The affinity of Tullimonstrum gregarium , a pincer‐mouthed, soft bodied bilaterian, has been subject to debate since its recovery from Carboniferous coal deposits at Mazon Creek, Illinois. After decades of impasse focused on mollusc, arthropod and annelid attributes, two recent, yet conflicting, high‐profile studies concluded that the ‘Tully Monster’ is a vertebrate, a relative of lampreys or jawed fishes. Here, we find that structures described as supporting vertebrate, and particularly crown vertebrate, affinity face significant challenges from biological, functional and taphonomic perspectives. Problems with comparator choice, interpretation of taphonomic processes at Mazon Creek and estimation of convergence within the bilaterian tree may have misled these recent studies, leading to conclusions which do not accommodate current understanding of the vertebrate record. For example, the absence of taphonomically‐expected synapomorphies in Tullimonstrum (e.g. otic capsules, body pigment) calls into question vertebrate identity and implies that convergence or deeper origins are responsible for vertebrate‐like traits. Further, phylogenetic placement within vertebrates is only made possible by the constraints of a chordate‐only dataset with limited outgroups and use of selective characters. Long‐discussed alternative placements among molluscs (e.g. heteropod gastropods), arthropods (e.g. anomalocarids) or elsewhere within non‐vertebrate deuterostomes are more congruent. Indeed, many of these lineages independently evolved vertebrate‐like traits, including complex eyes and ‘teeth’. Thus, given the totality of evidence, Tullimonstrum should be excluded from the vertebrate crown. Potential assignments for aberrant bilaterians, common throughout the Palaeozoic fossil record, need to be considered in light of the number and likelihood of required exceptions to established schemes.

Tullimonstrum gregarium Richardson, 1966 is a well‐known problematic bilaterian that only occurs in concretions recovered from marine Essex fauna deposits within the Carboniferous Francis Creek Shale, most notably at Mazon Creek, Illinois (307 Ma; Foster 1979; Shabica & Hay 1997; Sallan & Coates 2014). It is notable for its combination of traits uncommon in fossil and living animals, such as long‐stalked eyes and an elongate proboscis with a pincer‐like mouth (Richardson 1966; Johnson & Richardson 1969; Foster 1979). Like so many Palaeozoic soft‐tissue fossils with divergent morphology and a single documented occurrence in time (Donoghue & Purnell 2009), the affinity of Tullimonstrum was immediately subject to debate. It has been serially attributed to almost every bilaterian group, including arthropods, molluscs and ‘worms’ of all sorts (Richardson 1966; Johnson & Richardson 1969; Foster 1979; Beall 1991). Continuing uncertainty regarding Tullimonstrum's classification has produced as much interest as the aberrant morphology at its source. Despite thousands of specimens surveyed by dozens of workers over the decades, one major bilaterian clade was never linked to Tullimonstrum: Vertebrata. Vertebrates were most likely excluded from consideration due to a general absence of evident synapomorphies in Tullimonstrum, appearance of conflicting traits such as broad superficial segments, and gross dissimilarity with the coincident crown cyclostomes and gnathostomes (jawless and jawed fishes, respectively) that have well‐preserved modern morphologies (e.g. Mayomyzon and Myxinikela; Gabbott et al. 2016). In fact, when a single twentieth century study discussed a potential connection with then‐problematic conodonts, it was wholly in the context of a conodont–mollusc clade (Beall 1991). This relationship was summarily dismissed by the discoverers of the conodont animal while making a case for conodont–vertebrate affinity (Aldridge et al. 1993). The recent announcements in Nature of a vertebrate affinity for Tullimonstrum were therefore unexpected (Clements et al. 2016; McCoy et al. 2016). Two simultaneous studies came to this conclusion on the basis of novel, yet distinct, interpretations of previously observed features (primarily eye pigments and axial structures respectively). However, these arguments for vertebrate attribution present major challenges in terms of taphonomy, morphology and parsimony, and furthermore contain major inconsistencies. McCoy et al. (2016) used the presence of a ‘notochord’ to designate Tullimonstrum as a lamprey, yet noted that such internal elements are not preserved in definitive Mazon Creek vertebrates. Indeed, Clements et al. (2016) found no such structure, but described discrete layers of melanosomes in the eye as a vertebrate synapomorphy (Bardack & Richardson 1977; Gabbott et al. 2016), while acknowledging the absence of other, expected characters. Here, we argue that a crown vertebrate, and in particular a lamprey, identity for Tullimonstrum is unlikely. For example, the absence of taphonomically‐expected vertebrate synapomorphies in Tullimonstrum (e.g. otic capsules, body pigment; Bardack & Richardson 1977; Janvier 1996; Sallan & Coates 2014; Clements et al. 2016; Gabbott et al. 2016; McCoy et al. 2016) suggests that convergence or deeper origins are responsible for vertebrate‐like traits. Alternative, previously proposed, placements among molluscs (e.g. heteropod gastropods; Johnson & Richardson 1969; Foster 1979; Beall 1991), arthropods (e.g. Johnson & Richardson 1969; Foster 1979) or new attributions elsewhere within deuterostomes or chordates are more congruent for the reasons laid out below.

