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Accepted Manuscript Journal of the Geological Society

Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota, and the evolution of seriation in Ecdysozoa

Xiaomei Shi, Richard J. Howard, Gregory D. Edgecombe, Xianguang Hou & Xiaoya Ma

DOI: https://doi.org/10.1144/jgs2021-060

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This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion

Received 26 May 2021 Revised 2 August 2021 Accepted 7 August 2021

© 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

Supplementary material at https://doi.org/10.6084/m9.figshare.c.5551565

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Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota, and the evolution of seriation in

Ecdysozoa

Xiaomei Shi1,2,5, Richard J. Howard2,3,4,5, Gregory D. Edgecombe2,4*, Xianguang Hou1,2, Xiaoya

Ma1,2,3*

1. Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University,

Chenggong Campus, Kunming 650504, China

2. MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan

University, Chenggong Campus, Kunming 650504, China

3. Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall TR10

9TA, UK

4. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK MANUSCRIPT 5. These authors contributed equally

*Correspondence: [email protected]; [email protected]

ACCEPTED Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Abstract

Cricocosmiidae is a clade of palaeoscolecid-like worms from the Chengjiang Biota, China (Cambrian

Stage 3). In contrast to palaeoscolecids sensu stricto, which exhibit tessellating micro-plate trunk ornamentation, cricocosmiids possess larger, serially repeated sets of trunk sclerites bearing resemblance to lobopodian trunk sclerites (e.g., Microdictyon spp.). Cricocosmiidae were therefore proposed as stem-group Panarthropoda in some studies but are recovered as stem-group Priapulida in most phylogenetic analyses. The affinity of cricoscosmiids within Ecdysozoa is therefore of much interest, as is testing the homology of these seriated structures. We report four new specimens of the rare cricocosmiid Tabelliscolex hexagonus, yielding new details of the ventral trunk projections, sclerites and proboscis. New data confirm T. hexagonus possessed paired ventral trunk projections in a consistent seriated pattern, which is also reported from new material of Cricocosmia jinningensis

(Cricocosmiidae) and Mafangscolex yunnanensis (Palaeoscolecida sensu stricto). Even when the seriated sclerites and ventral projections of cricocosmiids are coded as homologous with the seriated trunk sclerites and paired appendages, respectively, of lobopodian panarthropods, our tree searches indicate they are convergent. Cricocosmiidae is nested within a monophyletic ―Palaeoscolecida sensu lato‖ clade (Palaeoscolecidomorpha nov.) in stem-group Priapulida. Our study indicates that morphological seriation has independent origins inMANUSCRIPT Scalidophora and Panarthropoda. Keywords: Cambrian, Ecdysozoa, palaeoscolecids, Cricocosmiidae, phylogenetics, palaeobiology

ACCEPTED Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Palaeoscolecida Conway Morris and Robinson, 1986 is a group of marine benthic ecdysozoan worms with a spiny anterior proboscis, an elongate trunk with striking ornamentation of small, annular- arranged, tessellating polymorphic plates, and bilaterally orientated terminal posterior hooks (Harvey et al. 2010). Cricocosmiidae Hou et al. 1999 are otherwise morphologically similar to palaeoscolecids, but exhibit a different, though also highly characteristic trunk ornamentation. The cricocosmiids

Cricocosmia jinningensis Hou and Sun, 1988 and Tabelliscolex hexagonus Han et al. 2003a possess larger sclerites that are serially repeated in groups along the length of the trunk, with a striking similarity in structure, position and composition to the trunk sclerites of some lobopodian panarthropods such as Onychodictyon spp. and Microdictyon spp. (Han et al. 2007a). Cricocosmiids and a few other vermiform taxa such as Maotianshania cylindrica Sun and Hou, 1987 and Tylotites petiolaris Luo et al. 1999 have been informally grouped with palaeoscolecids as ―Palaeoscolecida sensu lato‖; in this framework, ―Palaeoscolecida sensu stricto” is reserved exclusively for those taxa bearing the distinctive polymorphic tessellating plate ornament (see Harvey et al. 2010).

Palaeoscolecida sensu stricto has a broad biogeographic and stratigraphic range from the early

Cambrian to the late Silurian, whereas Cricocosmiidae, Maotianshania and Tylotites (i.e.,

Palaeoscolecida sensu lato) are known only from the Cambrian Stage 3 Chengjiang Biota of Yunnan, China. MANUSCRIPT Individual plates of Palaeoscolecida sensu stricto comprise the form-genera Milaculum,

Hadimopanella, Kaimenella and Utahphospha (see Hinz et al. 1990). These small shelly fossils

(SSFs) are known from the Cambrian to Silurian across a wide geographic distribution and are sometimes recovered in association with the sclerotome (Hinz et al. 1990; Müller and Hinz-

Schallreuter 1993; Brock and Cooper 1993; Zhang and Pratt 1996; Conway Morris 1997; Topper et al.

2010; Harvey et al. 2010; Duan et al. 2012; Streng et al. 2017). Individual plates of the form-genus

Hadimopanella have also been identified as small carbonaceous fossils (SCFs), alongside possibly conspecificACCEPTED pharyngeal teeth, introvert scalids and terminal posterior hooks from the early Cambrian of Scandinavia (Slater et al. 2017). Some SSF cuticle fragments even preserve in-situ the posterior terminal hooks of Palaeoscolecida sensu stricto (e.g., Harvey et al. 2010, Fig. 2E-G). Cricocosmiids remain unidentified from microfossil assemblages. Articulated two-dimensional compression fossils from Konservat-Lagerstätten demonstrate the complete cuticular morphology and partial soft-tissue anatomy of both groups however, revealing many similarities. Palaeoscolecida sensu stricto Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

described in detail from such articulated material include Scathascolex minor (Smith 2015),

Wronascolex antiquus (García-Bellido et al. 2013) and W. yichangensis (Yang and Zhang, 2016),

Mafangscolex yunnanensis (Hou et al. 1988; Luo et al. 2014; Yang et al. 2020); Guanduscolex minor

(Hu et al. 2008), and Utahscolex ratcliffei (Whittaker et al. 2020). The best preserved of these

Palaeoscolecida sensu stricto exhibit a three-part proboscis divisible into the traditional zonation system common to extant and fossil priapulans (Conway Morris 1977). Zone I is represented by an introvert with rings of scalids, Zone II is the collar – representing a diastema between introvert and pharynx, and Zone III is the eversible pharynx with rings of teeth. This is mirrored by the sometimes exquisitely preserved proboscis morphology of Cricocosmia jinningensis and Maotianshania cylindrica

(Huang 2005), and in addition is partially known in Tabelliscolex hexagonus (Han et al. 2003a) and

Tylotites petiolaris (Han et al. 2003b, 2007b).

Palaeoscolecid-like worms have an interesting and varied history of phylogenetic interpretation.

Palaeoscolecida was originally attributed to Annelida when first described (Whittard 1953), but later authors favoured an affinity among the cycloneuralian phyla due to the annulated trunk region (Dzik and Krumbiegel 1989; Conway Morris 1993), especially after the discovery of the anterior proboscis

(Hou and Bergström 1994). Alternative cycloneuralian interpretations of palaeoscolecids have been postulated, the group being allied to either nematomorphsMANUSCRIPT (Hou and Bergström 1994) or priapulans (Conway Morris 1997), or as stem-group Cycloneuralia (Zhurvalev et al. 2011). Another interpretation is that palaeoscolecids represent a basal branch within Ecdysozoa, which was popularized by the influential hypothesis of Dzik and Krumbiegel (1989) – wherein palaeoscolecids represent a transitional locomotor strategy linking priapulans and lobopodians. This was advocated by several authors (Budd 2001a, 2001b, 2003; Budd and Jensen 2003; Zrzavý 2003; Webster et al. 2006) on the basis that palaeoscolecids appear to possess a suite of characters that are expected in the ancestral ecdysozoan, such as an annulated cuticle and an armoured proboscis. Similarities between palaeoscolecidsACCEPTED and priapulans were therefore viewed as ecdysozoan plesiomorphies by these authors. This reconstruction of the ancestral ecdysozoan has been corroborated to a degree by ancestral state reconstructions (Howard et al. 2020a), but morphology-based phylogenetic analyses consistently retrieve a stem-group Priapulida affinity for palaeoscolecid-like worms (Wills 1998; Dong et al. 2004, 2005, 2010; Harvey et al. 2010; Wills et al. 2012; Liu et al. 2014; Ma et al. 2014a; Zhang et al. 2015; Shao et al. 2016). As such, Harvey et al. (2010) dismissed the idea that palaeoscolecid- Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

like worms are basally diverging ecdysozoans. Nevertheless, a recent study once again supported the basal-ecdysozoan interpretation after reporting hexaradial symmetry of the rings of introvert scalids

(i.e., circumoral armature) in a member of Palaeoscolecida sensu stricto (Yang et al. 2020).

