which outgroup taxa were coded with a known state had their ingroup inapplicables changed back to ‘?’. This approach represents a compromise between preserving the strength of the phylogenetic signal in characters for which inapplicability was probably plesiomorphic (coding inapplicable entries as additional states) and avoiding known spurious groupings (coding the inapplicable entries where the outgroup was coded with observed states as uncertainties). No substantial differences resulted from this alternative coding, however, and we decided to keep the additional state (‘9’) for all inapplicable entries without distinction for the final results.

We used paup* v.4.0b10 software (Swofford 2002 ) to generate trees and tnt v.1.1 (Goloboff et al . 2008a ) to calculate values of node robustness (i.e. Bremer support), jackknife partitioning and bootstrap values using traditional search with 100 replicates. We used a morphological matrix composed of 35 taxa including three outgroup taxa (Aria et al . 2015 ) coded for 50 unordered characters (see Aria et al . 2015 for a detailed list with comments and sources).

Morphological groups were identified using principal component analysis and regression analyses of head length and telson length, with residuals extracted and positive values separated from negative ones. Colour‐coding and the assignment of symbols were then based on the contingency of occurrences of the positive and negative residuals for the two traits.

Five measurements (head length, head width, trunk length, trunk width and telson length) were taken from photographs of 22 specimens using ImageJ freeware (Abràmoff et al . 2004 ; Aria et al . 2015 ). In a few cases (Aria et al . 2015 ), small sections of the body were absent or covered by sediment, and the full length or width extrapolated using ratios obtained using more complete material. Statistical analyses were performed using r (R Core Team 2014 ) and included the use of the vegan package (Oksanen et al . 2013 ) for multivariate analyses.

The 42 studied specimens come from the upper part of the basinal Stephen Formation (Aitken 1997 ) and were collected in situ within a 2‐m‐thick interval, near Marble Canyon (Kootenay National Park, British Columbia) during the 2012 field season (Caron et al . 2014 ). Preparation and observation methods were similar to previous studies (e.g. Conway Morris and Caron 2012 ). Elemental maps were created with an environmental scanning electron microscope (FEI Quanta 200 FEG) equipped with an energy scanning spectroscopy (EDS) X‐ray detector and octane plus silicon drift detector (SDD, using team software, Version 4.1) under low vacuum conditions (70 Pa, 15 kV, 400 μs dwell time) at the University of Windsor's Great Lakes Institute for Environmental Research. All fossil material is deposited at the Royal Ontario Museum in Toronto for Parks Canada.

We have adopted a conservative approach in attributing the sampled specimens to the same morphotype. It should be noted, however, that one specimen in particular from Marble Canyon that we have assigned to Leanchoiliida indet. is of a similar size and habitus as Yawunik but differs in having a conspicuous spatulate telson (Aria et al . 2015 ) that is more similar to, for example, Alalcomenaeus, though apparently lacking fringing spines. These features suggest the possibility of cryptic species/sexual dimorphism within the sample set (but see 6 below).

Telson ca. one‐sixth of body length, bipartite, composed of an articulating socket and flat telsal blade (Figs 2 A, 8 C–D, 9 ). Articulating socket largely hollow, triangular in transverse view, with a somewhat pointed postero‐median angle. Telsal blade proper lanceolate in dorsal view with rounded lateral margins and sub‐acute termination (Figs 2 A, 8 D, 9 ), traversed dorso‐medially in all its length by a tapering carina (Figs 2 A, 8 D, 9 ); margins of telson fringed from a third of their length posteriorward by alternating tiny stout and slender teeth (Figs 2 A, 8 D, 9 ).

Paddle‐shaped with triangular distal section and rounded apex, reaching approximately the base of the last endopod segment (Figs 8 E, 9 ). Divided into proximal and distal portions, the distal margin of the proximal part roughly aligned with the distal margin of the basipod (Fig. 8 E). Attachment with basipod uncertain. Margins of the distal triangular portion adorned with lanceolate lamellae up to one‐third the length of exopod; remaining margins of the exopod possibly bearing shorter lamellae (Figs 2 B, 8 C, E, 9 ).

Composed of seven segments, excluding terminal claw (Figs 2 A, 8 E, 9 ), and bipartite, divided into a proximal portion of one segment and a distal portion (Figs 2 A, 8 E, 9 ). Proximal part of endopod consists of an enlarged segment (ca. one‐third of the length of endopod), trapezoidal in lateral view; ventral side forming a crest and triangular in cross section (Fig. 2 A); adornment unclear. Arthrodial membrane present between proximal and distal portions of the endopod (Figs 8 E, 9 ). Distal portion of the endopod composed of five segments only slightly decreasing in width and length distalward, and an elongated distalmost segment, ca. twice as long as penultimate segment, sub‐segmented in the last third of its length – considered to be the seventh segment of the endopod – ending in a single terminal claw (Figs 8 E, 9 ). Each of the segments of the distal portion drawn out into a pair of stout spines on the ventral distalmost margins (Figs 8 E, 9 ). Terminal claw somewhat elongate distally, curved (Figs 2 A, 8 E, 9 ), only slightly smaller than last segment if parallel to bedding (Fig. 8 E); first half of claw bears two secondary spines, close together, the proximal one notably smaller (Figs 2 A, 8 E, 9 ).

Trunk divided into 12 segments, excluding telson (Figs 2 , 6 A, 9 ). Tergo‐sternites overlapping for approximately one‐quarter of their length when body is straight (Figs 2 , 8 C, 9 ), forming a short spine postero‐medially. Pleurae well‐developed; similarly overlapping, decreasing in size antero‐posteriorly; lateral aspect of the animal generally compact in dorsal view (Fig. 2 C), suggesting limited lateral expansion; slight increase in curvature within the posteriormost segments; antepenultimate trunk segment with pleurae forming a ‘skirt’ in outline with a rounded posterior angle, while other pleurae have sharp posterior angles; penultimate trunk segment with pleurae as sharp ‘blades’ pointed posteriorly; last trunk segment without pleurae, its spine appressed against the cuticle (Figs 2 A, 8 C–D, 9 ).

Probably biramous, composed of a short but broad basipod, the rest of the appendage highly reduced (less than 1/3 of the size of first trunk limb) in the second pair, possibly reduced by half the size of first trunk limb in the third (Figs 4 A–B, 8 A–B, 9 ). Nature of exopods unknown in second pair, probably lamellate in the third (Figs 4 A, 8 A–B).