Anatomical interpretations Clements et al. (2016) proposed total‐group vertebrate affinity for Tullimonstrum based on two traits: (1) a camera‐like eye; and (2) the presence of two organelles, cylindrical and spheroid melanosomes, in the eye. These were taken as evidence for a multilayered retinal pigmented epithelium (RPE), a vertebrate trait (although presence of the RPE was marked as equivocal in their fig. 4). With respect to the vertebrate nature of these characters, an alternative interpretation, based on three‐dimensionally preserved specimens, is that the eyes are pigment cup type (Johnson & Richardson 1969), which are far more phylogenetically widespread but not found among vertebrates (Clements et al. 2016). Second, the distribution of pigment cell structures and melanin within the eye has not been well surveyed among other bilaterians, living or extinct (Lamb et al. 2007; Schoenemann et al. 2009; Clements et al. 2016). This suggests other occurrences are possible and limits the current utility of these traits in placing Tullimonstrum. Presence of these two melanosome types is variable even among vertebrates: the RPE, along with the lens and iris, has been lost in extant hagfish (Locket & Jørgensen 1998; Gabbott et al. 2016), and only spheroid melanosomes are preserved in the eyes of the chondrichthyan Bandringa (Clements et al. 2016, extended data fig. 5l). No other vertebrate traits were identified by Clements et al. (2016), yielding two additional possibilities beyond vertebrate affinity: (1) cylindrical melanosomes and/or the RPE evolved deeper within deuterostomes with genetic prerequisites for RPEs (see below; Lamb et al. 2007); or (2) Tullimonstrum belongs to an entirely different group of bilaterians and exhibits convergent eye structures (Foster 1979). Eye structures and characters exhibit high levels of homoplasy, convergence and parallel evolution (Ogura et al. 2004; Serb & Eernisse 2008; Schoenemann et al. 2009; Clements et al. 2016). While independent gain of this structure (or its loss in proximate crown vertebrate outgroups) may appear unparsimonious, this must be balanced against the absence of expected vertebrate synapomorphies in Tullimonstrum, not to mention its incongruous body plan. In contrast, McCoy et al. (2016) identified a host of putative vertebrate traits in their description of Tullimonstrum, which were used to support a lamprey designation. This began with a ‘notochord’ represented by a mid‐body light‐coloured stain or gap in segmentation (Johnson & Richardson 1969; Fig. 1A, ax). ‘Notochord’ identity was assigned in Tullimonstrum only through comparison with an indented band in Gilpichthys, a poorly‐described putative chordate mistakenly referred to as a stem‐hagfish by McCoy et al. (2016; see Sansom et al. 2010; Janvier 2015; Janvier & Sansom 2015; Gabbott et al. 2016; Fig. 2B). Notochords are unpreserved in definitive Mazon Creek vertebrates (Bardack & Richardson 1977; Shabica & Hay 1997; Sallan & Coates 2014; Clements et al. 2016; Gabbott et al. 2016). Secondary mention of a notochord in Mayomyzon (Aldridge & Donoghue 1998) stems from misinterpreted pigmentation, such as an intermittent gap between dorsal stripes on dorsally preserved specimens or latero‐ventral line on laterally preserved specimens, which were never interpreted as such in primary descriptions (Bardack & Zangerl 1971; Bardack & Richardson 1977; Fig. 2A). Finally, expansion of this band in Tullimonstrum anterior to the eye bars, used to rule out gut identity despite connection with the anterior mouth, definitively rules out notochord interpretation. Notochords terminate posterior to the optic chiasma or the hypophysis in all vertebrates (Janvier 1996; Kardong 2011; Fig. 1B, no). Figure 1 Open in figure viewer PowerPoint Tullimonstrum and potential comparators. A, schematic of Tullimonstrum structures in lateral and