Hexaradial symmetry of the anterior armature is interpreted as plesiomorphic for Ecdysozoa in contrast to the pentaradial symmetry of crown-group Priapulida (see Liu et al. 2014).

Cricocosmiids, whilst mostly recovered within stem-group Priapulida as Palaeoscolecida sensu lato in the phylogenetic studies cited above, are also of great phylogenetic interest. Despite their major similarities to Palaeoscolecida sensu stricto, cricocosmiids have been implicated in hypotheses of panarthropod origins. This is because they exhibit a dorso-ventrally differentiated trunk, equipped with serially repeated sclerites that bear considerable similarity to some lobopodian trunk sclerites in both position and composition – thus prompting hypotheses of homology (Han et al. 2007a; Steiner et al. 2012). As such, cricocosmiids are of much evolutionary significance, which has not been explored in the context of ecdysozoan-wide phylogeny. Furthermore, it is unclear which palaeoscolecid-like taxa actually belong in Cricocosmiidae beyond Cricocosmia and Tabelliscolex, and what the exact phylogenetic relationship is between Cricocosmiidae and Palaeoscolecida sensu stricto. Cricocosmia jinningensis is one of the most common Chengjiang taxa, known from thousands of specimens (see Huang 2005; Hou et al. 2017). However, material MANUSCRIPTand documentation of Tabelliscolex are far sparser, with only one complete specimen of T. hexagonus described (Han et al. 2003a), and a single incomplete specimen of T. chengjiangensis apparently distinguished by sclerite morphology (Han et al. 2007a). Tylotites petiolaris is a similarly scarce palaeoscolecid-like worm from the Chengjiang biota (Luo et al. 1999; Han et al. 2003b, 2007b) also with serially repeated large sclerites, suggesting a cricocosmiid affinity, though the sclerites are spinose and arranged in annular rings in contrast to the paired lateral sclerites of Cricocosmia and Tabelliscolex.

In this study, we present new material of Tabelliscolex hexagonus from the Chengjiang Biota, preservingACCEPTED new anatomical details in high fidelity. These specimens facilitate a re-description of Tabelliscolex, and comparison to new material of Cricocosmia and Mafangscolex provides insight into evolution of the palaeoscolecid-like worms. Furthermore, we conducted an extensive phylogenetic analysis of Ecdysozoa to better resolve the systematics of ―Palaeoscolecida sensu lato‖ (sensu

Harvey et al. 2010). Our results facilitated a discussion of the evolution of seriated morphology in

Ecdysozoa, suggesting that seriation independently evolved in Panarthropoda and Scalidophora. Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Materials & Methods

Geological setting

All specimens studied here belong to the Yunnan Key Laboratory for Palaeobiology (YKLP) collection, which were collected from various localities of the Chengjiang Biota (see Hou et al. 2017), eastern

Yunnan, Southwest China. The Chengjiang Biota is found within the Maotianshan Member of the

Yu‘anshan Formation at Cambrian Series 2, Stage 3 (circa 518 Ma). The hypothesized location of the

Chengjiang Biota during the early Cambrian is near the shore of the South China continent, which was close to the equator (see Hou et al. 2017; Holmer et al. 2018).

Fossil material

Three specimens of Tabelliscolex were collected from Haikou County (all with counterparts; see Fig.

1a-e and 1g-h) and an additional specimen missing the counterpart from Anning City (Fig. 1f). One specimen (YKLP 11428) is complete with proboscis and armature, and all specimens show details of the ventral trunk projections. 7131 specimens of Cricocosmia jinningensis and 703 specimens of

Mafangscolex yunnanensis were also investigated for comparative analysis of the ventral projections

(Fig. 5). All specimens are deposited in the Yunnan Key Laboratory for Palaeobiology, Yunnan

University, Kunming, China. MANUSCRIPT

All specimens are preserved in fine-grained, yellowish mudstone. All are essentially flattened compressions in a lateral, dorsolateral or ventrolateral perspective, though retaining a low degree of three-dimensional relief in some structures such as the sclerites, scalids and gut tract. The preservation of the trunk sclerites in Tabelliscolex and Cricocosmia is similar to that investigated by previous authors (see Han et al. 2007a and Steiner et al. 2012). Dark carbon films represent gut tracts. Observation, preparation and camera lucida drawings were performed under a Nikon

SMZ800N microscope. Photographs were taken using a Leica M205 microscope, and a Canon EOS 6D MarkACCEPTED II digital camera equipped with either 100 mm or 65 mm macro lens. A Keyence VHX-6000 3D imaging microscope was used to create topographic models of the ventral projections in

Tabelliscolex (Fig. 6), and sclerites in various taxa were imaged under an FEI Quanta 650 scanning electron microscope (Fig. 4).

Phylogenetic methods

Phylogenetic analyses were performed to resolve the position of palaeoscolecid-like worms within Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Ecdysozoa (Fig. 7). Tree searches used four alternative optimality criteria (equal weights parsimony, implied weights parsimony, maximum likelihood and Bayesian inference) to account for the debate over the most suitable method to analyse discrete morphological characters (O‘Reilly et al. 2016,

2018a, 2018b; Puttick et al. 2017, 2019; Goloboff et al. 2018a, 2018b).

Parsimony tree searches were conducted in TNT 1.5 (Goloboff et al. 2008; Goloboff and Catalano

2016), using the New Technology Search function with default settings. A strict consensus of six most parsimonious trees (MPT) is presented from equal character weighting (Fig. S1), and clade support was assessed by Jack-knife resampling (Fig. S1; Table 2) (Farris et al. 1996). A strict consensus of four MPTs (Fig. S2) is presented for implied character weighting (using the default concavity constant k=3), and clade support was assessed by symmetric resampling (Fig. S2) (Goloboff et al. 2003).

Consistently recovered nodes across parsimony analyses are summarized in Figs. 7b-c. Character transformations within Palaeoscolecidomorpha were optimized in a parsimony context using

WinClada (Nixon 2002) (see Fig. 7c).

Probabilistic tree searches utilized the MK model for discrete morphological character data (Lewis

2001). The maximum likelihood implementation was conducted in IQ-Tree (Nguyen et al. 2015) (Fig.

7a), with nodal support assessed by 1000 Ultrafast bootstrap (UFBoot) replicates (Minh et al. 2013; Hoang et al. 2018). The Bayesian implementationMANUSCRIPT was conducted in MrBayes 3.2 (Ronquist et al. 2012). The Bayesian analysis was run until convergence of the MCMC chains after 6,000,000 generations. Convergence was assessed from the average standard deviation of split frequencies

(<0.01), ESS scores (>200), and PSRF values (approximately 1.00). 25% of samples were discarded as burn-in, and a majority rule consensus was output (Fig. S3).

Results

Systematic palaeontology

Phylum (stem-group) Priapulida Delage and Hérouard, 1897 ACCEPTEDClade Palaeoscolecidomorpha nov. (=Palaeoscolecida sensu lato in Harvey et al. 2010) Included taxa:

Palaeoscolecida Conway Morris and Robinson, 1986 (=Palaeoscolecida sensu stricto in Harvey et al.

2010)

Cricocosmiidae Hou et al. 1999 (emended to include Tylotites Luo et al. 1999)

Maotianshania Sun and Hou, 1987 Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Markuelia Val‘kov, 1983

Diagnosis: Annulated worm-like body with an anterior armoured proboscis, an elongate trunk of consistent width, and terminal posterior hooks. Anterior proboscis divided into three sections from proximal to distal: introvert with circumoral longitudinal rows of spines (presumed scalids); collar region with or without spines; pharynx covered in rings of pharyngeal teeth. Introvert at least partially retractable into the trunk, pharynx elongate and fully eversible. Posterior terminal hooks arranged bilaterally about the sagittal plane of the trunk.