Composed of a two‐segmented base, an enlarged peduncular portion and three long flagellate rami with rigidified bases (=shafts; Figs 2 A–B, 5 A–E, 6 A, 7 , 8 A, 9 , 12A–B), the distalmost (p5, ramus 3) bearing a complex claw system (Figs 7 , 9 ). Base approximately as long as the enlarged peduncular portion, inserted ventrally between the eyes and just anterior to the mouth (Figs 4 A, C, 5 A, 6 A, 8 A) through a putative arthrodial membrane (Fig. 9 ). Two basal segments approximately equal in length: proximal segment (p1) sub‐rectangular; distal basal segment (p2) rounded (Figs 5 A, 6 A). Peduncular portion sub‐cylindrical and two‐segmented, with the first (proximal) segment (p3) greatly enlarged, and second segment (p4) smaller, articulated with p3 segment so that their respective sclerites overlap extensively (Figs 2 A–B, 4 A, C, 5 A, C–E, 6 A, 7 A, C, 8 A, 9 ). Segment p3 displays a large dark spot just proximally to its junction with p4 (Figs 5 A, 6 A, 7 C, 9 , 12A–B) and extending into p4 and p5. The first and second rami represent outer outgrowths of p3 and p4, respectively (Figs 7 A, C, 8 A, 9 ); these extend into micro‐segmented flagella at about the level of the terminal claw complex on p5 (Figs 7 , 9 ). Segment p5 is about two‐thirds of cephalon length and connects to p4 with a relatively large arthrodial membrane (Figs 7 C, 9 ). Its proximalmost portion is shaped like the tapering hand‐guard of a jousting lance (Figs 7 A, C, 8 A, 9 ) and swells somewhat distally, like a femoral bone, near the claw complex. Claw complex composed of a large and proximally curved claw element with possibly blunt tip, on the basal inner margin of which are two successive smaller teeth, with a pair? of straight, medium‐long spine(s) (relative to main claw) attached at the base of the main claw element and pointing distally (Figs 7 A–B, D, 9 ). Like p3 and p4, p5 extending into flexible and finely sub‐segmented flagella, which in p5 are attached above the claws (on outer margin), into a small but clear socket abutting the base of the claw (Figs 7 A–B, D, 9 ). Shaft and proximalmost flexible part of flagellum of p4 adorned internally with minute teeth of alternating longer and shorter sizes; internal margin of p5 also dentate (Figs 7 A–B, D, 9 ). Total length of rami with flagella reaching and possibly largely exceeding half of body length in largest specimens (Figs 2 B, 5 B–E, 9 ). Inward curvature of rami varying among specimens, but rami always sub‐straight.

Intestine covered from third cephalic appendage to at least the tenth trunk segment by contiguous tridimensional inflations in the shape of alternately inverted striated triangles, substantially reduced in size in the ninth and tenth segments (Figs 2 B, 3 A, 5 F, 6 , 9 ). These elements are consistent with the midgut glands previously recognized in Leanchoilia (Butterfield 2002 ). Traces of intestine present in the tenth and possibly 11th trunk segment (Figs 2 C, 9 ) – we here construe that the anus was at the base of the telson, in the 12th trunk segment (Figs 2 A, C, 9 ) based on the observation of small ‘dark stains’ at this location and on previous interpretations of the leanchoiliid body plan. The midgut glands are preceded anteriorly, almost from the anteriormost margin of the body, by a contiguous, large organic body, shallower than the glands, that may represent a stomach (Figs 2 A–B, 4 A, C, 6 A). Additional serial carbonaceous structures of uncertain nature project within the base of the limbs (Figs 2 B, 5 C–E, 6 A, 9 ).

Four eyes, all with very reduced peduncles (Figs 2 B, 4 C, 9 ) and all located in the anteriormost portion of the head: two larger antero‐lateral eyes and two smaller antero‐dorso‐medial eyes; the latter oriented forward, located in front of the larger eyes (Figs 2 B, 4 C, 9 ) and slightly closer together than they are to lateral eyes (Figs 2 B, 4 C, 9 ). Lateral eyes protruding and sub‐elliptical with major axis directed antero‐posteriorly (Figs 2 A–B, 4 A, C, 6 A–B, 9 , 12A); dorso‐medial eyes at the very front of the animal, mushroom‐like in dorsal view, with diameter approximately half that of lateral eyes (Figs 2 B, 4 C, 9 ).

Length of cephalic shield ca. one‐quarter of total body length; slightly wider than anteriormost thoracic segments; margins smooth and devoid of dorsal carinae (Figs 2 , 4 A, C, 9 ). Anterior portion sub‐polygonal: anteriormost margin sub‐straight, extended approximately between the pair of antero‐dorso‐medial eyes (Figs 2 B–C, 4 A, C, 9 ), forming an angle with antero‐lateral margins; the latter are followed by a gentle notch from which extend long lateral margins, curving around the head proper. Lateral margins posteriorly forming a near 90° angle with postero‐lateral margins in lateral view (Figs 2 , 9 ). Postero‐dorsal margins of cephalic carapace concave (‘reflexed’) in lateral view; postero‐lateral margins arched around the body axis (Figs 2 , 9 ).

Biramicity of cephalic and trunk appendages ofgen. nov. sp. nov. A–B, specimen preserved ventrally, with most of the body tissues absent except for part of the ‘great appendages’ and cephalic limbs, ROM 63087 (see also Fig. 5 B top specimen). C–E, habitus of finely preserved moult, ROM 63068 and close‐up of tailpiece (D), showing the lanceolate telson with minute spines of alternating thicknesses and the posteriorward oriented blade‐like pleurae of trunk segment 11. Trunk segment 12 not preserved. E, close‐up of biramous trunk appendages on the counterpart.: ba, basipod; c2–3, cephalic appendage (2–3); ca, carina; cl, claw(s); ds, differential spines; en, endopod; ex, exopod; exs, exopodial shaft; lam, lamellae; p1–7, podomere (1–7); pd, podomeres. Scale bars represent 5 mm (A, D–E); 2.5 mm (B); 10 mm (C).

Morphology of the ‘great appendages’ ofgen. et sp. nov. A, paratype ROM 63066; B, close‐up of distal portion (insert in A) showing teeth along podomeres 4 and 5. C, ROM 63069, preserved in frontal view with ‘great appendages’ turned slightly inward; note the stout round structure inside p4. D, holotype ROM 62977 (see also Fig. 2 A), showing teeth along podomeres 4 and 5, terminal claw and secondary spines along podomere 5 and flagellum insertion. E, ROM 63088 showing finely sub‐segmented flagellum.: am, arthrodial membrane; if, insertion point of flagellum; fce, flanking claw element; hg, hand‐guard; mce, main claw element; p3–5, podomere of the ‘great appendage’ (3–5); pn, (proximal) peduncular node; sf, sub‐segmented flagellum; sp3–5, shaft of the ‘great appendage’ distal podomere (3–5); ss, secondary spines; te, teeth. Scale bars represent 2.5 mm (A, C, E); 1 mm (B, D).

Morphology of the midgut glands ofgen. et sp. nov. A–B, ROM 63070 (see counterpart of bottom specimen in Fig. 5 A; see also Fig. 11 A) showing the anterior extent of the midgut glands (B, close‐up of framed area in A). C, paratype ROM 63066 showing the fan‐like texture of the glands (see also Figs 2 B, 3 A). D, ROM 63088 showing the pattern of successive inverted triangles characterizing the leanchoiliid midgut glands (see also Fig. S2). B–D, specimens coated in ammonium chloride sublimate.: aeg, anterior extension of midgut glands; atg, alternate triangular glands; fg?, foregut?; ga, ‘great appendage;’ le, lateral eye; m?, mouth?; mgg, midgut glands; ol, oral lumen; pn, (proximal) peduncular node; sit, sub‐intestinal tonguelets. Scale bars represent 10 mm (A); 5 mm (B–C); 2.5 mm (D).