dorsoventral views, based on figured specimens in McCoy et al. ( 2016 et al. ( 2016 et al. ( 2016 Opabinia. E, Heteropod gastropod (Pterotrachiodea) mollusc anatomy; form based primarily on Pterotrachea and Ralph ( 1957 . Abbreviations: ac, annular cartilage; ad, adhesion surface; af, anal fin; an, anus; ap, atriopore; arc; arcualia; at; anterior tectal; ax, axial band; bc, buccal mass; bm, body margin; br, brain; ca, caudal appendage; cg, cerebral ganglion; cp, circular pigment; dn, dorsal nerve cord; df, dorsal fin; dl, dark line; dt, denticle; eb, eye bar; en, endostyle; es, eye stalk; eso, esophagous; fl, fin lines; fr, fin ray; gf, gill filament; gg, gut gland; gn, gonad; gs, gill slit; ht, heart; in, intestine; jj, jaw joint; jk, jaw knob; kt, keratin teeth; ln, lens; lsc, large semicircle; lv, liver; ms, myoseptum; mt, mouth; my, myomere; nc, neurocranium; nh, nasohypophyseal; no, notochord; ns, nasal capsule; og, optic ganglion; oh, oral hood; on, optic nerve; ot, otic capsule; pb, proboscis; pc, piston cartilage; pe, pericardial cartilage; ph, pharynx; pn, pincer; pt, posterior tectal; rc; raised circular patch; rd, radula; re, retina; ri, round indentation; rt, respiratory tube; sc, spinous cartilage; seg, segment; sep, septum; sk, suction disk; ssc, smaller semicircle; st, stomach; sy, statocyst; tf, tail fin; vm, visceral mass; vn, ventral nerve cord; wl, white lines; wp, white patch. and potential comparators. A, schematic ofstructures in lateral and dorsoventral views, based on figured specimens in McCoy. (). See Clements. () and McCoy. () for trait assignments by those authors. B, generalized lamprey musculoskeletal and internal anatomy. C, non‐vertebrate chordate anatomies. D, Palaeozoic marine arthropod anatomy based on. E, Heteropod gastropod (Pterotrachiodea) mollusc anatomy; form based primarily onand Ralph (: ac, annular cartilage; ad, adhesion surface; af, anal fin; an, anus; ap, atriopore; arc; arcualia; at; anterior tectal; ax, axial band; bc, buccal mass; bm, body margin; br, brain; ca, caudal appendage; cg, cerebral ganglion; cp, circular pigment; dn, dorsal nerve cord; df, dorsal fin; dl, dark line; dt, denticle; eb, eye bar; en, endostyle; es, eye stalk; eso, esophagous; fl, fin lines; fr, fin ray; gf, gill filament; gg, gut gland; gn, gonad; gs, gill slit; ht, heart; in, intestine; jj, jaw joint; jk, jaw knob; kt, keratin teeth; ln, lens; lsc, large semicircle; lv, liver; ms, myoseptum; mt, mouth; my, myomere; nc, neurocranium; nh, nasohypophyseal; no, notochord; ns, nasal capsule; og, optic ganglion; oh, oral hood; on, optic nerve; ot, otic capsule; pb, proboscis; pc, piston cartilage; pe, pericardial cartilage; ph, pharynx; pn, pincer; pt, posterior tectal; rc; raised circular patch; rd, radula; re, retina; ri, round indentation; rt, respiratory tube; sc, spinous cartilage; seg, segment; sep, septum; sk, suction disk; ssc, smaller semicircle; st, stomach; sy, statocyst; tf, tail fin; vm, visceral mass; vn, ventral nerve cord; wl, white lines; wp, white patch. Figure 2 Open in figure viewer PowerPoint et al. ( 2016 Tullimonstrum. A, Mayomyzon showing oral disk and otic capsules (Royal Ontario Museum V56800a immersed in alcohol with rebalanced colour levels). B, potential placements of Tullimonstrum in amended analysis of McCoy et al. ( 2016 Mayomyzon (71), tail equivocal for Mayomyzon (52, 53) and tectal character removed (117); blue line indicates position of Tullimonstrum in original analysis (NB not recovered in the amended analysis); red lines indicate the possible placements of Tullimonstrum in the amended analysis (Tullimonstrum A–E). Euconodonta was included in the analysis but has been pruned here in order to enable visualization of the underlying signal. Abbreviations: gp, gill pigment; od, oral disk; ot, otic capsule. Recoded phylogenetic analysis of McCoy. () shows uncertainty in