Remarks: The division of the proboscis is likely to be a plesiomorphy for Priapulida (see Conway

Morris 1977). The reliable autapomorphies of Palaeoscolecidomorpha are the elongate body profile with consistent trunk width (i.e., the trunk does not vary in width throughout the length of the body and the lateral margins are virtually parallel), and the terminal posterior hooks. The number of hooks may vary across taxa, but are always arranged bilaterally, with a left hook and a right hook (or a left and right pair etc.). Serially repeated paired ventral trunk projections are present in select Cricocosmiidae and Palaeoscolecida (see Fig. 5 and Fig. 8) and represent another possible autapomorphy for

Palaescolecidomorpha.

Family Cricocosmiidae Hou et al. 1999 Included genera: MANUSCRIPT Cricocosmia Hou and Sun, 1988

Tylotites Luo and Hou, 1999

Tabelliscolex Han et al. 2003a

Revised diagnosis: Palaeoscolecidomorphs with serially repeated arrangements of macroscopic trunk sclerites and corresponding paired ventral projections (at least in Cricocomsia and

Tabelliscolex), a single pair of terminal posterior hooks, and a posterior unarmed region of the introvert. Trunk sclerites composed of many tubercles of equal size on the outer surface with correspondingACCEPTED pits on the inner surface (at least in Cricocosmia and Tabelliscolex). Remarks: In the previous revision of Cricocosmiidae (Han et al. 2007a), Tylotites was not included, but our phylogenetic analysis recovered Tylotites within this clade (see Fig. 7). Therefore, the family

Tylotitidae Han et al. 2003b is a synonym of Cricocosmiidae. The serially repeated trunk sclerites are distinct in shape and arrangement in each genus, but their presence is autapomorphic for

Cricocosmiidae. Tylotites lacks data on sclerite fine structure, so the presence of tubercles and Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

corresponding pits is unknown. Tylotites sclerites differ considerably from Cricocosmia and

Tabelliscolex in arrangement, being long conical spines along transverse rings. Cricocosmia exhibits lateral pairs, and Tabelliscolex exhibits lateral pairs with an additional dorsal sclerite for each pair.

Tylotites and Cricocosmia are sister taxa in our parsimony and maximum likelihood analyses (see

Figs. 7, S1, S2) and share the synapomorphy of an acute distal termination to their sclerites (i.e., an overall spinose shape with a distinct apex), although in Tylotites the spines are elongated. The presence of paired ventral projections is unknown in Tylotites, but likely considering this character is present in Cricocosmia, Tabelliscolex and Mafangscolex (Palaeoscolecida), and therefore is likely to be a plesiomorphy for Cricocosmiidae. The posterior unarmed introvert region (i.e., a smooth region between the Zone I armature and the trunk annulations) is present in all cricocosmiids and also in

Maotianshania (see Huang 2005). This character is also possibly present in Markuelia (see reconstruction in Dong et al. 2010), but it is inferred from coiled late-stage embryos and is therefore less reliable. Regardless, Maotianshania and Markuelia are resolved proximally to Cricocosmiidae to the exclusion of Palaeoscolecida (see Fig. 7), which appear to lack this character (e.g., Figs. 1A, 1H,

1I and 3 in Smith 2015; Fig. 3A in García-Bellido et al. 2013; and Figs. 1A, 1B and 2O in Yang et al.

2020). The phosphatic cuticle fragment taxon Houscolex Zhang and Pratt, 1996, from the early Cambrian of Shaanxi, was also referred to CricocosmiidaeMANUSCRIPT by Han et al. (2007a). Although it possesses ventral projections resembling those of Cricocosmia and Mafangscolex, it lacks large sclerites, and accordingly is excluded from the Cricocosmiidae as delimited here.

Genus Tabelliscolex Han et al. 2003a

Type species: Tabelliscolex hexagonus Han et al. 2003a.

Diagnosis: (emended from Han et al. 2003a and Huang 2005) Cricocosmiid worm with two lateral rows of ellipsoidal sclerites, and a single corresponding dorsal row of ellipsoidal sclerites. Typical 3- zoned proboscis exhibits at least 7 circlets of elongate spines on the introvert (Zone I), stout triangular spinesACCEPTED on the collar (Zone II) and many circlets of short, spinose pharyngeal teeth (Zone III). Paired ventral projections are preserved as teardrop-shaped from above, or spine-shaped in lateral profile, curving posteriorly, and are present beneath every third set of sclerites.

Species Tabelliscolex hexagonus Han et al. 2003a

2003, Tabelliscolex hexagonus Han et al. 2003a, pl. 1 Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

2005, Tabelliscolex hexagonus. Huang 2005, pl. 46

2007, Tabelliscolex hexagonus. Han et al. 2007a, pl. 1, 4, 6

2007, Tabelliscolex chengjiangensis Han et al. 2007a, pl. 1, 4, 6, syn. nov.

Diagnosis: As for the genus.

Holotype: Eli-0001218A, pl. 1.1, Han et al. 2003a.

Paratypes: Eli-0001219A, Eli-0001220A, Eli-0001221A, Eli-0001222A, pl. 1.2-5, Han et al. 2003a.

Description: Two of the four new specimens preserve the full length of the trunk, YKLP 11428 and

YKLP 11431 (Fig. 1a-c and 1g-h). YKLP 11428 measures 7.9 cm in length and up to 0.3 cm in width, whereas YKLP 11431 measures 3.4 cm in length and up to 0.2 cm in width, indicating either different stages of maturity or intraspecific variation. Body is slim, cylindrical and divided into an anterior proboscis, annulated trunk with net-like ellipsoidal sclerites, and a pair of terminal posterior hooks on the trunk.

Proboscis: YKLP 11428 shows the completely everted proboscis (Fig. 3) and facilitates a more detailed description. The proboscis is subdividedMANUSCRIPT into three sections (Zone I-III) from anterior to posterior in typical fashion for palaeoscolecidomorphs (introvert, collar, and pharynx). The width of the everted introvert is almost equal to the width of the trunk. The length of the fully everted pharynx reaches around 5.00 mm, and the pharynx features numerous circlets of tiny spinose teeth. The pharynx comprises three sections: section 1 is ca. 2.2 mm in length and is slightly wider than the collar, covered with teeth pointing anteriorly; section 2 is swollen, 1.1 mm in length, and also covered with teeth pointing anteriorly; section 3 is at least 1.6 mm in length with a uniform width, and covered with teeth pointing laterally (see Fig. 3b). The collar is roughly conical, and ornamented by stout, triangularACCEPTED spines. The collar spines sit immediately below the everted pharynx and are around ~0.1 mm in length. The introvert follows the collar with a clear boundary and is ornamented by elongate curved spines (presumably scalids). Spines are arranged circumorally, forming longitudinal rows.

From anterior to posterior, each longitudinal row of spines become shorter in length. The spines of the first three circlets are about 0.7 mm in length, and are elongate and curved with sharp tips (Fig. 3); the spines in the following circlets become shorter and their lengths are about one-third that of scalids in Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

the first three circles, with at least seven circlets in total. The fidelity of preservation is not sufficient to determine the exact number of spines per row or their mode of radial symmetry.

Overall, the detailed morphology of the proboscis in T. hexagonus remains incomplete but is improved by the present study. Our observations reported here show YKLP 11428 appears to exhibit three distinct fields of pharyngeal teeth, as in Cricocosmia jinningensis (see Huang 2005). However,

C. jinningensis appears to have a markedly different tooth morphology and arrangement in those distinct fields, but that cannot be confirmed or rejected in T. hexagonus due to the quality of preservation. The pharynx of the putative cricocosmiid Tylotites petiolaris (see Han et al. 2007b) is too poorly preserved/figured to be compared for this character. Other palaeoscolecidomorphs with detailed pharyngeal morphology data available do not share this configuration. The pharynx of

Maotianshania cylindrica certainly differs, with simple uniform spinose teeth covering most of it, and some more elongate basal teeth (Huang 2005). Mafangscolex yunnanensis exhibits two fields of teeth with differing arrangement, but seemingly uniform morphology (Yang et al. 2020), and Scathascolex minor exhibits just one field with cusped morphology (Smith 2015).

Trunk: The annulated trunk is cylindrical and elongate with no discernible subdivision, with about 4 annulations per millimetre. YKLP 11428 shows thatMANUSCRIPT the annulations in the anterior-most portion of the trunk are denser and lacking in sclerites (Fig. 2a). T. hexagonus exhibits a single bilateral pair of hooks at the posterior termination of the trunk, each hook being curved and bent laterally with a broad base.