Antero‐ventral morphology and gut‐related structures ofgen. et sp. nov. A, ventral view of anterior section showing probable position of mouth on the right, ROM 63070 (see counterpart Fig. 6 A, bottom specimen). B–G, ROM 63087; B, slab with two large specimens (see Fig. 8 A–B for close‐up of top specimen); C–E, close‐ups of bottom specimen under different illuminations and photo‐treatments: C, dry, cross‐polarized light; D, wet, direct light; E, superimposed images of both part and counterpart using Image Apply and Difference blending mode in Adobe Photoshop™ CS6; F–G, close‐ups of framed areas in B and E, specimens coated in ammonium chloride sublimate showing midgut glands in dorsal (F) and lateral (G) views with preserved details of the fan‐like texture (F) and structures afferent to the gut possibly internal to the limbs (G).: aa, adjacent arc (identity unclear); app, appendages; bp1–2, basal podomere (1–2); le, lateral eye; mgg, midgut gland(s)/gut; ol, oral lumen; pds1–2, peduncular segment (1–2); pn, (proximal) peduncular node; scr, shafts of chelate rami; sit, sub‐intestinal tonguelets. Scale bars represent 5 mm (A, G); 20 mm (B); 10 mm (C–E); and 6.5 mm (F).

Eyes, cephalic appendages and anterior anatomy ofgen. et sp. nov. A, image of anterior section in latero‐oblique view with breakages in the head shield revealing the base of cephalic appendages, holotype ROM 62977 (see Fig. 2 A). B–C, paratype ROM 63066 (see Fig. 2 B); B, second cephalic (postoral) limb, showing probable biramicity (insert in C). Superimposed images of both part and counterpart using Image Apply and Difference blending mode in Adobe Photoshop™ CS6). C, anterior section in latero‐oblique view showing lateral and median eyes, a possible stomach, one basilar sclerite of the left ‘great appendage’ and the base of the cephalic limbs (C2–C4).: bra, bradorid; c2–4, cephalic appendage (2–4); en?, endopod?; ex, exopod?; le, lateral eye; me, median eye; mgg, midgut glands; pga, peduncle of ‘great appendage’; pn, (proximal) peduncular node; s/g/gl, stomach/gut/digestive glands; st?, stomach?; t1, trunk appendage 1. Scale bars represent 5 mm (A–B); 3.5 mm (C).

Elemental maps ofgen. et sp. nov. A, paratype ROM 63066 (see Fig. 1 B). B, paratype ROM 63067 (see Fig. 1 C). Note the patchy carbonaceous remains found throughout the specimen, and the presence of an Fe–Mg–O phase templating the midgut glands and eyes. The intestine and midgut glands are primarily enriched in Ca and P, in the form of CaPO: ACS, ammonium chlorite sublimate; BSE, backscatter scanning electron micrograph image;, intestine; le, lateral eye; mg, midgut gland. All scale bars represent 5 mm.

Habitus ofgen. et sp. nov. in latero‐oblique (A–B) and dorso‐oblique (C) views. A, holotype ROM 62977 (see close‐ups of anterior section and ‘great appendages’ Figs 4 A, 7 D, respectively). B, paratype ROM 63066 (see close‐ups of anterior section, midgut glands and ‘great appendages’ Figs 4 B, C, 6 C, 7 A, B, respectively). C, paratype ROM 63067. Dashed rectangles indicate the locations of areas used for elemental mapping (see Fig. 3 ). B–C, superposed images of part and counterpart pairs using Image Apply and Darken blending mode in Adobe Photoshop™ CS6.: c1–4, cephalic appendage (1–4); ga, ‘great appendage’; le, lateral eye; me, median eye; t1–12, trunk segment (1–12); te, telson; ten, trunk appendage endopod; tex, trunk exopod. All scale bars represent 10 mm.

Leanchoiliid arthropod with 17 segments (head: 4; trunk: 12; tailpiece: 1). Cephalic shield with a sub‐polygonal anterior margin and a distinct, sub‐straight, anteriormost margin; lateral margin with a slight notch about one‐third the length of the cephalon from the anterior. Shafts of rami 2 and 3 of the ‘great appendages’ adorned on their inner margins with teeth of slightly varying sizes. Body ending in a lanceolate telson traversed dorso‐medially by an acute carina. Posterior two‐thirds of telson margin adorned with minute postero‐lateral spines alternating between stout and thin.

Latinized spelling of Yawu?nik’ , legendary creature of the Ktunaxa First Nation whose people live in the South Kootenay National Park area (today's part of south‐eastern British Columbia) . Yawu?nik’ is central in the creation story of the Ktunaxa and is described as a ‘huge sea monster who killed many of the animals’ ( http://www.ktunaxa.org/who-we-are/creation-story/ ).

The regions of the midgut glands that show an association with the Fe–Mg–O phase have relief, indicating that these parts of the structures were also originally replaced by calcium phosphate. It is possible that this association represents partial replacement of the phosphate minerals, but the opposite distributions of C and Fe–Mg–O in this region suggests instead that the latter phase formed at the expense of carbonaceous remains that lay atop the structures replicated in calcium phosphate. Detailed study of the early diagenetic processes surrounding fossilization at Marble Canyon and the metamorphic overprint on fossil composition is ongoing.

The Fe–Mg–O phase exhibits a preferential association with the eyes and midgut glands. Such selective replacement of certain tissues in secondary mineral phases can be explained in part by differences in the thickness of the original carbonaceous remains in different anatomical aspects of fossils, as seen in graptolites (Page et al . 2008 ). During the heating accompanying metamorphosis, carbonaceous remains lose some fraction of their original volume, leaving small voids around the fossils, which served as loci for secondary mineral precipitation. This process continued as the temperature increased towards peak metamorphic conditions, and the precipitation of different secondary mineral phases was favoured in new void spaces produced by continued volatilization of carbonaceous remains (Page et al . 2008 ). Mineral‐replacement reactions were also involved (Butterfield et al . 2007 ), as indicated by the partial replacement of vein calcite by both the Fe–Mg–O phase and the K–Al–Si–O (muscovite) phase (see Fig. 3 ).

Specimens show various orientations consistent with rapid burial following brief entrainment in turbulent mudflows. Several obliquely or laterally preserved specimens are bent dorsally, a posture we attribute to rigor mortis contraction of the dorsal musculature (Fig. 2 A). Elemental mapping (Fig. 3 ) reveals that the taphonomic expression is consistent with that of Burgess Shale fossils from the Walcott Quarry (Orr et al . 1998 ; Butterfield 2002 ; Butterfield et al . 2007 ). Specimens were originally preserved as carbonaceous compressions (Butterfield 1995 ; Gaines et al . 2008 ), with aspects of the digestive organs of Yawunik , viz. intestine and midgut glands, typically preserved three dimensionally in calcium phosphate (Figs 3 , 5 C–G, 6 ), as previously described in Leanchoilia (Butterfield 2002 ). Original carbonaceous remains still occur throughout the specimens (Fig. 3 ). Pyrite, clearly visible as bright spots in BSE (Fig. 3 ), is also present in the intestine and midgut glands as isolated small (<30 μm) crystals and clusters, but does not replicate anatomical structures, and therefore is incidental to preservation. Like fossils of the Walcott Quarry, specimens exhibit thin coatings of mineral templates that precipitated at elevated temperature during greenschist facies metamorphism (Orr et al . 1998 ; Powell 2003 ; Butterfield et al . 2007 ; Page et al . 2008 ), long after fossilization was complete. In our specimens, two such phases are present. An Fe–Mg–O phase, not previously documented in Burgess Shale fossils, occurs prominently. The second phase, K–Al–Si–O (muscovite), which is consistently associated with Walcott Quarry fossils (Butterfield et al . 2007 ), is also present, but exhibits a very limited distribution. The bulk composition of this phase is very similar to that of the matrix and is best identified from the elemental maps by slight depression in Si relative to the matrix, in regions where K, Al, Si and O are all present.

