The affinities of the cosmopolitan arthropod Isoxys and its implications for the origin of arthropods DAVID A. LEGG AND JEAN VANNIER Legg, D.A. & Vannier, J. 2013: The affinities of the cosmopolitan arthropod Isoxys and its implications for the origin of arthropods. Lethaia, Vol. 46, pp. 540–550. Isoxys is a cosmopolitan bivalved arthropod genus known almost exclusively from Cambrian Konservat-Lagerst€atten. Despite its wide geographical distribution in such sites of exceptional preservation, little was known of its soft-part anatomy until recently when remains of eyes and raptorial frontal appendages were discovered. This absence has precluded determination of affinities. The new discovery of soft parts led to two important hypotheses: (1) that Isoxys was related to the ‘great-appendage’ arthropods and (2) that its contained species were not congeneric. Neither has been tested using a detailed cladistics analysis. The morphology of Isoxys is re-evaluated and coded into an extensive cladistics analysis. Our results indicate that Isoxys was indeed a monophyletic genus with all representatives united by the presence of an expansive dorsal shield with prominent antero- and posterolateral cardinal spines. It also indicates that Isoxys occupies a crucial role in arthropod evolution, resolving at the base of Arthropoda. The ‘great appendages’ of Isoxys are interpreted as innovating from either the protocerebral or deutocerebral somite and are therefore not homologous to those of other ‘great-appendage’ arthropods, which are interpreted as originating from the tritocerebral somite of the head. □ Anomalocaridid, arthropodization, congeneric, great appendage, phylogeny. David A. Legg [d.legg10@imperial.ac.uk], Department of Earth Science and Engineering, Imperial College London, London SW7 5AW, UK; David A Legg [d.legg10@imperial.ac.uk], Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK; Jean Vannier [jean.vannier@univ-lyon1.fr], Laboratoire de geologie de Lyon: Terre, Planetes, Environnement, Universite Lyon 1 UMR 5276 du CNRS, b^atiment GEODE, 2 rue Rapha€el Dubois 69622 Villeurbanne, France; manuscript received on 09/10/2012; manuscript accepted on 30/04/2013. The characteristic bivalved carapace of Isoxys (Fig. 1), with prominent antero- and postero-dorsal spines, is a common component of many Cambrian Konservat-Lagerst€atten (Vannier & Chen 2000; Garcıa-Bellido et al. 2009a,b; Stein et al. 2010; Fu et al. 2011). It is known from 16+ species distributed over 14 such localities (Wang et al. 2010) and was one of the first Burgess Shale-type arthropods described (Walcott 1890). Despite its ubiquity and long history of study its affinities have remained obscure, this is attributed to the lack of information regarding soft-part anatomy of this taxon, which is mostly preserved as isolated dorsal shields (sensu Stein et al. 2010), often referred to as a carapace. Recently, new collections in China, Australia and Canada have recovered specimens of Isoxys with soft anatomy preserved, specifically visual organs and limb morphology (Vannier & Chen 2000; Garcıa-Bellido et al. 2009a, b; Vannier et al. 2009; Stein et al. 2010; Fu et al. 2011; Schoenemann & Clarkson 2011). These new discoveries have prompted a reconsideration of the affinities of Isoxys. In particular, the recent recovery of raptorial frontal appendages in specimens from the Chengjiang biota (Vannier & Chen 2000; Fu et al. 2011), Burgess Shale (Garcıa-Bellido et al. 2009a) and Sirius Passet Lagerst€atten (Stein et al. 2010) has led some (Vannier et al. 2009) to consider Isoxys related to the ‘great-appendage’ arthropods, more formally known as Megacheira (senus Hou & Bergstr€ om 1997). This hypothesis has been challenged based on a lack of consensus regarding the segmental affinities of the ‘great appendage’ of other taxa (Fu et al. 2011). The difference in limb morphology between different species of Isoxys has also led some to propose that these taxa may not be congeneric (Stein et al. 2010; Fu et al. 2011). Few have either tested the monophyly of this genus or tried to determine its affinities using a cladistics analysis, a notable exception being Vannier et al. (2009), which resolved Isoxys as sister-taxon to the anomalocaridid, another clade of so-called ‘great-appendage’ arthropods. To test the monophyly of this genus and determine the affinities of its contained species, we undertook a re-evaluation of its morphology and coded these taxa into an extensive cladistics analysis (Legg et al. 2012) including megacheirans, bivalved arthropods, anomalocaridids and a large number of other arthropod groups. DOI 10.1111/let.12032 © 2013 The Authors, Lethaia © 2013 The Lethaia Foundation LETHAIA 46 (2013) The affinities of Isoxys 541 A B C D E F G Fig. 1. The cosmopolitan arthropod Isoxys. A, reconstruction of Isoxys acutangulus Walcott 1908, from the Middle Cambrian Burgess Shale (British Columbia, Canada), in oblique orientation. Based on the description of Garcıa-Bellido et al. (2009a) and additional morphological interpretations presented herein. B–G, variations in dorsal shield morphology (redrawn from Vannier & Chen 2000) drawn in lateral view and represented by B, I. acutangulus; C, I. auritus; D, I. volucris; E, I. curvirostratus; F, I. chilhoweanus and G, I. longissimus. Institutional abbreviation: ROM, Royal Ontario Museum, Toronto, Ontario, Canada; USNM, Smithsonian National Museum of Natural History, Washington DC., USA.; YDKS, Yunnan Institute of Geological Sciences, Kunming, Yunnan Province, China. Previous work on the affinities of Isoxys The paucity of soft part remains known for Isoxys has precluded determination of its affinities (Delle Cave & Simonetta 1991; Williams et al. 1996). The earliest descriptions of Isoxys were generally vague and contained little discussion of possible affinities (Walcott 1890, 1908). There was a trend amongst many early arthropod workers to align Cambrian bivalved arthropods with either branchiopod or phyllocarid crustaceans (e.g. Walcott 1912), and the latter was also true of Isoxys (Richter & Richter 1927; Brooks & Caster 1956), although little if any justification was given for this assignment. Rolfe (1969) noted similarities in carapace morphology between Isoxys and archaeostracan phyllocarids, such as weak mineralization and an entire dorsal hinge; however, none of the proposed characters is exclusive to these taxa, and a similar morphology may have evolved many times within the Arthropoda (Hou & Bergstr€ om 1997). The first detailed description of the soft anatomy of Isoxys was by Shu et al. (1995), although possible body segments had previously been noted for Isoxys communis from Emu Bay (Glaessner 1979). Shu et al. (1995) considered Isoxys to be a crustacean; however, their interpretations were latter challenged as ‘speculative’ (Vannier & Chen 2000:296), and the putative crustacean features, such as two pairs of antennae, could not be verified. Later Vannier et al. (2006) tentatively assigned Isoxys to the enigmatic Thylacocephala, a clade of crustacean-like arthropods with enlarged raptorial appendages, although later rejected this opinion (Vannier et al. 2009). The discovery of elongate prehensile raptorial appendages in Isoxys prompted comparison with ‘great-appendage’ arthropods (Garcıa-Bellido et al. 2009a) and also prompted a reassessment of its monophyly (Stein et al. 2010; Fu et al. 2011). The status of ‘great-appendage’ arthropods has been a contentious issue (Legg 2013), with some considering them stem-group chelicerates (Cotton & Braddy 2004; Dunlop 2006), whilst others consider them stem-lineage euarthropods (Budd 2002; Daley et al. 2009; Legg et al. 2012; Legg 2013). The only cladistic analysis to date to include Isoxys (Vannier et al. 2009) resolved it amongst ‘great-appendage’ arthropods, as sister-taxon to a clade composed of Occacaris Hou 1999 and Forfexicaris Hou 1999; bivalved arthropods with ‘great appendages’ from the Chengjiang biota; this more inclusive group resolved as sister-taxon to the anomalocaridids Anomalocaris Whiteaves 1892; and Parapeytoia Hou et al. 1995 (although see Daley et al. 2009; Stein 2010; Legg et al. 2012; and Legg 2013; for an alternative interpretation of Parapeytoia). Close relationships between Isoxys and other ‘great appendage’ bearing bivalved arthropods were chal- 542 D. A. Legg & J. Vannier lenged by Fu et al. (2011) who questioned the segmental affinities of many putative ‘great appendage’ and argued that it carried little phylogenetic significance. Morphological