Sclerites: T. hexagonus bears 110-120 sets of serially repeated sclerites along the majority of the length of the trunk. Each set is composed of three ellipsoidal flat sclerites arranged transversely (one dorsal, two lateral). The dorsal sclerite is equal in size to its corresponding lateral sclerites (Figs. 2c,

2d, 4a). The length-width ratio of the sclerites decreases gradually from anterior to posterior (Fig. S4). Each scleriteACCEPTED is ornamented with outer tubercles (ca. 120) and corresponding inner pits. Pit/tubercle diameter ranges 20-30 μm. There is a clear and smooth gap between each set of sclerites, with boundaries running either side of the gap transversely across the trunk to the body margin, which is best seen in some parts of YKLP 11428 and 11429 (Fig. 1a, b, d and Fig. 8c; also seen in Plate I, Fig.

1d in Han et al. 2003a, and Fig. 46a-d in Huang et al. 2005). If each set of sclerites is supposed to be situated in the middle of every annulation, as shown in the reconstructions of Fig. 45 in Huang (2005) Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

or Fig. 6B in Han et al. (2007a), the annulation boundaries should run across the middle of the gap between each set of sclerites, but this is not observed. Therefore, each of these gaps is interpreted as a non-sclerite bearing annulation, and sclerite sets occur on every two annulations (Fig. 1a, b, d, 8c).

It appears that the sclerite-bearing annulations are slightly narrower than those without sclerites, indicating heteronomous annulation, which is known from varied ecdysozoan taxa.

Ventral projections: Pairs of projections are serially repeated along the length of the ventral trunk

(see Figs. 2 and 5g-I), seemingly occurring beneath every three sets of sclerites (Fig. 8c). The pair is usually preserved as teardrop-shaped with one convex projection and one concave projection (Fig. 6).

The depth/height of ventral projections ranges from 25 μm to 90 μm, and the length of the projection can reach up to 0.5 mm. Laterally compressed projections reveal that these ventral projections are spine-shaped in profile, curving posteriorly, with a strong base (Fig. 5i).

Remarks: Han et al. (2007a) described a second species, Tabelliscolex chengjiangensis, differing from T. hexagonus by having concentric lamina in some pits and possibly lacking dorsal sclerites. No complete or articulated fragmentary specimens of T. chengjiangensis have been reported yet. This additional species is described from suspected moulted sclerites with a concentric lamina pit interpreted as absent in the type species T. hexagonusMANUSCRIPT. In general, a concave structure is more likely to gather clay, pyrite particles and other minerals in late diagenesis, and therefore concentric lamina may not be primary biogenic structures. As such, the moulted sclerites described as T. chengjiangensis may be synonymous with T. hexagonus, and more specimens are needed to confirm the validity of T. chengjiangensis.

New observations in Cricocosmia and Mafangscolex

In addition to Tabelliscolex hexagonus, we report similar paired ventral structures in Mafangscolex yunnanensis (Palaeoscolecida sensu stricto) and Cricocosmia jinningensis (Cricocosmiidae). The ventralACCEPTED projections of Mafangscolex are spike-like (Fig. 5b), 0.20－0.25 mm in length, with an expanded base (Fig. 5c), occurring about every two annulations (Figs. 5a-b, 8a). Mafangscolex‘s ventral projections occur on the median zone between two transverse plate bands. The ventral projections of Mafangscolex are perpendicular to the body wall (Fig. 5b). The shape of the ventral projections of Cricocosmia appears similar to those of Mafangscolex in dorsal view, but there are Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

marked differences. Cricocosmia ventral projections correspond to paired lateral sclerites per annulation, and in lateral view it can be observed that the tip of ventral projection of Cricocosmia points posteriorly (Fig. 5d-f). The ventral projections of Tabelliscolex are obviously larger than those of

Cricocosmia and Mafangscolex, are preserved in high relief (Fig. 6), and overall are more similar in shape to those of Cricocosmia than Mafangscolex.

Phylogenetic characters

The phylogenetic character matrix (included in NEXUS format as a supplementary data file) was used in all tree searches, comprising 95 fossil and extant ecdysozoan taxa scored for 179 morphological characters. This character sample derives primarily from two previous matrices – the cycloneuralian- focused matrix of Harvey et al. (2010), and the panarthropod-focused matrix used and updated in

Smith and Ortega-Hernández (2014), Smith and Caron (2015), Yang et al. (2015) and Zhang et al.

(2016). The full list of character descriptions is included as an appendix in the Supplementary

Material. The following newly defined/modified characters are of specific relevance to the present study (i.e., pertaining to the seriated structures common to palaeoscolecid-like worms and lobopodians) and are listed in the order in which they appear in the character list:

77. Trunk with serially repeated paired ventral/ventrolateral structures (absent or present).

This character suggests the homology of the pairedMANUSCRIPT ventrolateral trunk lobopods of panarthropods with the paired ventral projections in Mafangscolex, Cricocosmia and Tabelliscolex. This is justified by the consistent metameric pattern of the ventral projection pairs relative to the trunk annulations (Fig.

5), and in the case of Cricocosmia and Tabelliscolex, relative to lateral/dorsolateral sclerites shared by lobopodians (see reconstructions in Fig. 8). As such, the projections are consistent with lobopods in topological position, occur in a serially repeated sequence, and share a metameric relationship to lateral/dorsolateral sclerites with many lobopodian taxa. Tylotites is coded as unknown (?), as it has serially repeated ventral spines (Han et al. 2007b), but it is unknown if these are distinct paired structuresACCEPTED that can be discriminated from the transverse rings of spinose sclerites that are unique to this taxon.

An alternative to this character formulation would draw a homology between the ventral projections of palaeoscolecidomorphs and the claws of panarthropods. We regard this as less plausible than a homology with the lobopod/appendage as a whole for the following reasons: 1) in terms of character ontology, the whole:part relationship between an appendage and its claw is Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

appendage (whole): claw (part). The claw evolving before the appendage violates the character ontology, whereas a proposal that appendages existed in palaeoscolecidomorph ancestry but were lost apart from the claws is ad hoc; 2) evidence for claws – a recalcitrant morphological element

(Murdock et al. 2016) – is missing from many lobopodian taxa, challenging an assumption that claws are a panarthropod plesiomorphy. Some taxa are missing claws includinh Diania (see Ma et al.

2014b) and Xenusion (see Dzik and Krumbiegel 1989), which are potential stem-group

Panarthropoda according to of some of our phylogenetic analyses; 3) the ventral projections of palaeoscolecidomorphs are not sclerotized, but rather are preserved in the same colour and style as the body wall, whereas sclerotized claws are darker and opaque compared to the lobopod.

Additionally, claws are usually paired (except Aysheaia, Hallucigenia hongmeia and Luolishaniidae), whereas the ventral projections are single – one left and one right.

78. Form of serially repeated paired ventral/ventrolateral trunk structures (simple projections or limbs):

This character discriminates the paired spine-like projections of Mafangscolex, Cricocosmia and

Tabelliscolex from lobopods. Whilst the paired ventral projections of the worms mirror lobopods in a topological context, lobopods are markedly different in their more flexible, conical form with a broad circular attachment to the body wall (Whittington 1978;MANUSCRIPT Hou et al. 2004; Liu et al. 2008; Howard et al.

2020b).

79. Serially repeated epidermal specializations (absent or present):

In addition to many lobopodian taxa, this is coded present for Cricocosmia (as in Yang et al. 2015) and Tabelliscolex, which have serially repeated pairs/triplets of lateral/dorsolateral sclerites (Han et al.

2007a); and Tylotites, which has serially repeated transverse rings of spinose sclerites (Han et al.

2007b). Mafangscolex and other palaeoscolecid-like worms are coded as absent.

80. Position of serially repeated epidermal specializations (longitudinal rows, incomplete transverseACCEPTED rings or complete transverse rings): In most lobopodians, Cricocosmia, and Tabelliscolex the serially repeated epidermal specializations

(nodes, spines, sclerites, etc.) are arranged in longitudinal rows in a dorsal, lateral or dorsolateral perspective. Tylotites (see Han et al. 2007b) and the luolishaniid lobopodian Acinocricus (see Caron and Aria 2020) however exhibit their respective sclerites in rings. In the case of Tylotites, the sclerites form complete rings around the body, whereas in Acinocricus the rings do not conjoin ventrally. Two Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

taxa therefore exhibit serially repeated spinose sclerites in rings, Tylotites (complete ring) and

Acinocricus (incomplete ring). This character also differentiates the two by the dorso-ventral differentiation exhibited by Tylotites, wherein the more ventral spines of the transverse ring are considerably shorter than more dorsal spines (see Fig. 1.1 in Han et al. 2007b).