These results emphasize that the trend in reduction of relative head length (RHL) and telson length (RTeL) is consistent only within a subset of the measured specimens. If relative telson length (RTeL) may be subject to a large natural variation, four specimens at least exhibit isometry of RTeL and RHL relative to trunk length (Fig. 11 C–D). These findings indicate that the observed allometric patterns may result from ontogenetic changes or from sexual dimorphism.

The dispersion of points for the regression of RHL reflects some of the bimodality observed in the density distribution. Such a dissociation between smaller and larger specimens is also observable for RTeL. When applying a residual‐based colouring of PCA results (Fig. 11 C, see 3 ), we obtain a segregation along the RTL gradient of specimens with large RHL (green triangles), RTeL (red crosses) or both (purple Xs), and those specimens – the largest in absolute size – with both small RHL and RTeL (black spots). A nonlinear regression analysis of the entire data set excluding RTL does not find any function that fits significantly better than a linear one (Fig. 11 D), but there is a pronounced asymmetry of the point distribution around the regression line, indicating that the possible size gradient is discontinuous.

Linear regression analyses between measures of shape and the total length show two opposite trends (Fig. 11 B). Although adjusted R 2 values are weak, the positive correlation (aR 2 = 0.30) of RTL with the total length is in contrast to the negative relationships of the other measured body parts, that is RHL, RTeL, RTW and RHW. This would mean that growth is allometric, such that the body tends to become increasingly slender and the head relatively shorter as the overall length of the animal increases. The strong positive correlation between relative trunk and head widths (RTW and RHW, aR 2 = 0.79) demonstrates that tight relationships between measurements can be significant despite the small sample size and differences in taphonomy. The small and dispersed distributions of points overall around regression curves are indicative of small residuals, suggesting that measurement error due to variation in burial angle is limited.

Morphometric analyses for 22 specimens of Yawunik kootenayi gen. et sp. nov. A, standardized density distributions of TL (black), RTL (blue), RHL (green), RTeL (red), RTW (orange) and RHW (cyan). Note how RTeL, RTW and RHL deviate from the Gaussian‐like distribution of trunk lengths. B, linear regressions of relative lengths by total lengths. Trunk width, head width and head length are negatively correlated to trunk length. Some specimens, however, show identical proportions with change in size, explaining results in (A). C, principal component analysis (PCA) of the specimens. Colouring and choice of symbols for the points determined by the residual value of regression analyses for respectively (RHL/RTeL). Black dots: (−/−); red crosses: (−/+); green triangles: (+/−); purple Xs: (+/+). The specimens represented by purple Xs, keeping similar proportions across sizes, may be sexually or ontogenetically different from those represented by black dots, whose habitus tend to be more slender with trunk size. D, regression of all the combined data by total length using a generalized fitting model. Fit with lowest residual sums of squares (4.82): log(138.47x −2.94 ). Linear fit (4.84): −0.698x + 3.40. Abbreviations : RHL, relative head length; RHW, relative head width; RTeL, relative telson length; RTL, relative trunk length; RTW, relative trunk width; TL, total length.

Shape components, as captured by the logarithm of body part lengths and widths divided by total body length (see 3 ), deviate from normal distributions to various degrees (Fig. 11 A). Relative trunk length (RTL) and relative head width (RHW) follow the pseudonormal distribution of total length, while relative telson length (RTeL) and relative trunk width (RTW) contain clusters of marginal values and show a translation of their respective medians away from the distribution of trunk lengths (Fig. 11 A). The density distribution of relative head length (RHL) is a prominent outlier, its quasi bimodality suggesting that some specimens may exhibit head proportions diverging from the general trend in shape between smaller and larger specimens.

Yawunik is consistently grouped with the Australian Oestokerkus based on the shared presence of a pair of enlarged and elongated ‘great appendages’, but this feature is also characteristic of Leanchoilia superlata . The enlarged and elongated ‘great appendages’ distinguish these taxa from other leanchoiliids that have frailer ‘great appendages’, a distinction that can be congeneric, as in L. persephone (García‐Bellido and Collins 2007 ) and L. illecebrosa (Liu et al . 2007 ; see the discussion on the definition and boundaries of this character in Aria et al . 2015 ). Optimization based on overall parsimony considers the large ‘great appendage’ trait convergent in L. superlata , a well‐supported sister‐taxon to L. illecebrosa based on the serration of pleural margins and the presence of axial ridges. Further resolution within leanchoiliids seems to be hampered by conflicting characters or simply by the lack of clear apomorphies. A consistent signal was retrieved in the continuity of telson shape, placing Alalcomenaeus as sister‐taxon to all other leanchoiliids (Fig. 14 ), following that a spatulate telson is a feature of the ancestor Yohoia . Using the ‘newstate’ coding method, a change in cephalic shape from flat to convex/rostral then places Yawunik + Oestokerkus as sister‐group to the remaining taxa (Fig. 14 B, D). It remains to be determined, however, whether such characters vary in more complex ways within this group, for instance, if they may be associated with sexual dimorphism. The poor resolution overall demonstrates that, beyond the description of new specimens or the redescription of several taxa, a revision of all leanchoiliids is critically needed.

The monophyly of the leanchoiliid clade is almost exclusively supported by ‘great appendage’ characters: the flagellate condition, the swollen state of the peduncular podomeres and the presence of a claw complex at the tip of the distalmost ramus (Fig. 14 C, D). The presence of median eyes, the ornamentation of trunk endopods and the fixation of an 11‐segmented trunk also support the monophyly of the group, but the importance of each of these characters is method dependent and/or homoplastic at other nodes. Median eyes also characterize crown chelicerates and only represent a local synapomorphy with leanchoiliids if the clades are not directly related, whereas paired spinose projections on the trunk endopods are present in some artiopods and in Martinssonia Müller and Waloszek ( 1986 ). The number of trunk segments is a particularly variable character overall, but there seems to be a consistent reduction from 13 to 11 within the Cheiromorpha. Yawunik 's 12‐segmented trunk shows that this condition, however, may be prone to reversals, whereas the lower number of trunk segments in Oelandocaris oelandica Müller ( 1983 ) is possibly related to its developmental stage.