interpretation of Isoxys spp. This section is not intended as a complete reinterpretation of the morphology of Isoxys but rather a discussion of key characteristics – specifically the dorsal shield, frontal appendages, trunk appendages, posterior tail fan and digestive glands – which may help to determine the affinities and interrelationships of its contained species. Dorsal shield The dorsal shield of Isoxys shows considerable intraspecific variation, although species assigned to this taxon always possess extensive antero- and posterolateral spines (Fig. 1). The dorsal shield of some species possesses an extensive dorsal fold, for example Isoxys auritus (Shu et al. 1995), reminiscent of other bivalved arthropods; whereas others lack such a fold, for example I. curvirostratus (Vannier & Chen 2000). The dorsal shield may also differ in micro-ornament, which may be reticulate, for example I. auritus (Shu et al. 1995) or striate, for example I. curvirostratus (Vannier & Chen 2000). This has led some (e.g. Fu et al. 2011) to infer that all species referred to Isoxys may not be congenic. Although a diagnostic characteristic of Isoxys (Vannier & Chen 2000; Garcıa-Bellido et al. 2009a, b; Stein et al. 2010; Fu et al. 2011), the phylogenetic significance of the antero- and posterolateral spines is unclear. Similar spines occur in larval malacostracans (Garcıa-Bellido et al. 2009b; Vannier et al. 2009) and halocypridid ostracods (Vannier & Chen 2000) and may represent a convergent adaptation to a pelagic lifestyle (Vannier & Chen 2000; Garcıa-Bellido et al. 2009b; Vannier et al. 2009), the slender nature of the spines reducing drag during swimming (Vannier & Chen 2000). Although the latter taxa were not included, this character was coded as present for all species assigned to Isoxys. Frontal appendages Much of the debate regarding the affinities and monophyly of Isoxys has concerned the morphology of its frontal appendages. The frontal appendages of Isoxys were originally compared with the raptorial appendages of ‘short great-appendage’ arthropods, LETHAIA 46 (2013) such as Leanchoilia Walcott 1912; Alalcomenaeus Simonetta 1970; Yohoia Walcott 1912; Jianfengia Hou 1987; Haikoucaris Chen et al. 2004; and Fortiforceps Hou & Bergstr€ om 1997; the ‘great appendages’ of which are composed of a proximal unit consisting of a two-segmented peduncle separated from a distal group of two to four chelate to subchelate podomeres by a distinctive ‘elbow’ joint (Fig. 1A, B; Haug et al. 2012). Supposed similarities between the appendages of ‘short great-appendage’ arthropods and Isoxys appear to be overstated. Although a two-segmented peduncle was reported in specimens of I. acutangulus (Garcıa-Bellido et al. 2009a; Vannier et al. 2009), a re-examination of the figured material could not identify such structures (see Fig. 1C, D), and those identified as peduncular elements (see for example Garcıa-Bellido et al. 2009a; Fig. 3A) are indistinguishable from the more distal spinose elements. Frontal appendages are preserved in at least five species: I. acutangulus Walcott 1908; I. auritus Jiang 1982; I. communis Glaessner 1979; I. curvirostratus Vannier & Chen 2000; and I. volucris Williams et al. 1996; and although they differ in podomere count and overall length they have a conservative morphology. They are generally elongated with podomeres reducing in size towards the distal end and lack chelate or sub-chelate spines (Fig. 2C–F). Instead, the perpendicular spines project from the centre of each podomere, like those of anomalocaridids, particularly Anomalocaris (Fig. 2G,H). In I. volucris, the spines appear to bifurcate, like those of the putative anomalocaridid Tamisiocaris Daley & Peel 2010 (cf. Stein et al. 2010; Fig. 2; Daley & Peel 2010; Fig. 1). Podomere boundaries are poorly delineated in many specimens of Isoxys although podomere number may be inferred from the position of the spines. There are at least five spinose podomeres in I. acutangulus (Fig. 2C, D) and I. curvirostratus, seven in I. volucris and as many as 13 in juvenile specimens of I. auritus (Fig. 2E,F), a number comparable to Anomalocaris (n = 14; Fig. 2G,H). The spines of Isoxys are located on the concave inner surface of the anteriorly curved appendage (Fig. 2C–F), comparable