placement of. A,showing oral disk and otic capsules (Royal Ontario Museum V56800a immersed in alcohol with rebalanced colour levels). B, potential placements ofin amended analysis of McCoy. () performed using PAUP* and MrBayes; amended characters: oral disc present for(71), tail equivocal for(52, 53) and tectal character removed (117); blue line indicates position ofin original analysis (NB not recovered in the amended analysis); red lines indicate the possible placements ofin the amended analysis (A–E). Euconodonta was included in the analysis but has been pruned here in order to enable visualization of the underlying signal.: gp, gill pigment; od, oral disk; ot, otic capsule. Designation of a notochord was crucial to McCoy et al.’s (2016) description as it was used to justify choice of the vertebrate bodyplan for subsequent reconstruction of all Tullimonstrum's other characters. First, it should be noted that notochords are at least a chordate, and more likely a deuterostome, synapomorphy rather than a vertebrate identifier (Annona et al. 2015; Holland et al. 2015). Second, and more importantly, this initial selection of a single comparator inflated similarities, even when many of Tullimonstrum's traits do not fit a vertebrate comparison (see Donoghue & Purnell (2009) for commentary on this approach). For example, widely‐spaced indentations and white lines in the anterior body were reinterpreted as vertebral arches (cartilaginous arcualia), despite the latter being identified as dorsal fin posteriorly (McCoy et al. 2016; Fig. 1A, wl). Arcualia are normally more numerous, closely‐packed and have paired extensions alongside the dorsal nerve cord and/or notochord (separate in lamprey, joined as arches in gnathostomes; Janvier 1996; Kardong 2011) rather than being self‐contained, stout and rounded as reconstructed in Tullimonstrum (McCoy et al. 2016). Moreover, they are not readily preserved in either the jawless or jawed vertebrates of the Mazon Creek Essex fauna (Nitecki 1979; Shabica & Hay 1997; Sallan & Coates 2014; Gabbott et al. 2016; Table 1). Table 1. Comparative taphonomy among Mazon Creek vertebrates from the Essex Fauna and Tullimonstrum as described by Clements et al. ( ) and McCoy et al. ( ) Lateral line traces Otic capsules Notochord RPE Myomeres Arcualia Gut Tullimonstrum (McCoy et al. 2016 ✗ ✗ ✓ ? ✓ ✓ ✓ Tullimonstrum (Clements et al. 2016 ✗ ✗ ✗ ✓ ? ✓ ? ✗ Mayomyzon ✓ ✓ ✗ ✓ ✗ ✗ ✗ Myxinikela ✓ ✓ ✗ ✓ ✗ ✗ ✗ Esconichthys ✓ ✓ ✗ ✓ ✗ ✗ ✗ Bandringa ✓ ✓ ✗ ✗ ✗ ✗ ✗ Laterally‐rounded segments divided by vertical septa (Johnson & Richardson 1966), were termed gill pouches anteriorly and w‐shaped muscle blocks (myomeres) posteriorly (McCoy et al. 2016), despite a lack of differentiation (Fig. 1A, seg, sep). These interpretations likewise present challenges. Vertebrate myomeres are relatively much thinner, posteriorly‐angled and overlapping, and extend the full length of the body and pharynx (Bardack & Richardson 1977; Janvier 1996; Donoghue & Purnell 2009; Kardong 2011; Sansom et al. 2011; Fig. 1B). Furthermore, recent decay experiments on priapulids have demonstrated that rings of circular muscles can taphonomically transform into weakly sigmoidal shapes (Sansom 2016). As such, instances of subtle curvature of serial segments (e.g. Tullimonstrum; Pikaia: Conway Morris & Caron 2012) may not be reliable evidence for the presence of w‐, z‐ or v‐shaped chordate myomeres. Lamprey respiratory tubes, or ‘gill pouches’, are paired lateral extensions underlying all gill slits on each side (Fig. 1B), rather than segmented units, each underlying a single gill pore (McCoy et al. 2016). The reconstructed respiratory anatomy is also dissimilar to gnathostomes, which have gill openings directly off the expanded buccal cavity or connected pharynx. The gill openings are separated only distally by thin tissues and filaments attached to skeletal