Throughout our analyses, the Leanchoiliidae are retrieved consistently as a monophyletic clade with Yohoia and Haikoucaris as its sister‐taxa within another larger monophyletic megacheiran clade we call Cheiromorpha (Fig. 14 ). This clade can be further expanded to include chelicerates, Jianfengia Hou ( 1987 ) and Fortiforceps (Fig. 14 A, C); we refer to this configuration informally as the Cheliceromorpha, as the validity of this group remains in doubt. Jianfengia and Fortiforceps are otherwise retrieved as sister‐taxa to the clade of all euarthropods that share a ground pattern of a stable number of seven podomeres on each of their trunk endopods. We call this clade the Heptopodomera, and we hence propose:

Discussion

Hypostome It has remained unclear whether or not leanchoiliids possessed a hypostome and, if so, whether it was a sclerotized or a soft clypeo‐labrum/hypostome (sensu Waloszek and Müller 1990). The counterpart of specimen ROM 63070, consisting of a head preserved in ventral view, clearly shows the area below the proximal portion of the ‘great appendage’ above the mouth, and no trace of an intermediate sclerite can be detected (Figs 5A, 6A; see also ROM 63087, Fig. 8A). Haug et al. (2012a) reported evidence for a hypostome in a specimen of Leanchoilia superlata with a cephalic stain preserved in dorsal view, but no specimen so far has exhibited the characteristic bulge of a hypostome, as preserved in trilobitomorphs. A clear hypostome/labrum has otherwise not been observed in either Leanchoilia (Bruton and Whittington 1983; García‐Bellido and Collins 2007) or Alalcomenaeus (Briggs and Collins 1999). The presence of a hypostome in fuxianhuiids (Hou and Bergström 1997; Waloszek, et al. 2005; Yang et al. 2013), should, given a basal phylogenetic placement of these taxa (among bivalved arthropods (Budd 2002) or derived from them (Legg et al. 2012)) with respect to Euarthropoda Lankester (1904), predict the presence of this structure in more derived taxa (given a homology throughout, see e.g. Scholtz and Edgecombe 2006). Budd (2008) interpreted some ‘stem bivalved’ arthropods as probably having a hypostome/labrum as well, although it is much less clear than the presence of an anterior sclerite. Our reassessment of Oelandocaris oelandica, if correct, may shed light on the matter and suggests that a hypostome/labrum is present but not easily preserved in leanchoiliids. It may be that the ventral depression posterior to the frontal appendages in ROM 63070 (Fig. 5A) does not correspond to the mouth but, instead, to the location of a former, taphonomically removed hypostome – this part is indeed damaged and the posterior end of the specimen is lacking. Additional evidence is required to support such a view.

Nature of the cheiral internal ‘organ’ Several specimens of Yawunik display dark/reflective spots in the middle of their ‘great appendage’ peduncles (e.g. Figs 5A, 6A, 7B, 12A–B). These structures were identified in Leanchoilia by Bruton and Whittington (1983) and interpreted by the authors as an ‘excretory organ’ without further mention or discussion, although they noticed the apparent similarity to digestive organs based on purported phosphate mineralization. The ‘dark spots’ were recently mentioned and illustrated by Haug et al. (2012a, fig. 6D), but again without further analysis. Figure 12 Open in figure viewer PowerPoint Yawunik kootenayi gen. et sp. nov.; A, ROM 63070 (see also Fig. Leanchoilia superlata in which two successive spots are clearly visible in each peduncle, connected by thick tissues which become thin filaments as they enter each distal spine ROM 61900. D, dissected chelicera of the tarantula Brachypelma smithi showing connective filaments and basilar sclerite (photograph courtesy of David Hubble). Abbreviations: bs, basilar sclerite; dn, distal node; fct, filament of connective tissue; fr1–3, filament of ramus 1–3; pf, proximal filament; pn, (proximal) peduncular node; pnc, (proximal) peduncular node centre. Scale bars represent 0.5 mm (A); 2.5 mm (B–D). Evidence for internal tissues preserved within the ‘great appendages’ of Burgess Shale leanchoiliids. A–B,gen. et sp. nov.; A, ROM 63070 (see also Fig. 6 A, B, top specimen); B, ROM 63069, close‐up on right ‘great appendage’ (see also Fig. 7 C). C,in which two successive spots are clearly visible in each peduncle, connected by thick tissues which become thin filaments as they enter each distal spine ROM 61900. D, dissected chelicera of the tarantulashowing connective filaments and basilar sclerite (photograph courtesy of David Hubble).: bs, basilar sclerite; dn, distal node; fct, filament of connective tissue; fr1–3, filament of ramus 1–3; pf, proximal filament; pn, (proximal) peduncular node; pnc, (proximal) peduncular node centre. Scale bars represent 0.5 mm (A); 2.5 mm (B–D). These structures are also present in the peduncles of Yohoia's ‘great appendages’ (Haug et al. 2012b, fig. 5C, F) and, given that they seem to be only rarely preserved, they may well be associated with ‘great appendage’ peduncles in general. Interestingly, however, a similar carbonaceous compression was found in the postoral chelae of a new branchiocaridid from Marble Canyon (under study), which leads to the conclusion that this feature may correspond to a general anatomical property of grasping appendages. Fortunately, one particularly well‐preserved specimen of Leanchoilia superlata reveals in great detail the entire afferent system to which this structure belongs (Fig. 12C). There are, in fact, two ‘bulbs’, with the larger one – the one more commonly preserved – located approximately at the junction between podomeres 3 (= peduncle) and 4, and the smaller one, about half the size of the other, located at the junction between podomeres 4 and 5 (= the distalmost ramus). The two bulbs are not only interconnected by a large band of tissue, but three filaments ramify from them and project into each ramus of the ‘great appendages’, one from the ‘peduncular bulb’ and the other two from the ‘distal bulb’. Similar connective filaments (or ‘tendons’) are present, for instance, in chelicerae, where they support muscle insertions (D. Hubble and J. Dunlop, pers. comm. 2014; Fig. 12D). This view is consistent with the presence of such tissues across analogous structures and would confirm the eudesmatic nature of such segments (Couzijn 1976), such as those in the chelicerae, as well as the flexibility of the three rami. These connective bands and additional muscle fibres are attached to internal cuticular outgrowths usually referred to as ‘basilar sclerites’ or ‘sclerotized nodes’ in, for example, insects and acarines. Such sclerites could correspond to the dark rings at the junctions between podomeres, but this would imply that these sclerites were conspicuously thicker than the cuticle itself. The combination of the sensory function of the flagella with the raptorial motion of the terminal podomeres could also suggest that neural connections between the brain and the deutocerebral appendage should be especially developed. Accordingly, it could be hypothesized that the filaments are apparent nerves, and the intermediary organs are ganglia and somata, both particularly large with respect to those of modern arthropods (e.g. in chelicerates, Hjelle 1990; Foelix 1996). This view could be supported by the fact that the filaments seem to project very deep within the podomeres and might even reach their tips. Notwithstanding, this explains neither the peculiar intensity of the dark spots, nor the relatively inconspicuous imprint of a large neural centre such as the brain in the same specimens. Given the occurrences of preservational similarity between this spot and the broad circum‐intestinal features, the hypothesis of a secretory gland cannot be definitively ruled out. It is also possible that the intensity of the dark spot and the nodes with their filaments represent in fact more than one type of tissue. A more thorough comparison of this feature across Burgess Shale arthropods and additional elemental mapping could help resolve this question.