to those of ‘short great-appendage’ arthropods, whereas those of anomalocaridids are typically located on concave inner margin of a posteriorly curved appendage (Fig. 2G,H). This is arguably not a big difference and could come about through torsion of the limb. The variation in limb morphology may imply different feeding strategies achieved via differences in the prehensile motion of the limbs. Fu et al. (2011) considered the frontal appendages of Isoxys to originate from the deutocerebral somite and rejected homology with frontal appendages of LETHAIA 46 (2013) The affinities of Isoxys A B C D E F G H 543 Fig. 2. Frontal appendages in arthropods. A, the ‘short great appendages’ of Yohoia tenuis (USNM 155621) with B, interpretive sketch. C, Isoxys acutangulus (ROM 51211), with D, interpretive sketch. E, detailed view of the ‘great appendages’ of I. auritus (YDKS 43), with F, interpretive sketch. G, the ‘great appendage’ of Anomalocaris (ROM 51213), with H, interpretive sketch. Abbreviations: als, anterolateral spine; as, anterior sclerite; ds, dorsal shield; ex, exopods; fa, frontal appendage; fap, frontal appendage peduncle; ga, ‘great appendage’; gap, ‘great appendage’ peduncle; le, lateral eye; pls, posterolateral spine; te, trunk endopod; ts, trunk somite. other bivalved ‘great-appendage’ arthropods, namely Occacaris and Forfexicaris. The hypothesis of Fu et al. (2011) was based on a misinterpretation of head organization in Occacaris and Forfexicaris, which were thought to possess a pair of appendages anterior of the ‘great appendages’, which were considered to originate from the deutocerebral somite and tritocerebral somite, respectively. This 544 LETHAIA 46 (2013) D. A. Legg & J. Vannier Fig. 3. Head organization in aiolopods (based on Scholtz & Edgecombe 2006, fig. 3). Nervous material is yellow; structures associated with the protocerebrum are in blue including the mouth, which is embryologically associated with the protocerebrum but later may move posteriorly to the deutocerebral somite; deutocerebral somite and associated structures in red; and tritocerebrum and associated structures in green. Black lines in the central mass indicate nerve connections. Abbreviations: AS, anterior sclerite; CH, chelicerae; GA, ‘great appendage’; PA, Primary antenna; SA, Secondary antenna; SPA, Specialized post-antennal appendage. was based on comparisons with extant mandibulate arthropods, which typically possess an antenniform deuterocerebral appendage (Fig. 3). Antenniform appendages are completely unknown from Forfexicaris, and the supposed antenna of Occacaris is indistinguishable from the trunk endopods and has most likely been misidentified. The exact morphology of the ‘great appendage’ of Occacaris is hard to determine from the one poorly preserved specimen but that of Forfexicaris resembles the ‘great appendage’ of other ‘short great-appendage’ arthropods. ‘Short great appendages’ were generally considered homologous to the chelicerae of chelicerates (pygnogonids, horseshoe crabs, eurypterids and arachnids) and would therefore supposedly belong to the deutocerebral (Fig. 3; Damen et al. 1998; Telford & Thomas 1998; Dunlop 2006; Haug et al. 2012); however, recent descriptions of neural tissues in fuxianhuiids suggests the ‘great appendages’ innovate from the tritocerebral neuromeres of the brain (Ma et al. 2012; Yang et al. 2013). The pedunculate eye stalks and the ‘great appendages’ of Isoxys are situated beneath an anterior sclerite (sensu Budd 2008), a weakly sclerotized anterior plate (Fig. 2C,D). The eyes of extant arthropods and their nearest extant outgroup, onychophorans, originate from the protocerebrum, the anteriormost ganglion of the arthropod brain (Fig. 3; Strausfeld 2012). The close association of the ‘great appendage’ and ocular sclerite of Isoxys may indicate that they originate from the same somite, that is, both from the protocere- brum, or possibly the deutocerebrum. Either way, the ‘great appendages’ of Isoxys, and by extension anomalocaridids, would not be segmentally homologous to the ‘great appendages’ of ‘short greatappendage’ arthropods (Fig. 3). Trunk appendages Although the anterior appendages clearly consist of discrete segments, the morphology of the trunk limbs is less clear. The