elements, rather than solid septa (Randall 1982; Kardong 2011). Further, lamprey respiratory tubes are buried deep within a complex gill skeleton (Fig. 1B), as is the case for the general vertebrate pharynx (not reconstructed in Tullimonstrum: Bardack & Richardson 1977; Randall 1982; Janvier 1996; Donoghue & Purnell 2009; Kardong 2011; McCoy et al. 2016). Finally, the faint circles described as gill slits in Tullimonstrum sit on septa rather than associated gill tissue or pouches (McCoy et al. 2016), precluding a respiratory function, and also lack the expected pigmentation evident in Mazon Creek cyclostomes (Bardack & Richardson 1977; Gabbott et al. 2016; Fig. 1A). An expanded central oval was designated as part of a naked tripartite brain otherwise preserved as stains, despite its distinct hard‐tissue composition and continuity with the eye bars (Johnson & Richardson 1969). Associated skeletal (e.g. neurocrania) and nervous tissues (e.g. cranial nerves, placodes; Janvier 1996; Kardong 2011) are entirely missing. The absence of otic capsules filled with hard material (e.g. statoliths) in Tullimonstrum is both phylogenetically and taphonomically inconsistent with the lamprey interpretation. As decay‐resistant structures (Sansom et al. 2011), they are frequently preserved in demonstrable Mazon Creek vertebrates, including lampreys (Bardack & Richardson 1977; Sallan & Coates 2014; Gabbott et al. 2016; Fig. 2A, Table 1). A dark circle under the ‘gills’ was interpreted as the liver (McCoy et al. 2016), despite the liver's universal placement in vertebrates posterior to the pharynx (Kardong 2011; Fig. 1B, lv). Finally, a thin, ventral line shared with Gilpichthys was identified as the gut, yet lacks distinct units (divided intestine; Kardong 2011; Figs 1A, dl; 1B, in) and exhibits a split appearance in dorsoventral view. Specialized traits in Tullimonstrum present raise additional issues. For instance, McCoy et al. (2016) reconstructed a mouth with associated ‘buccal apparatus’, and opposing ‘tooth’ rows at the anterior of the proboscis. Articulated jaws with marginal tooth rows are a trait of gnathostomes (Janvier 1996; Clements et al. 2016). They are not homologous with muscular lamprey mouth structures, which comprise extensive, multi‐surface ‘tooth’ fields and depend on internal elements (Kardong 2011; Fig. 1B). This interpretation would necessitate an exceptional instance of convergent evolution of grasping jaws. Further, such a reconstruction is functionally improbable; most aquatic vertebrates depend on the movement of large volumes of water through the mouth into enlarged cavities for prey transport (ram and suction feeding) and/or respiration (ram ventilation) at some point (e.g. during active swimming; Roberts 1975; Randall 1982). This is particularly true for fusiform, typically pelagic and mid‐water types like Tullimonstrum (Roberts 1975). The narrow, jointed, elongate proboscis in Tullimonstrum would severely limit such flow, at odds with a greatly enlarged pharyngeal region and long distance to the gut, as inferred from placement of gill slits along half the body length (McCoy et al. 2016). A perpendicular arrangement for the gill pouches, as reconstructed from the segments (McCoy et al. 2016), would likewise present an obstacle (Kardong 2011). Some fishes have circumvented these issues through the pumping of enlarged buccal cavities, spiracular openings above the gill chamber (e.g. rays), piston cartilages and velum (e.g. lampreys), and/or muscular opercula (e.g. teleosts) which suction water directly into the gills (Roberts 1975; Kardong 2011). None of these solutions are apparent in Tullimonstrum; the buccal cavity itself is greatly reduced and separated from the gills, a jointed proboscis would disrupt a piston cartilage, and the designated gill openings are very small and round, without any apparent cover.