93. Correspondence of serially repeated dorsolateral epidermal specializations to ventral paired structures (1:1, 2:1, 3:1 or 4:1):

In most lobopodians and Cricocosmia, there is a 1:1 relationship between trunk lobopod pairs and the metameric epidermal specializations – i.e., for every serially repeated group of epidermal specializations (nodes, spines etc.) there is one pair of ventral structures (lobopods or ventral projections, see Fig. 5d and 8b for Cricocosmia). In Tabelliscolex the ventral projection pairs occur beneath every third serial sclerite triplet (see Figs. 1, 2 and 8c), in Luolishania beneath every second

(Ma et al. 2009), and in Thanohita beneath every fourth (Siveter et al. 2018). Coded 1:1 for

Collinsovermis (Caron and Aria 2020), inapplicable (-) for Mafangscolex, and uncertain (?) for

Tylotites and Acinocricus.

Phylogenetic trees

The majority of our phylogenetic analyses (all parsimony tree searches and maximum likelihood) recovered an exclusive monophyletic grouping of MANUSCRIPTthe palaeoscolecid-like worms, located within stem- group Priapulida that we name Palaeoscolecidomorpha (see Fig. 7). This clade is essentially equivalent to ―Palaeoscolecida sensu lato‖ as defined by Harvey et al. (2010), which was ambiguous in terms of monophyly in that study. Only Bayesian inference failed to recover

Palaeoscolecidomorpha, but it also did not resolve Scalidophora beyond the extant priapulan taxa and Palaeoscolecida sensu stricto. Full topologies with 95 taxa and support values for all tree searches are presented in the Supplementary Material.

When Palaeoscolecidomorpha is recovered, it always contained a monophyletic clade comprising WronascolexACCEPTED, Mafangscolex and Scathascolex (representatives of Palaeoscolecida = ―Palaeoscolecida sensu stricto” in Harvey et al. 2010) and another containing Cricocosmia, Tylotites and Tabelliscolex (Cricocosmiidae). The position of Maotianshania and Markuelia within

Palaeoscolecidomorpha was slightly variable across optimality criteria, but they were always closer to

Cricocosmiidae rather than to Palaeoscolecida. Maximum likelihood resolved Maotianshania and Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Markuelia as sister taxa, and that clade in turn sister taxon to Cricocosmiidae. However, in the parsimony strict consensuses, Markuelia and Maotianshania form a polytomy with Cricocosmiidae.

No tree search yielded a relationship between Palaeoscolecidomorpha and Panarthropoda. Stem- group Panarthropoda comprises only Aysheaia under equal weights parsimony and maximum likelihood, whereas Aysheaia is resolved in the stem-group of Tardigrada (as sister to Onychodictyon) under implied weights parsimony. The stem-group of Panarthropoda comprises Microdictyon,

Xenusion, Diania and Paucipodia under implied weights parsimony, whereas other methods recovered these lobopodians in stem-group Onychophora. Bayesian inference did not resolve the stem group of Panarthropoda.

Discussion

Systematics of palaeoscolecidomorph worms

According to our phylogenetic analyses, Palaeoscolecidomorpha is most likely a clade within the stem-group of Priapulida, forming the sister-group to the clade comprising crown-group Priapulida and the paraphyletic assemblage of ―archaeopriapulids‖ (e.g., Ottoia see Conway Morris 1977; Eximipriapulus see Ma et al. 2014a). The MANUSCRIPT hexaradially ornamented phosphatic microfossil Eopriapulites (Liu et al. 2014) was usually the most stemward branch of total-group Priapulida, lending support to the hypothesis that Ecdysozoa is ancestrally hexaradial in circumoral armature (Liu et al. 2014). The armature of Mafangscolex yunnanensis has been determined to be hexaradial (Yang et al. 2020), and therefore it is inferred the transition to pentaradial circumoral armature occurred in the more crownward paraphyletic ―archaeopriapulid‖ assemblage. The crown-group Priapulida plus

―archaeopriapulids‖ clade is constrained to a minimum divergence point of the Ediacaran-Cambrian boundary by the ichnospecies Treptichnus pedum (Vannier et al. 2010; Kesidis et al. 2019), which indicates Palaeoscolecidomorpha had also diverged by this point. As such, the fossil record shows

PalaeoscolecidomorphaACCEPTED probably existed from at least the terminal Ediacaran until at least the

Silurian.

Palaeoscolecidomorpha is supported by two unambiguous autapomorphies: bilaterally arranged terminal posterior hooks, and an elongate trunk of consistent width (see systematic palaeontology).

Bilateral terminal posterior hooks are present in all included taxa though with variation in the number Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

of hooks/spines. Duan et al. (2012) rejected a palaeoscolecid affinity for Markuelia on the basis of the number of posterior spines (3 pairs) and the absence of polymorphic tessellating plate ornament. Our analysis partly supports the hypothesis of Duan et al. (2012) in that Markuelia is not a member of

Palaeoscolecida. Markuelia is recovered here in the more inclusive Palaeoscolecidomorpha, in which all taxa share the presence of posterior bilateral spines/hooks – including taxa also with seemingly derived numbers of hooks (e.g., the triplet set of Wronascolex, the double pair of Scathascolex). The inclusion of Markuelia within Palaeoscolecidomorpha suggests palaeoscolecidomorphs were direct developers (see Dong 2007; Dong et al. 2004, 2005, 2010), but are bracketed by loricate taxa in our phylogeny (Loricifera, Sicyophorus, crown-group Priapulida), suggesting direct development is a derived trait in Markuelia/Palaeoscolecidomorpha. The elongate trunk of consistent width is also present in all included palaeoscolecidomorph taxa and presents a reliable character for identifying poorly preserved palaeoscolecidomorphs fossils that preserve little or no cuticular ornamentation

(e.g., Conway Morris and Peel 2010). One additional potential palaeoscolecidomorph autapomorphy is the seriated paired ventral projections (see Fig. 5). Ventral projections are clearly present in the cricocosmiids Cricocosmia and Tabelliscolex, but also apparently in the palaeoscolecids

Mafangscolex and Houscolex (see Fig. 2 in Zhang and Pratt 1996), which implies this character is plesiomorphic to both groups (hence an autapomorphyMANUSCRIPT of Palaeoscolecidomorpha). However, some caution is required, as this character is clearly absent in all known stages of development in Markuelia

(Dong et al. 2010). Furthermore, it is unknown if this character occurs in the remaining 2D compression taxa, as it is rarely preserved even in the taxa in which it definitely occurs other than

Tabelliscolex, but the presence of ventral structures is also not precluded in these without further study. For example, only 21 of 703 specimens of Mafangscolex preserved the ventral projections in the present study (see Table 1), and most palaeoscolecid studies include substantially fewer specimens than the present one.

PalaeoscolecidaACCEPTED is supported as a group within Palaeoscolecidomorpha by a single, but highly distinctive autapomorphy – the annular arranged tessellating polymorphic plate ornamentation. Within

Palaeoscolecida, Scathascolex and Wronascolex each show a derived number of posterior hooks.

Whereas Mafangscolex exhibits a single pair of hooks like cricocosmiids and Maotianshania,

Scathascolex possesses a double pair of hooks (Smith 2015, Fig. 1J), and W. antiquus possesses a triplet (García-Bellido et al. 2013, Fig. 3F). Furthermore, a single ring of introvert spines/scalids is Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

autapomorphic to S. minor. The introverts of all other palaeoscolecidomorph taxa (and the vast majority of scalidophorans) comprise multiple transverse rings. Smith (2015) speculated that the single ring in Scathascolex may be indicative of a nematoid affinity, as rings of circumoral structures on nematoid proboscises are much fewer than those of scalidophorans, but our analyses do not support this. Scathascolex exhibits several other characters allying it to Scalidophora and

Palaeoscolecida (see Table 3). Cricocosmiidae is supported by at least one distinctive autapomorphy, the variable seriated large trunk sclerites (see systematic palaeontology), and appears to share the unarmed posterior proboscis region at least with Maotianshania.