limbs are biramous, however, endopods are sometimes absent, which may indicate that they are more weakly sclerotized than the exopods. In cases where the endopods are preserved, individual podomeres can rarely be distinguished; this may also be due to their weak sclerotization; notable exceptions are I. curvirostratus, in which the endopods display more than 10 well-delineated podomeres (Fu et al. 2011 Fig. 4A), and I. volucris, which has really weakly sclerotized appendages, at least when compared to the frontal appendages, although approximately 10 podomeres may be discerned (Stein et al. 2010; :Fig. 2). A high number of endopod podomeres is regarded as a primitive characteristic of arthropods, with seven representing the crown-group ground pattern (Boxshall 2004). Such weakly sclerotized endopods are also present in other Cambrian bivalved arthropods, particularly basal taxa such as Nereocaris Legg et al. 2012; Jugatacaris Fu & Zhang 2011; and Pectocaris Hou 1999 (see also Hou et al. 2004). The exopods are best preserved in I. volucris; LETHAIA 46 (2013) The affinities of Isoxys A B C D F H 545 E G J K L I M Fig. 4. Posterior trunk and telson of dinocaridids and basal arthropods. A, Isoxys acutangulus (ROM 57899) preserved in lateral orientation with B, interpretive sketch. C, close-up view of the telson and lateral telson processes of I. acutangulus (ROM 57907), with D, interpretive sketch. E, the posterior tail fan of Opabinia (USNM 131217) preserved in lateral view. F, close-up of the tail fan of Anomalocaris (ROM 51211), with G, interpretive sketch. H–M, reconstructions of the posterior tail fan and telsons of dinocaridids and basal arthropods. H, Anomalocaris (based on ROM 51211). I, Opabinia (based on USNM 131217). J, I. acutangulus (based on ROM 57907 and 57899). K, Nereocaris (based on Legg et al. 2012, figs. 1 and 2). L, Jugatacaris (based on Fu & Zhang 2011; fig. 8.2) and M, Odaraia (based on Briggs 1981, fig. 47). Abbreviations: as, anterior sclerite; ds, dorsal shield; fa, frontal appendage; fl, fluke; le, lateral eye; ltp, lateral telson process, pls, posterolateral spine. 546 LETHAIA 46 (2013) D. A. Legg & J. Vannier they are sub-ovoid and have a fringe of fine setae. Similar exopods are common amongst basal arthropods and have been reported from other bivalved arthropods, for example Nereocaris, Pectocaris and Perspicaris Briggs 1977; and fuxianhuiids, for example Fuxianhuia Hou 1987; and Shankouia Waloszek et al. 2005, but are distinct from ‘short great-appendage’ arthropods, some of which, for example Fortiforceps, have fine setae but lack sub-ovoid exopods, and others of which, for example Alalcomenaeus and Leanchoilia, possess sub-ovoid exopods but have spinose setae. Posterior trunk and telson The posterior trunk and telson are known exclusively from I. acutangulus (Fig. 4A–D,J). The posterior was originally described as consisting of a single segment, a fluke-like telson, with posteriorly deflected exopods abutting it in some specimens (Garcıa-Bellido et al. 2009a); a re-examination of this material indicates that the putative exopods are actually distinct from the other exopods of the body and are better interpreted as lateral telson processes (sensu Legg et al. 2012). Unlike the exopods, the lateral telson processes are sub-triangular and lack a setal fringe (Fig. 4C,D). A similar arrangement is seen in both the dinocaridids Opabinia Walcott 1912 (Fig. 4E,I), and Anomalocaris (Fig. 4F–H), and basal bivalved arthropods, particularly Nereocaris (Fig. 4K), Jugatacaris (Fig. 4L), A B Pectocaris and to a lesser degree Odaraia Walcott 1912 (Fig. 4M), and was used to support a basal position within Arthropoda for the latter group (Legg et al. 2012). In the dinocaridids, the lateral flaps project dorsally (Fig. 4E), as observed in laterally preserved specimens. The majority of specimens of I. acutangulus with lateral telson processes are dorsoventrally compressed, however, a single laterally preserved specimen indicates that the telson processes also projected dorsally (Fig. 4A,B). Digestive glands Both I. acutangulus (Fig. 5A,C) and I. communis preserve an extensive series of bulbous midgut glands; similar glands have also been reported from a diversity of extant and Cambrian panarthropods, including