Functionality of the ‘great appendages’ The flexibility of the three ‘great appendage’ rami was discussed by Haug et al. (2012a) for Leanchoilia superlata based on segmental anatomy and taphonomy. Haug et al. concluded that the flexibility of the appendage was limited in the posteriorward position by the size of arthrodial membranes and the shape of the proximal peduncular segment (p3) in particular. Contrary to these expectations, however, in Yawunik, neither the bodies of the specimens nor their appendages are disarticulated in specimens in which the appendage projects posteriorward. Where disarticulation is observed, is it not obvious that the cause is a biomechanical issue rather than simply the result of taphonomy (see, e.g. specimen Fig. 2B, whose frontal appendage is still in position but twisted laterally). Our own observations on Yawunik and also on Leanchoilia do not suggest an angular shape of the proximal peduncular segment (p3) but, rather, support a rough cylindrical shape, which would have been much easier to rotate antero‐posteriorly. The remaining biomechanical issue is that the rotation is not only antero‐posterior, but also lateral, as the position of the distal, claw‐bearing ramus (p5) is inverted in the forward position (see Haug et al. 2012a, fig. 12L). This movement, however, requires only some degree of additional lateral flexibility of the basal segments, as is particularly explicit in ROM 63070, which is preserved in ventro‐frontal view (Figs 5A, 6A). The appendages are neither disarticulated nor particularly damaged and still are in an intermediate position, allowing for both frontal and reversed posterior positions, providing an outward rotation (Figs 5A, 6A). This mechanism remains conjectural, but thanks to the range of preserved orientations of the ‘great appendages’ in Leanchoilia and Yawunik, it is now possible to reconstruct the different steps of the full motion (see Supporting Information, Figs S3–S6). In addition to evidence of possible internal muscle attachment and the presence of small teeth on the margins of the two distalmost rami, these observations on the flexibility of the peduncular segments emphasize the functionality of the leanchoiliid ‘great appendages’. The combination of such a flexible appendage and a well‐developed sensory system probably allowed leanchoiliids to capture a wide range of prey. This may have helped compensate for the lack or limitation of additional appendage differentiation at this early stage of arthropod evolution.

Eyes The number of eyes and even their presence have been subjects of significant debate in the descriptive history of leanchoiliids. Recently, Haug et al. (2012a) proposed that the presence of eyes should not be regarded as a diagnostic feature of the group. Even though this character is controversial, it constitutes a critical asset for the classification of leanchoiliids (García‐Bellido and Collins 2007), especially because of the evidence for additional, median ‘ocelli’, which are also characteristic of chelicerates and are therefore of phylogenetic significance (Cotton and Braddy 2004). Yawunik clearly demonstrates that these ‘ocellar’ spots, preserved in the same manner as its lateral eyes, are, in fact, fully developed median eyes. They are mushroom‐shaped, as is the case for instance in arachnids. The median eyes might have been poorly preserved in other morphotypes due to different qualities of preservation, or possibly their smaller size. The presence of two median eyes is in fact likely to be diagnostic of known leanchoiliids. The presence of a possible fifth eye in Alalcomenaeus has been discussed (Briggs and Collins 1999; García‐Bellido and Collins 2007), but the evidence remains unclear. The plesiomorphic condition for the group would have been a single pair of eyes (see phylogenies below), but the peculiar condition of Opabinia Walcott (1912b), along with uncertainties regarding the eyes and frontal organs in a number of bivalved stem arthropods, as well as the presence of four median eyes in pycnogonids, suggests that variation in number of eyes might have been greater among stem taxa and that this plasticity – rather than an exact number – might be the plesiomorphic condition retained in chelicerates.