Seriated sclerites in Cambrian Ecdysozoa

Dzik and Krumbiegel (1989) predicted that palaeoscolecid-like worms represented a transitionary locomotor grade between infaunal priapulans and errant epibenthic lobopodians (Dzik and

Krumbiegel, 1989, Fig. 6). This was presented as evidence of the shared ancestry of lobopodians

(―xenusiids‖) and priapulans, which was confirmed in the following decade by the advent of molecular phylogenetics and the Ecdysozoa hypothesis (Aguinaldo et al. 1997; Giribet and Edgecombe 2017).

In the model of Dzik and Krumbiegel (1989), a hypothetical worm is depicted to represent the ―missing-link‖ between palaeoscolecids and the relativMANUSCRIPTely simple lobopodian Xenusion auerswaldae – which was resolved in stem-group Panarthropoda by implied weights parsimony in the present study

(Fig. S2). This hypothetical worm had paired, seriated, dorsolateral and ventrolateral structures. This depiction therefore resembles our current interpretation of cricocosmiids in cross-section and lateral profile (see Fig. 8e). Cricocosmiids were poorly known at the time of publication of Dzik and

Krumbiegel (1989), but subsequent studies recognised the similarity of the seriated sclerites of

Tabelliscolex, Cricocosmia and several lobopodians (Han et al. 2007a; Steiner et al. 2012). This naturally provoked hypotheses in these studies of panarthropod ancestry among cricocosmiids, seemingly corroborated by Dzik and Krumbiegel‘s predictions. This has not however been supported by previousACCEPTED morphology-based phylogenetics (e.g., Harvey et al. 2010), and is not supported by our phylogenetic analyses (see Fig. 7), even with the ventral projections newly accounted for, and a significantly better representative taxon and character sample for Ecdysozoa as a whole.

Furthermore, there is evidence that cricocosmiids were infaunal burrow dwellers (Huang et al. 2014), whilst Dzik and Krumbiegel (1989) expected such a worm to be epibenthic. Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Like Tabelliscolex, Cricocosmia bears two lateral rows of sclerites (Figs. 5d, 8b) with two distinct surfaces (Figs. 4a-f). However, Cricocosmia lacks the additional dorsal row of sclerites, and rather than ellipsoidal plates as in Tabelliscolex, Cricocosmia sclerites are posteriorly directed, smoothly curved, hollow spines with an apex and a broad base. Regardless, the outer surface in both taxa bears hexagonally arranged tubercles, with the inner surface covered by corresponding pits – which are interpreted together as holes ornamenting the sclerite. This gives cricocosmiid sclerites a distinctive ―net-like‖, porous appearance (see Fig. 4) that is at least superficially similar to sclerites of the lobopodians Onychodictyon spp. (Topper et al. 2013), Microdictyon spp. (Topper et al. 2011; Pan et al. 2018) and seemingly Hallucigenia hongmeia (Steiner et al. 2012), but not H. sparsa or H. fortis;

(see Caron et al. 2013). In addition, many other lobopodian taxa exhibit seriated nodes, spines and plates but have not been investigated under SEM. There are however several aspects where cricocosmiid sclerites differ from lobopodian sclerites in structure. In Microdictyon the ―net-like‖ appearance of the sclerite is similarly produced by hexagonally arranged pore holes, but they vary in size towards the periphery (whereas cricocosmiid pores are consistent all over), and each pore is ornamented by a further hexagonal ring of nipple-like tubercles (Figs 4g-h; see Topper et al. 2011;

Pan et al. 2018). This can also be observed in Onychodictyon, though the secondary hexagons of ornamentation are not nipple-shaped, but scarpedMANUSCRIPT platforms (Topper et al. 2013). Our phylogenetic analyses suggest these differences in structure are explained by the fact these sclerites are a result of evolutionary convergence (analogy), as opposed to shared inheritance (homology). Cricocosmiidae is deeply nested in Scalidophora in our trees, and lobopodian taxa exhibiting seriated sclerites are a diverse polyphyletic assemblage of stem-group panarthropods, stem-group onychophorans, and stem-group tardigrades. The most parsimonious explanation appears to be that seriated dorsolateral trunk structures are plesiomorphic to the panarthropods but have also evolved independently within

Scalidophora. Our phylogenetic interpretation does not necessarily preclude some of the ideas postulatedACCEPTED by Dzik and Krumbiegel (1989) however, as the lower reaches of the panarthropod stem- group remain poorly resolved. The elongate, annulated trunks of lobopodians undoubtedly reveal that panarthropods are derived from worm-like ancestors, but no fossil has yet been constrained to stem- group Panarthropoda that lacks paired appendages, with a recent study rejecting Facivermis yunnanicus as a cycloneuralian-panarthropod transitionary example (Howard et al. 2020b). Our study does conclude however, that cricocosmiids, like Facivermis, are also not representative of the Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

cycloneuralian-panarthropod transition (i.e., stem-group Panarthropoda). Cricocosmiids and other palaeoscolecidomorphs exhibit an array of scalidophoran synapomorphies, in addition to the ecdysozoan plesiomorphies and homoplastic characters they share with lobopodian panarthropods.

These scalidophoran and/or priapulan synapomorphies including the retractable anterior introvert, multiple transverse rings of spinose circumoral armature on that proboscis (though their identity as hollow scalids is technically unconfirmed), the eversible pharynx, and at least in some

Palaeoscolecida the array of characteristic scalidophoran external sensory structures (flosculi, tumuli, tubuli, etc., see Harvey et al. 2010).

Ventral projections in palaeoscolecidomorphs

Ventral spines and nodes were originally described in Tabelliscolex (Han et al. 2003a), and subsequently reported to occur in a sequence beneath every third set of sclerites (Huang 2005) – though fossil material supporting this hypothesis was not figured. Han et al. (2007a) also reported ventral spines in Cricocosmia, but did not figure them, and compared them to the ventral spines of the extant meiofunal priapulan Tubilichus, interpreted as an indication of sexual dimorphism. We have demonstrated here that the hypothesis of Huang (2005) is likely to be the most correct, but the spines of Tabelliscolex are paired, curved, and posteriorlyMANUSCRIPT directed (see Figs. 5g-I, 6 and 8c). In addition, we report that similar, smaller paired ventral projections are also present in Cricocosmia and

Mafangscolex. We interpret the ventral projections as a probable autapomorphy for

Palaeoscolecidomorpha (see above discussion and Table 3). Incompletely preserved members of

Palaeoscolecida from other localities also appear to demonstrate corroborating evidence of this character (Zhang and Pratt 1996; Hu et al. 2012). Hu et al. (2012) described large protuberances being irregularly present in the posterior ventral of Yunnanoscolex magnus and Wudingscolex sapushanensis from the Guanshan Lagerstätte. The Wuliuan Burgess shale ―archaeopriapulid‖

Louisella pedunculata also has two longitudinal rows of long filamentous ventral spines/papillae

(ConwayACCEPTED Morris 1977). The shape and arrangement of these structures in Louisella have obvious differences however, running only along a posterior portion of the trunk, and with no corresponding relation to the trunk annulations. As is the case for seriated sclerites, the phylogenetic distance between cricocosmiids (stem-group Priapulida) and lobopodians (stem-group Panarthropoda) means that even when a primary homology between ventral projections and lobopods is coded, these Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

seriated ventrolateral structures optimise on the phylogeny as homoplastic. The ventral projections of

Cambrian scalidophoran worms are convergent with panarthropod appendages, even to the degree that both show a correlated seriality with dorsolateral sclerites.

Palaeobiology of Tabelliscolex

Locomotion. Ventral projections of palaeoscolecidomorphs have previously been discussed with reference to sexual dimorphism, to increase friction in burrowing, or for mucus secretion from glands

(Han et al. 2007a). Given that ventral projections are observed in all specimens of Tabelliscolex but are seen in only a minute fraction of the samples for Cricocosmia and Mafangscolex, relative abundance data are inconsistent with sexual dimorphism. We regard it as most plausible that the ventral projections of T. hexagonus were used to increase friction, as these structures are larger and harder than that of other palaeoscolecidomorphs. The ventral projections of T. hexagonus might therefore have had an ability to support the body and be employed in locomotion.