Opabinia (Fig. 5B,D), Anomalocaris, a variety of bivalved arthropods, ‘great-appendage’ arthropods (Butterfield 2002), fuxianhuiids (Zhu et al. 2004) and trilobitomorphs (Vannier & Chen 2002), and are thought to function as a storage organ in organisms with an infrequent but rich diet (Butterfield 2002; Vannier & Chen 2002), that is, carnivores and scavengers. Their presence therefore has major implications for the ecology of the earliest arthropods; Legg et al. (2012) recently argued that the most basal arthropods were filter or depositfeeders based on gut morphology and frontal limb structure and were not carnivorous as has been advocated elsewhere (Maas et al. 2004). The pres- C D Fig. 5. Digestive glands of Isoxys and Opabinia. A, Isoxys acutangulus (ROM 57904). B, Opabinia (ROM 59874). C, close-up view of the gut glands of I. acutangulus (ROM 57904). D, close-up view of the gut glands of Opabinia (ROM 59874). LETHAIA 46 (2013) ence of midgut glands in Isoxys, as well as the large pedunculate eyes and prehensile frontal appendages, indicates it was predator with an infrequent source of nutrition. Phylogenetic analysis To determine the affinities of Isoxys, this taxon was coded into an extensive phylogeny of arthropods and their kin, including 312 (84 extant, 228 extinct) aiolopods (arthropods, onychophorans, tardigrades and closely related fossil taxa, i.e. lobopodians) and two non-aiolopod (universal) outgroups. This data set was based on a published matrix (Legg et al. 2012). Both Occacaris and Forfexicaris were included in earlier versions of the data set but were excluded from the current analysis due to their poor preservation and lack of code-able characters. To test the monophyly of Isoxys, all known species with sufficiently preserved soft anatomy were included, namely I. acutangulus, I. auritus, I. curvirostratus and I. volucris. To the original list of 580 characters, an additional 175 were added (see online Supporting Information - Data S1). Cladistic analysis was undertaken using TNT (Tree searches using New Technology) v. 1.1. (Goloboff et al. 2008a). The large size of the data set necessitates the use of New Technology search options, this was undertaken using 100 Random Addition Sequences with Parsimony Ratchet (Nixon 1999), Sectorial searches, Tree Drifting and Tree Fusing (Goloboff 1999). Experimentation was able to determine that default settings for these options were sufficient to find the most parsimonious trees. Multi-state characters were treated as non-additive (unordered) and weighted using both equal weighting and implied weighting with a variety of concavity constants (k = 2, 3 and 10). Implied weighting is the favoured weighting option from a philosophical standpoint (Legg et al. 2012; Ortega-Hernandez et al. 2013; Legg & Caron in press) and has been shown to increase character support and reduce the sensitivity of the data set to the inclusion of additional taxa and/or characters (Goloboff et al. 2008b). Few methods of determining nodal support are unaffected by character weighting; however, Symmetric Resampling can be used in such instances (Goloboff et al. 2003). Symmetric Resampling used 100 replicates, using New Technology search options including Parsimony Ratchet, Sectorial searches, Tree Drifting and Tree Fusing, with a change probability of 33 per cent. Nodal support is expressed as group present/contradicted (GC) frequency differences. The affinities of Isoxys 547 Results and discussion The position of Isoxys was consistent under all search options and character weighting schemes, resolving as a monophyletic clade at the base of a paraphyletic assemblage of bivalved arthropods (Fig. 6), that is, as sister-taxon to all other arthropods (sensu Bergstr€ om et al. 2008; Legg et al. 2012). The monophyly of Isoxys was supported by a single synapomorphy, the presence of a dorsal shield with extensive anteroand posterolateral spines, a diagnostic characteristic of the genus. Although this resolved as the only unequivocal synapomorphy of this clade, all members also possess anteriorly curved frontal appendages with non-chelate spines on the concave inner margin. Features thought to indicate the non-monophyly of the genus instead support smaller clades within Isoxys; a sister-taxon