Systematic boundaries of the leanchoiliids and the affinities of Actaeus and Oelandocaris Extensive systematic (Størmer 1944; Bruton and Whittington 1983; Briggs and Collins 1999; García‐Bellido and Collins 2007; Haug et al. 2012a) and phylogenetic (Bergström 1992; Budd 2002; Cotton and Braddy 2004; Edgecombe et al. 2011) studies have contributed to anchor three genera with flagellate frontalmost appendages – namely Leanchoilia, Alalcomenaeus and, recently, Oestokerkus – as monophyletic within the yohoiid megacheirans (Cotton and Braddy 2004; Edgecombe et al. 2011; Haug et al. 2012a; Legg et al. 2012). Some other taxa with leanchoiliid affinities might, however, have been overlooked. An immediate example is Actaeus armatus Simonetta, known from only one specimen and only marginally considered in the literature (USNM 155597), although the holotype is rather well‐preserved (Fig. 13). Available evidence would argue against considering Actaeus as an oddly preserved Leanchoilia, viz. the reduced relative size of the ‘great appendage’ peduncles (somewhat closer to the Alalcomenaeus condition; Fig. 13B) and the wide, spatulate lamellae present on the trunk exopods (Fig. 13C). Under water, the specimen reveals additional details such as the segmentation and claws of ‘great appendages’ (Fig. 13B, D) and outlines of cephalic exopods. The presence of flagella and also of claws on one of the rami confirms the leanchoiliid affinity of Actaeus, even if a thorough anatomical analysis is for now impossible. Figure 13 Open in figure viewer PowerPoint Anatomical details of the leanchoiliid Actaeus armatus, holotype USNM 155597. A–B, dry, cross‐polarized light; C–D, wet, direct light. A, C and D are from the part, and B is from the counterpart. A, whole specimen. B, right ‘great appendage’ showing part of the peduncle and the rigidified shaft, claw complex and flagellum of the distal ramus. C, paddle‐shaped trunk exopod, featuring wide spatulate lamellar projections. D, left lateral eye and left ‘great appendage’ showing the division into three rami. Abbreviations: cc, claw complex; ex, exopod; fl, flagellum; lam, lamellae; lle, left lateral eye; rlga, ramus of left ‘great appendage’; plga, peduncle of left ‘great appendage’; rdr, right distal ramus. Scale bars represent 10 mm (A); 2 mm (B); 2.5 mm (C); 1 mm (D). Less obvious, and certainly controversial, would be the case of Oelandocaris oelandica Müller. The Cambrian animal is minute, as is characteristic of all ‘Orsten’ fossils, and even the larger of the two forms proposed to belong to this morphotype (Stein et al. 2008) could be an immature stage, as is probably the case for numerous arthropod taxa from this locality (Müller and Waloszek 1987; Waloszek and Müller 1990). In their report and later in their detailed restudy, Stein et al. (2008) argued for the assignment of Oelandocaris to stem crustaceans, along with the other stem crustacean morphotypes from the ‘Orsten’ based on two characters: the presence of sub‐segmented (or ‘multi‐annulated’) cephalic exopods and that of a proximal endite, thought to later form the coxae and other basal elements of the masticatory appendages. Waloszek and Müller argued for such endite to be a strong synapomorphic character of Eucrustacea (or rather, Phosphatocopina (Müller, 1964) + Eucrustacea (Waloszek and Müller, 1990)). The piece of integument alleged to be a proximal endite in Oelandocaris is tiny and not sufficiently detailed (Stein et al. 2008, fig. 5B2). Nevertheless, the presence of such an inconspicuous feature would be extremely difficult to verify in specimens from Burgess Shale type deposits, in spite of the detailed preservation, and the prevalence of such a proximal endite may therefore be questionable in the entire stem‐group. As for the sub‐segmentation of cephalic exopods, Stein et al. (2008) noted that this is a functional character present in free larvae of extant eucrustacean taxa, and also in stem ‘Orsten’ immature morphotypes, in phosphatocopines, and even in the meraspid stages of Agnostus Brongniart (Brongniart and Desmarest 1822; Müller and Waloszek 1987). The general lack of information on larval stages in other stem taxa is a good reason to be cautious about using this character too strictly for analysis of the crustacean lineage. Although Oelandocaris exhibits only up to five trunk segments (Stein et al. 2008) in its larger morph (possibly due to its immature state), it features an habitus and several characters reminiscent of leanchoiliid anatomy. For instance, the antennules are composed of three articulated rami. The shafts, nevertheless, appear to be sub‐segmented, and the peduncle is directly attached to the body via an arthrodial membrane, in the apparent absence of a two‐segmented base. Distal portions of the shaft are unknown, as are flagella, but the flexibility of the sub‐segmented shafts makes the presence of a claw complex and complementary flagella unlikely. If correctly reconstructed, the reduced endopod of the second cephalic appendage of Oelandocaris is similar to that described in Leanchoilia superlata by Haug et al. (2012a), that is four main short podomeres, the exopod of which is unfortunately still unknown. The exact configuration is currently unknown in Yawunik, but the reduced condition of the anterior cephalic appendages leaves open the possibility of a similar condition. If homologous, these appendages could represent plesiomorphic mouthparts, like those in the Cephalocarida Sanders (1955; Addis et al. 2007). The biramous appendages of the trunk are phyllopodous and equivalent to the leanchoiliid type, and to Leanchoilia in particular, except for the contiguity of the distal portion of the exopod with the first endopodial podomere. The exopodial setation was reconstructed as spinose by Stein et al. (2008), but, given the type of preservation, this seems rather putative. Notable differences include the ventral position of putative median eyes, the presence of a large hypostome, and a head incorporating five pairs of appendages. The extent to which resemblances or differences are ontogenetic features cannot be determined at present, but it is possible that, in a scenario where leanchoiliids are the sister‐group to mandibulates (not supported herein) or, possibly, to a somewhat convergent clade containing Oelandocaris, certain ancestral characters may be retained in the larval stages of the more derived taxa. This hypothesis seems more tenable than, for example, the possible loss of hypostome and fifth cephalic pair in adult leanchoiliids, if Oelandocaris is considered to belong to this group. The intriguing arthropod Enalikter aphson Siveter et al. (2014) from the Silurian Herefordshire Lagerstätte has been introduced as a megacheiran, but, given the level of detail available, it could well be associated with an ‘intermediate’ taxon such as Oelandocaris. Whether Enalikter is a ‘lamellipedian’ is not clear, insofar as the appendages of Enalikter seem to be only partially reconstructed; the ‘rays’ of the exopods are possibly parts of wider flat structures (Siveter et al. 2014). It has been associated, however, with Bundenbachiellus giganteus Broili (1929) of the Hunsrück Slate Lagerstätte, which distinctly shows lamellar exopod setae (Moore et al. 2008). Apart from trilobitomorphs, lamellar exopod setae are present in a range of stem taxa including Canadaspis Walcott (1912b) and megacheirans, with wider lamellae possibly present in Fortiforceps Hou and Bergström (1997), intermediate lamellae in Actaeus and smaller ones in Leanchoilia. Only a more detailed reconstruction of Enalikter could shed light on the affinity of its exopod ornamentation. Enalikter apparently also possesses a bulging, albeit modified hypostome, a trait present in Oelandocaris but so far not clearly documented in leanchoiliids. In addition, both morphotypes are relatively small, although Enalikter is still much larger than the large Oelandocaris morph. Unlike in both leanchoiliids and Oelandocaris, however, the three rami of the frontal appendages in Enalikter do not seem to be imbricated and articulated within one another, but rather jointed together to the peduncle (Siveter et al. 2014) – a major difference in the context of the megacheiran ‘short great appendages’. Because of the incomplete and insufficiently detailed information, we have decided to include neither Bundenbachiellus nor Enalikter in our phylogenetic analysis, but only Oelandocaris. Bundenbachiellus and Enalikter should, however, be considered of potential importance to further clarify the position of Oelandocaris with respect to leanchoiliids and stem crustaceans.