All priapulans move using the hydrostatic skeleton to push the body via peristalsis, but the movement of different species reveals some minor differences. This behaviour may result in distinctive probing burrows, which are comparable between present-day priapulans and late Ediacaran-Cambrian trace fossils called treptichnidsMANUSCRIPT (Vannier et al. 2010; Kesidis et al. 2019), such as Treptichnus pedum, the occurrence of which defines the boundary between the Ediacaran and

Cambrian (Buatois 2018). The complete movement cycle of Priapulus caudatus has four stages: (1) trunk peristaltic contraction, then propagating rapidly from posterior to anterior; (2) invagination; (3) powerful eversion; and (4) proboscis inflation (Vannier et al. 2010). The complete movement cycle of the Cambrian priapulan Eximipriapulus globocaudatus was inferred to involve seven stages (the introvert, trunk, and terminal region in different stages have different shapes) (Ma et al. 2014a). The movement of the cricocosmiid T. hexagonus is hypothesised to have involved a double anchor fixing strategyACCEPTED like other palaeoscolecidomorphs, making use of both the proboscis scalids and the posterior hooks. T. hexagonus has a long trunk and an obvious difference between the dorsum and abdomen.

This species is therefore regarded as engaging in a demersal and epifaunal lifestyle. When it burrowed, the parts of the trunk close to the proboscis or the posterior end would undergo peristaltic contraction. The ventral projections pointing posteriorly would increase friction to prevent sliding and accelerate movement, as well as control the direction of movement. Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Ontogeny and moulting. Information on moulting in fossil scalidophorans is often lacking, with fossils preserving an animal and its exuvium together being rare, as moult events were of short duration and the exuviae are thin (Daley and Drage 2016). Little is known about ontogeny and moulting of palaeoscolecidomorphs specifically, aside from some isolated cuticular fragments apparently showing a double-layer interpreted as having been buried shortly before ecdysis (Müller &

Hinz-Schallreuter 1993). However, the moulting process has been described in phosphatic scalidophoran cuticle fragments from Fortunian-aged Kuanchuanpu Formation of Shaanxi Province,

China (Wang et al. 2019). These scalidophorans are thought to occupy a position in stem-group

Scalidophora, and to moult in a similar manner to extant priapulans, wherein the body is extricated smoothly from the old tubular cuticle, or the exuvium is turned inside out like the finger of a glove

(Wang et al. 2019). An early Cambrian stem-group loriciferan from Greenland appears to show a similar mode of ecdysis, with the body preserved emerging from an apparent tubular exuvium (Peel et al. 2013). Therefore, it may be inferred the process of ecdysis has been conserved across

Scalidophora. However, cricocosmiid trunk sclerites, though resolved as convergent here, are highly similar to those of lobopodians, which may indicate additional complexity in the moulting process in cricocosmiids. Isolated lobopodian sclerites with two conjoined elements, wherein a larger sclerite underlies a smaller one, have been reported in MANUSCRIPTMicrodictyon spp. (Zhang and Aldridge 2007) and Onychodictyon spp. (Topper et al. 2013) from the Cambrian of China and Greenland, respectively.

These conjoined specimens are interpreted as successive moults, indicating that the ecdysis of trunk sclerites may occur through a gradual replacement process (Topper et al. 2013).

T. hexagonus appears to be represented by at least one juvenile specimen (YKLP 11431, Fig. 1g).

It is apparently smaller than other specimens but similar in morphology. Figures 1d and 1e show specimens somewhat different from the others, and suspected outlines (long black arrow) of sclerites besideACCEPTED some sclerites may be indicative of moulting. Mode of feeding. In most specimens of T. hexagonus, the intestine is completely flat and preserved in dark colour, whereas in one specimen, the alimentary canal is preserved three- dimensionally (Fig. 1d-e, 2c, 2d). Three-dimensional gut structures found in Burgess Shale-type arthropods have been suggested to be a taphonomic artefact either due to sediment infilling caused by panic behaviour during live burial (Edgecombe and Ramsköld 1999) or combined early diagenetic Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

mineralization and severe weathering processes (Butterfield 2002; Vannier and Chen 2002).

However, more recent studies rejected these scenarios in Chengjiang fossil preservation, as geochemical analyses show that these three-dimensional gut contents consist of genuine sediment with no evidence of mineralisation (Hou et al. 2004; Bergström et al. 2007; Ma et al. 2014a). In the case of Chengjiang vermiform taxa, the three-dimensional structures often only occur partially in the gut, and hence they are unlikely to be the result of sediment infill during live burial, but instead are indicative of at least occasional sediment-digestion (Hou et al. 2004; Bergström et al. 2007; Ma et al.

2014a). In contrast to the complex digestive systems seen in Cambrian arthropods (e.g., the presence of midgut glands/diverticulae), the palaeoscolecids and other Cambrian scalidophorans have a relatively simple alimentary tract (Vannier et al. 2014; Ortega-Hernández et al. 2018). However, the detailed alimentary tract morphologies and gut contents can shed light on their feeding ecology. The alimentary tract of most Cambrian scalidophoran worms is composed of a terminal mouth, muscular pharynx lined with teeth and an elongated gut, while some species even have a division of oesophagus, midgut and hindgut (Ma et al. 2014a). Diverse skeletal animal fragments and cololites have been reported in the gut tracts of the stem-group priapulans Ottoia prolifica from the Burgess

Shale (Vannier 2012), Singuuriqia simoni from Sirius Passet (Peel 2017), and Selkirkia sinica from Xiaoshiba (Lan et al. 2015), each instance interpretedMANUSCRIPT as indicative of a generalist feeding regime including predation and deposit feeding – as in modern day macropriapulans. The data at hand do not allow inferring whether T. hexagonus was a deposit-feeder or a carnivore, or both, as no individual gut elements could be confidently identified.

Conclusions

Tabelliscolex hexagonus is a palaeoscolecidomorph (stem-group Priapulida), belonging to the subgroup Cricocosmiidae, along with at least Cricocosmia jinningensis and Tylotites petiolaris. TabelliscolexACCEPTED is distinguished from other cricocosmiids by the configuration and morphology of its sclerites, and the correspondence of its well-developed ventral projections with the serially repeated sets of trunk sclerites compared to other taxa. Palaeoscolecidomorphs, and especially cricocosmiids, demonstrate a remarkable parallel evolution of ectodermal seriation that is strikingly similar to that of lobopodian panarthropods. Paired ventral projections that occur in a seriated pattern relative to the lateral/dorsolateral trunk sclerites are a topological mirror of the seriated trunk nodes/sclerites of Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

lobopodians, which are positioned in a dorsolateral position above the ventrolateral lobopodous appendages in taxa ranging from stem-group panarthropods to stem-group onychophorans and stem- group tardigrades.

Seriation is more pervasive in Ecdysozoa than just Panarthropoda and Palaeoscolecidomorpha.

Kinorhynchs have seriated cuticular structures (unpaired tergal plates and paired sternal plates) that are in some cases, notably in the well-studied genus Echinoderes, correlated internally with seriated components of the trunk musculature and the nervous system. These include paired sets of dorsal, ventral, dorsoventral and diagonal muscles (Herranz et al. 2014) and seriated transverse neurites and ganglia (Herranz et al. 2019). As such, seriation is manifest in both the ectoderm and mesoderm, and kinorhynchs exhibit covariation of seriated organ systems to a degree that they are not uncommonly described as segmented (but see Scholtz 2020 for a cogent distinction between seriation and segmentation). Seriation has multiple origins in Ecdysozoa and its different instances involve different character systems. Kinorhynchs and arthropods shared striated tergites, sternites, ganglia and trunk muscles, whereas palaeoscolecidomorphs and lobopodians shared seriated dorsolateral sclerites and ventrolateral projections. Acknowledgements MANUSCRIPT We thank Javier Ortega-Hernández and another anonymous reviewer for constructive reviews that improved this manuscript.

Funding

Yunnan Provincial Research Grants (Grant Nos. 2015HA021, 2015HC029 and 2019DG050 for X-GH and X-YM) supported the Yunnan Key Laboratory for Palaeobiology group including fossil collecting, supporting students and research expenditure. NERC Independent Research Fellowship (Grant No. NE/L011751/1)ACCEPTED provided salary and research expenditure for X-YM. NERC GW4 + Doctoral Training Partnership provided stipend and research expenditure for RJH. The funding bodies played no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.

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Table 1. Details of new palaeoscolecidomorph material reported in this study.