relationship between I. acutangulus and I. curvirostratus was supported in some sub-sets of trees by the presence of a reduced frontal appendage with sub-pentagonal podomeres and a more inclusive clade including these taxa, and I. volucris was supported by the presence of a dorsal shield with a weakly defined dorsal fold. This analysis indicates that the presence of a bivalved dorsal shield and the presence of an elongate frontal appendage are plesiomorphic characteristics of Isoxys, also present in either the dinocaridids or a paraphyletic assemblage of bivalved arthropods. Legg et al. (2012) considered a bivalved dorsal shield a synapomorphy of Arthropoda Von Siebold 1848 (sensu Bergstr€ om et al. 2008) but considered the lateral plates of Hurdia (P-elements sensu Daley et al. 2009) a possible precursor; the similarities between Isoxys and other dinocaridids, such as the morphology of the frontal appendages and the posterior trunk region, may be taken as additional support for this hypothesis. The recovered topology (Fig. 6) indicates that although the frontal appendages of dinocaridids, Isoxys and possibly even the antennae of onychophorans are homologous, they are not homologous to the frontal appendages of ‘short great-appendage’ arthropods. Specialized frontal appendages are not present in some of the most basal bivalved arthropods, namely Nereocaris, Jugatacaris and Pectocaris, with ‘short great appendages’ appearing in more derived bivalved arthropods such as Odaraia (Legg et al. 2012) and Branchiocaris. A lack of specialized frontal appendages in Nereocaris is a genuine morphological absence rather than a taphonomic artefact (Legg et al. 2012; Legg & Caron in press), however, the condition in Jugatacaris and Pectocaris is less clear. A number of bivalved arthropods from 548 D. A. Legg & J. Vannier LETHAIA 46 (2013) Fig. 6. The phylogeny of Isoxys and stem-lineage arthropods. A summary of most parsimonious trees (MPTs) produced under different character weighting schemes (see text for discussion). Names in capital letters are suprageneric taxa, and numbers in square brackets refer to the number of terminal taxa within them. Italicized writing associated with nodes are key innovations (synapomorphies) acquired at these nodes. LETHAIA 46 (2013) the Burgess Shale were found to possess specialized post-antennal appendages (Legg et al. 2012), although previously they were considered absent (e.g. Briggs 1977). Experimental coding of the anterior appendages of the latter taxa had no influence on topology and would seemingly indicate a genuine absence of specialized frontal appendages. The loss of appendages appears to be associated with an ecological shift from a predatory habit to filter and deposit feeding, as proposed for many early bivalved arthropods (Hou 1999; Hou et al. 2004; Fu & Zhang 2011; Legg et al. 2012). Legg et al. (2012) excluded the radiodonts, large pelagic predators with strongly sclerotized frontal appendages, no trunk limbs and a scleritized oral circlet (Collins 1996), from their definition of Arthropoda, noting that the arthropodized frontal appendages of anomalocaridids may not be homologous to the trunk limbs of true arthropods, that is, they may have separate origins. The presence of an anomalocaridid-like frontal appendage in Isoxys, which also possesses arthropodized trunk limbs, may indicate that they have a common origin and are homologous; we therefore propose that the contents of Arthropoda should also include the radiodonts, but should still exclude those dinocaridids, which lack arthropodized appendages, namely Opabinia. Acknowledgements. – We would like to thank Jean-Bernard Caron and Peter Fenton for access to specimens deposited in the Royal Ontario Museum; Allison Daley, Diego Garcıa-Bellido, Javier Ortega-Hernandez and Greg Edgecombe for helpful discussion and providing photos used in this manuscript; and David Siveter and Graham Budd for their reviews. We would also like to thank Martin Stein who provided photos although they were not used in this version of the manuscript. DAL is funded by a Janet Watson scholarship (Imperial College London) and JV by an ANR (Agence Nationale de la Recherche) grant (ORECO and RALI). 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