Leanchoiliids as stem arthropods: polarization, inapplicables and implied weights The issue of outgroup selection and the polarization of characters may be major causes for inconsistent phylogenetic results in cladistic studies of the arthropods (Rota‐Stabelli, et al. 2011; Legg et al. 2012). The placement of megacheirans in particular has been affected by assumptions regarding their closest sister‐taxa. The selection of marellomorphs (Briggs and Fortey 1989) or hypothetical stem taxa (Cotton and Braddy 2004) has resulted in an arachnomorph solution, while the use of lobopodian or dinocaridid outgroups rather favoured the grouping of artiopods with mandibulates (Daley et al. 2009; Legg et al. 2012). Polarization based on fuxianhuiids has resulted in support for both hypotheses (Stein and Selden 2012; Lamsdell 2013). Although it seems appropriate to choose the most basal part of the stem – Cycloneuralia, the sister‐clade to lobopodians, onychophorans, tardigrades and all arthropods (Budd and Telford 2009; Rota‐Stabelli et al. 2011) – as the outgroup, it must also be stressed that any expansion of the cladogram at this level is accompanied by issues of homoplasy, analogy and, especially in the case of lobopodians and dinocaridids, numerous autapomorphies not informative for polarization. Our own results show that the traditional Arachnomorpha and Artiopoda + Crustacea scenarios can both, in fact, be generated from the same data set, using the same outgroups, but using different methods of coding for inapplicable states. The underlying reason is that these two coding methods also greatly affect polarization. This result puts particular emphasis on the fact that rooting character states are difficult in this context and that the historical debate of early arthropod evolution is strongly related to this problem, from which the same two scenarios persistently emerge. Nonetheless, consistency between polarizing hypotheses – and hence evolutionary scenarios – can still be attained for some ingroup nodes by strong sets of synapomorphies. In other words, the issue of polarization is first and foremost an issue of higher nodes. For situations in which the outgroup character state is also inapplicable due to the absence of the derived structure (a rather common condition for high level phylogenies), ‘newstate’ coding preserves the inapplicability as information rather than uncertainty. This, however, comes at the cost of redundancy and thus overemphasis on traits developed into character variations. ‘Newstate’ coding of our data set favoured a cladogram in which stem arthropod features gradually contribute to the assembly of the crown arthropod body plan (Fig. 14C). Under this scenario, the crustaceans diverge early from the rest of a large group, the Arachnomorpha, among which only the chelicerates survived beyond the Palaeozoic. In this configuration, Jianfengia and Fortiforceps are surprisingly derived, as it appears that the character of endopodial podomere number is optimized as homoplastic, in contrast to other characters such as the absence of ‘rounded tip’ at the apex of the frontalmost appendage that appear as questionable synapomorphies of so many taxa. Interestingly, this is not because the podomere number character is downweighted by the implied weights method, but because of the coding of inapplicables, as the same configuration is retrieved in the unweighted consensus tree. A probable explanation for this unlikely scenario is that repeated inapplicable states among the dependent characters describing the ‘short great appendages’ result in a strong signal for polarizing nonmegacheirans/nonchelicerates as plesiomorphic. Figure 14 Open in figure viewer PowerPoint et al. ( 2015 Strict consensus cladograms for leanchoiliids and selected panarthropods (35 total taxa). A–B, coding inapplicables as additional states; A, consensus of 945 trees with unweighted characters; best tree: length = 210, CI = 0.4905, RI = 0.7390, RC = 0.3625. B, consensus of 315 trees using implied weighting with concavity k = 4; best tree: Goloboff fit = −37.0857, CI = 0.4858, RI = 7341, RC = 0.3567. C–D, coding inapplicables as unknown states; C, consensus of 1110 trees with characters unweighted; best tree: length = 137, CI = 0.4672, RI = 0.6907, RC = 0.3227; D, consensus of 45 trees using implied weighting with concavity k = 4; best tree: Goloboff fit = −39.997, CI = 0.4612, RI = 6907, RC = 0.3227. On unweighted topologies, node robustness is evaluated by Bremer support (bold, top of branch), jackknife partitioning (below branch, left) and bootstrap sub‐sampling (below branch, right). Values below 25, or at nodes not retrieved by the sub‐sampling method, are marked with a dash. On weighted topologies, important synapomorphies are indicated at nodes and refer to the numbers of each on the list provided in Aria. (), as well as the sense of state change. Eurypterida and Cephalocarida were coded for states of characters that characterize or are generally present within those taxa. As a corollary, the ‘newstate’ consensus tree is comparatively more resolved than the consensus topology with inapplicables coded as missing (Fig. 14B), although it is still much less resolved than the topologies obtained under implied weights. Resolved nodes are weak (Fig. 14A), and small changes in character states can collapse branches into a polytomy similar to that shown in Figure 14B, but it is an indication that inapplicable coding somewhat retains more ‘information’ partly through redundancies. The repetition of the phylogenetic signal, in addition to isolating the megacheirans and chelicerates uptree, overemphasizes the absence of a number of characters, such as pleurae, in selected crustaceans. As a result, the number of steps is minimized and crustacean taxa are grouped as the sister‐group of arachnomorphs. The ‘stem bivalved arthropods’ possess several crustacean‐like characters (antennules, bivalved shield, furca, ringed trunk tergo‐sternites), which could be responsible for the attraction of crustaceans downtree. However, the removal of these taxa from the analysis does not result in a significant change in tree topology. The optimization of the ‘newstate’ tree reveals that only two characters clearly influence the phylogenetic grouping of the arachnomorphs: the presence of pleurae and the presence of lamellae on the exopods (Fig. 14C). Arguably, what features should be considered ‘lamellae’ is not clearly defined (Ortega‐Hernández et al. 2013), but as described by Briggs (1978), the exopods of Canadaspis perfecta Walcott clearly bear lamellate setae, casting doubt on the validity of this synapomorphy for arachnomorphs. As suggested by Ortega‐Hernández et al. (2013), we tentatively coded the absence or limitation of ‘imbrication’ of the lamellae in megacheirans, but this played a limited role in supporting the topology. Likewise, pleurae are also present in fuxianhuiids, interestingly retrieved here for the first time within the ‘stem bivalved arthropods’, as they are also in malacostracan crustaceans for example, and therefore poorly circumscribe the identity of arachnomorphs. As our data set is designed to focus on the phylogeny of leanchoiliids, it is by far less complete than the one used by Ortega‐Hernández et al. (2013), for example, to address the monophyly of trilobitomorphs, but emphasizes nonetheless that the existence of a monophyletic Arachnomorpha may be difficult to defend through the reconstruction of ancestral states of characters. It is important to stress that in spite of the caveats described, this phylogenetic configuration does not contradict a consensus on crown relationships involving chelicerates as sister‐clade to other arthropods (Rota‐Stabelli et al. 2011). Instead, it implies considerable morphological and genetic changes to the mandibulate lineage independent of the Arachnomorpha, which tend to be the focus of fossil‐based phylogenetic analyses. That this configuration is also retrieved using, for instance, marrellomorphs or artificial taxa as an outgroup with a ‘missing’ approach for inapplicables, suggests that there is a common and deeply rooted explanation for the emergence of the Arachnomorpha clade via multiple methodologies. The present evidence points to a polarization that places emphasis on the plesiomorphic nature of the ‘absence’ of characters. Furthermore, justification for the use of a marrellomorph or a single hypothetical taxon to root the characters of all arthropods appears to be lacking. In this respect, it is of interest to note that a large data set (Legg et al. 2012; Legg and Vannier 2013; Siveter et al. 2014), using the ‘missing’ coding option and considered to be reliably reconstructing higher node relationships (Edgecombe and Legg 2014), was recently able to produce a radically different topology supporting the Arachnomorpha hypothesis (Legg 2014). Certainly, resolution does not equal robustness, and it appears that the latter is not necessarily achieved using implied weighting, even when maximizing the number of characters. As an alternative, the coding of inapplicable states as ‘unknown’ or ‘missing’ avoids overemphasis on the equivalent ‘absence’ of the inapplicable state, although the impact of added uncertainties on a character's significance during optimization is not clear. The topology obtained here under implied weights is congruent with recent topologies found using lobopodian outgroups such as those described in Legg et al. (2012). Although the data are of course strongly pseudo‐replicated, plotting the number of inapplicable entries against the CI of the characters for both unweighted and weighted topologies reveals a different distribution between the ‘missing’ and ‘newstate’ methods (Fig. 15). The ‘newstate’ coding results overall in more average values of CI per character (although medians are identical (0.5)), meaning that characters overall are more homoplastic, but also that the CI values are generally not as low (Fig. 15B). The likely explanation for this difference comes from the additional ‘constraints’ that the ‘newstate’ coding imposes on the algorithm for tree building. Characters are more easily optimized when inapplicables are replaced by uncertainties, which can increase CI values, but also may decrease them. This ultimately affects the outcome of using implied weights, which are based on the CI of characters and are very conservative of the CI calculated without re‐weighting. Figure 15 Open in figure viewer PowerPoint Distribution of consistency index (CI) values for phylogenetic characters. A, CI values calculated using the ‘missing’ coding option for inapplicables. B, CI values calculated using the ‘newstate’ coding option for inapplicables. Note the lesser spread and greater number of intermediate values around 0.5, showing the more homoplastic topology constrained by the ‘newstate’ coding. Box‐plots are identical between the unweighted and weighted (implied weighting) analyses. However, cases of ‘illegitimate’ optimization that ensue can be easily tracked down by reconstructing ancestral states. This methodological plasticity can help highlight possible issues related to the original assumptions of homology between structures, or simply of their identity as a shared state of character. In other words, where ‘newstate’ coding adds weight to primary assumptions, ‘missing’ coding allows for the violation of these assumptions by sometimes breaking the dependency of characters between polyphyletic groups. For instance, it appears in this case of using ‘missing’ coding that the resolution of antennules between ‘stem bivalved arthropods’ (or at least some of them) and members of the Antennulata conflicts with our original coding assumption of a shared state of character and, likewise, that ‘short great appendages’ and chelicerae should therefore not be given the same state in a character describing the various types of deutocerebral appendages.