Phylogenetic relationships To reconstruct the phylogenetic relationships of Tullimonstrum following these anatomical interpretations, McCoy et al. (2016) utilized a chordate‐only dataset modified from previous studies (Sansom et al. 2011; Conway Morris & Caron 2014). They recovered Tullimonstrum as a lamprey, resolved crownward of the co‐occurring, anatomically‐modern Mayomyzon (Bardack & Zangerl 1971; Bardack & Richardson 1977; Sansom et al. 2011; Conway Morris & Caron 2014; Gabbott et al. 2016; Fig. 2A), on the basis of its asymmetrical tail. The lamprey clade was defined by characters that are either missing in Tullimonstrum (oral hood, annular and piston cartilages, see above), or equivocal in their interpretation (tectal cartilages, for which there is no evidence in Jamoytius; Sansom et al. 2011). Furthermore, the crownward placement of Tullimonstrum relative to Mayomyzon results from incorrect interpretation of an absence of an oral disc and tail asymmetry in the latter (Gabbott et al. 2016; Fig. 2A). Redressing just this coding for Mayomyzon and removal of only the most equivocal character for Tullimonstrum (tectals, see above) renders the placement of Tullimonstrum as ambiguous in an incongruent topology (Fig. 2B); this highlights immediate issues with the lamprey interpretation of McCoy et al. (2016), and with a vertebrate affinity more generally. Importantly, use of an all‐chordate dataset, excluding divergent non‐vertebrate forms and all non‐vertebrate fossils, renders a chordate and particularly vertebrate affinity as near‐inevitable, irrespective of codings applied to any taxon in question. Further, cyclostome monophyly was not supported in any analysis (McCoy et al. 2016; Fig. 2B), at odds with mounting evidence and the majority of recent topologies (Donoghue & Purnell 2009; Sansom et al. 2010; Kardong 2011; Conway Morris & Caron 2014; Janvier 2015; Gabbott et al. 2016). As such, the dataset employed by McCoy et al. (2016) does not present a test of chordate or vertebrate affinities.

Taphonomy Taphonomy was used to explain incongruent traits in Tullimonstrum (Clements et al. 2016; McCoy et al. 2016), despite established preservational modes at Mazon Creek (Shabica & Hay 1997; Sallan & Coates, 2014; Table 1). The definitive vertebrates found alongside Tullimonstrum in the Essex fauna (Nitecki 1979; Shabica & Hay 1997; Clements et al. 2016) preserve a consistent subset of cyclostome and gnathostome features: pigmented body outline, jaws, oral hood, fins, superficial gill structures, tooth impressions, eyes, statolith‐filled otic capsules and lateral line traces (Bardack & Richardson 1977; Sallan & Coates 2014; Gabbott et al. 2016; Fig. 2A, Table 1). The last two characters are vertebrate synapomorphies and have not been identified in Tullimonstrum, despite detailed study of over 1200 specimens (Clements et al. 2016; McCoy et al. 2016). This indicates true absence. Contra the explanation by McCoy et al. (2016), otic capsules are found in fishes preserved in dorsoventral view at Mazon Creek (Shabica & Hay 1997; Sallan & Coates 2014), including the cyclostome Mayomyzon (Bardack & Zangerl 1971); as decay‐resistant structures, their presence is expected (Sansom et al. 2011). Further, no Essex vertebrate, again including Mayomyzon (Bardack & Richardson 1977; Sansom et al. 2010), preserves internal musculoskeletal structures as distinct from external, delineating melanin pigment (Sallan & Coates 2014; Gabbott et al. 2016). The rare ‘myomeres’ and guts of some dubious agnathans like Gilpichthys (Shabica & Hay 1997; McCoy et al. 2016) are therefore likely to be external features, as are those of Tullimonstrum. Hence, it is highly unlikely that many of the structures described by McCoy et al. (2016) would be preserved in Tullimonstrum, even if a vertebrate affinity were favoured.

Alternative comparative models Considering the large number of biological and taphonomic issues presented by a lamprey or crown‐vertebrate identity for Tullimonstrum, alternative comparisons may provide a better‐fit diagnosis (e.g. multiple model comparisons for vetulicolians, Aldridge et al. 2007). Simply changing the interpretation of the axial band (described as a ‘notochord’ by McCoy et al. (2016) under their vertebrate interpretation) could alter downstream diagnoses enough to result in a different affiliation or suitable comparator. For example, similar axial bands, and ‘muscle blocks’, are widespread in exceptionally‐preserved Palaeozoic arthropods, representing guts with glands and exoskeletal segments (Zhang & Briggs 2007; Yang et al. 2016). Ventral nerve cords, pigmented, stalked eyes, brain and lateral tail‐fins are widely preserved in anomalocarids (Zhang & Briggs, 2007; Yang et al. 2016), with an elongate, jointed proboscis with pincer present in Opabinia (Foster 1979; Zhang & Briggs 2007; Fig. 1D). McCoy et al. (2016) rejected an arthropod affinity based on a taphonomic assumption that three‐dimensional preservation and cuticle carbonization was universal for Mazon Creek arthropods. This is demonstrably not the case based on available arthropod material (Shabica & Hay 1997). It is true that arthropods do not have melanosomes under a vertebrate‐based definition (Clements et al. 2016). However, they do have spheroid eye cells containing a related retinol‐based pigment among other cell types, which may be preserved similarly to the melanosomes and melanin of vertebrates (Eakin & Westfall, 1965; Hamdorf 1979; Schoenemann et al. 2009). Even positive identification of vertebrate‐like differentiated melanosomes does not preclude non‐vertebrate affinities. Similar structures have been identified in the eyes of other Bilateria (Clements et al. 2016). One example noted is molluscs, which independently evolved both melanin‐containing pigment cells and complex camera eyes without the extensive extrinsic musculature required by vertebrates, and thus are a plausible alternative based on these characters alone (Hamdorf 1979; Blumer 1998). In fact, McCoy et al. (2016) presented evidence supporting a molluscan identity for Tullimonstrum, in line with previous arguments for a heteropod gastropod affinity (Foster 1979). As McCoy et al. (2016) and previous workers such as Foster (1979) and Beall (1991) noted, molluscs share many inferred features of Tullimonstrum, including a multi‐lobed ‘brain’, complex eyes on hardened stalks, transverse muscle‐bands, guts ending before the posterior body, asymmetrical dorsoventral tail‐fin, proboscis, and bifurcate mouth with buccal mass and radular ‘tooth’ rows (the latter preserving like chordate teeth at Mazon Creek; Ralph 1957; Foster 1979; Fig. 1E). As noted above, since variation in eye microstructure has not been well surveyed in most non‐vertebrate bilaterians, and particularly their fossil members, alternate identities cannot be ruled out on these grounds (Clements et al. 2016). If the eye melanosome organization of Tullimonstrum is shown to be homologous with the RPE of vertebrates, it is still likely to be better supported among non‐vertebrate deuterostomes. Many invertebrate deuterostomes share, or have genetic precursors for, RPE elements (Lamb et al. 2007). They also lack many of the crown‐vertebrate characters absent in Tullimonstrum (Janvier 1996; Sansom et al. 2010; Conway Morris & Caron 2014). These include tunicates, which have pigmented eyes and tail ‘fins’ in larval form, as well as lancelets and hemichordate acorn worms, which have simple gill openings and a notochord extending past the ‘brain’ (Janvier 1996, 2015; Kardong 2011). That said, the most similar, segmented forms among extinct deuterostomes are Palaeozoic stem‐chordate vetulicolians, themselves of problematic affinity (Janvier 1996, 2015; Aldridge et al. 2007; Donoghue & Purnell 2009; Fig. 1C).

Conclusion Our review of the evidence suggests that a non‐vertebrate affinity for Tullimonstrum would produce fewer taphonomic discrepancies and require fewer apomorphies and Bauplan alterations than a vertebrate assignment. It is likely that a full accounting of the evidence (morphological, taphonomic, developmental and phylogenetic) with consideration of all well‐supported potential affinities, will be required to determine the most parsimonious attribution of Tullimonstrum (Aldridge et al. 1993; Donoghue & Purnell 2009). For the time being, crown vertebrate affinity appears to be particularly weakly supported.

Acknowledgements The authors thank the Editor, Martin Brazeau and Per Ahlberg for constructive comments and contributions that helped improve this manuscript.