Taxon Affinity Total With ventral Localities

projections

Tabelliscolex hexagonus Cricocosmiidae 4 specimens 4 specimens Haikou,

Anning

Cricocosmia jinningensis Cricocosmiidae 7131 115 specimens Haikou

specimens

Mafangscolex yunnanensis Palaeoscolecida 703 21 specimens Haikou

specimens

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Table 2. Support for palaeoscolecidomorph clades across phylogenetic optimality criteria. JK

= jack-knife resampling, SR = symmetric resampling, BS = bootstrapping, PP = posterior probability,

X = node was not resolved. Note that each method of clade support assessment is methodologically distinct, and scores should not be interpreted together all on a single scale. E.g., a low score for jack- knife/symmetric resampling indicates that few characters support that clade, whereas a low score for bootstrapping indicates the clade is recovered fewer times when the original analysis was repeated many times.

Clade Equal weights Implied weights Maximum Bayesian

parsimony parsimony likelihood inference

Palaeoscolecidomorpha JK = 7% SR = 13% BS = 63% X

Palaeoscolecida JK = 36% SR = 38% BS = 93% PP = 0.63

Cricocosmiidae JK = 67% SR = 70% BS = 99% X

Table 3. Comparison of key characters in Palaeoscolecidomorpha. Taxa included known from high quality proboscis, trunk and posterior hook data.

Taxon Age & Affinity Ventr Seria Tessellat Bilateral Rings of Poste Coll localit al ted ing posterio introvert rior ar y struct trunk micropla r spines/sc intro spi ures scler te hooks/s alids(?) vert nes ites MANUSCRIPTornamen pines unar tation med Tabellisc Camb Cricocos Yes Yes No Yes (1 Multiple Yes Yes olex rian miidae pair) rings hexagon Stage Hou et al. us 3, 1999 Han, Yunna Zhang n, and Shu, China 2003a Cricocos Camb Cricocos Yes Yes No Yes (1 Multiple Yes No mia rian miidae pair) rings jinningen Stage Hou et al. sis 3, 1999 Hou and Yunna Sun, n, 1988 China TylotitesACCEPTED Camb Cricocos ? Yes No Yes (1 Multiple Yes No petiolaris rian miidae pair) rings Luo et Stage Hou et al. al. 1999 3, 1999 Yunna n, China Mafangs Camb Palaeosc Yes No Yes Yes (1 Multiple No No colex rian olecida pair) rings sinensis Stage Conway Hou and 3, Morris Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Sun, Yunna and 1988 n, Robinson, China 1986 Scathas Wuliu Palaeosc ? No Yes Yes (2 Single ring No No colex an, olecida pairs) minor BC, Conway Smith, Canad Morris 2015 a and Robinson, 1986 Wronasc Camb Palaeosc ? No Yes Yes (1 ? No ? olex rian olecida triplet) antiquus Stage Conway Glaessn 4, S. Morris er, 1979 Austra and lia Robinson, 1986 Maotians Camb ? ? No No Yes (1 Multiple Yes No hania rian pair) rings cylindric Stage a 3, Sun and Yunna Hou, n, 1987 China Markueli Camb ? No No No Yes (3 Multiple Yes? ? a rian pairs) rings Valkov, (Siberi 1983 a, (5 China, species) Austra lia) to Ordovi cian MANUSCRIPT (USA) Figure 1. Four new specimens of Tabelliscolex hexagonus from the Chengjiang Biota. (a)-(c) complete specimen YKLP11428. (a) Part, inset shows that sclerites occur every second annulation;

(b) Counterpart, showing the whole body, with inset showing sclerites on every second annulation, the boundaries of which are indicated by black arrows; (c) Close-up of the posterior terminal hook. (d)

Part of YKLP11429, inset shows that sclerites occur on every second annulation, with sclerite-bearing annulations narrower than non-sclerite bearing ones; (e) Counterpart of specimen YKLP11429, black arrows and close-up inset indicating possible moulting. (f) Specimen YKLP11430, showing that the cuticleACCEPTED is more perishable than the sclerites. (g), (h) A complete specimen YKLP11431. (g) Part, the introvert and anterior section of the trunk were compressed and deformed. (h) Counterpart. Scale bars = 5 mm in (a), (b), (d), (e), (g), (h), 1 mm in (g), 500 μm in (c).

Figure 2. Camera lucida drawings of Tabelliscolex hexagonus. (a) Part and (b) counterpart of specimen YKLP 11428. (c) counterpart and (d) part of specimen YKLP 11429. An = annulation Dr = Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

degradation residues?, Gu = gut, Is = introvert scalids, Ph = pharynx, Sc = sclerites, Th = terminal hook, Vp = ventral projections. Scale bars = 6 mm in (a), (b), 5 mm in (c), (d).

Figure 3. Proboscis of Tabelliscolex hexagonus. (a) Proboscis of the part of YKLP 11428. (b)

Camera lucida drawing of part. The proboscis is subdivided into three sections: introvert, collar and pharynx. Three zones can be identified along the everted pharynx, according to the variation of width and pharyngeal teeth. Cs, Collar spines; Pt, pharyngeal teeth; Is, Introvert scalids. Scale bar = 1 mm.

Figure 4. SEM images of trunk sclerites. (a)-(c) Tabelliscolex hexagonus YKLP 11429. (a) Three rows of sclerites along the trunk, two lateral and one dorsal. (b) Close-up of left white box in (a), showing the outer surface of sclerite with many tubercles. (c) Close-up of right white box in (a), showing the inner surface of sclerite with tiny pits. (d)-(f) Cricocosmia jinningensis YKLP 11432. (d)

Two rows of lateral sclerites of Cricocosmia. (e) Close-up of left white box in (d), showing the outer surface of sclerite with many tiny tubercles and a central spine. (f) Close-up of right box in (d), showing the inner surface of sclerite with tiny pits. (g), (h) The lobopodian Microdictyon sinicum YKLP

11433. (g) Sclerite of Microdictyon. (h) Close-up of white box in (g), showing the nodes surrounding the holes. (i) Sclerite of Onychodictyon ferox YKLP 11434. Mg, Margin of sclerite. Scale bars = 1 mm in (a), (i); 500 μm in (b), (d), (g); 400 μm in (c), (h);MANUSCRIPT 200 μm in (e), (f). Figure 5. Ventral projections. (a)-(c) Mafangscolex yunnanensis. (a) YKLP 11435, the white arrows indicate laterally compressed ventral projections, and the black arrows show the vertically compressed ventral projections. (b) YKLP 11436, close-up of laterally compressed ventral projections, occurring every two annulations and perpendicular to the body. (c) YKLP 11437, close-up of vertically compressed ventral projections, preserved as nodes. (d)-(f) Cricocosmia jinningensis. (d) YKLP

11438, white arrows indicate paired ventral projections corresponding to each pair of sclerites. (e)

YKLP 11439, close-up of laterally compressed ventral projections, showing spine shapes with the tips pointingACCEPTED posteriorly (white arrows). (f) YKLP 11440, close-up of a pair of laterally compressed ventral projections in a ventrally preserved specimen. (g)-(i) Tabelliscolex hexagonus, YKLP 11428. (g) region of the trunk, showing that paired ventral projections are distributed repeatedly every two/three set of sclerites. (h) Close-up of white box in (g), showing a pair of vertically compressed ventral projections. (i) A laterally compressed ventral projection (white arrows), revealing its spine-shaped profile. Scale bars = 1mm in (a), (g); 500 μm in (c), (d); 250 μm in (h), (i); 100 μm in (b), (e), (f). Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

Figure 6. 3D detail of ventral projections of Tabelliscolex hexagonus (YKLP 11428). Paired vertically compressed ventral projections were imaged with a 3D Imaging Microscope (see methods) to reveal their topological information. (a)-(c) One ventral projection preserved as a convex node, with total height of 80.9 μm. (d)-(f) The other ventral projection preserved as a concave depression, with total depth of 84.0 μm. (g)-(i) Comparison between the paired ventral projections, with one convex and one concave. Scale bars = 50 μm.

Figure 7. Phylogenetic trees. (a) Full results of maximum likelihood tree search, with bootstrap support values. Full results for parsimony and Bayesian methods can be found in the Supplementary

Material (Figs. S1-3). (b) Summary tree of nodes consistently retrieved by parsimony tree searches, showing the relative position of Palaeoscolecidomorpha within stem group Priapulida. As equal weights failed to resolve the branching order or stem-group Priapulida, the position of Eopriapulites from implied weights is also shown (c) Optimization of character acquisitions in

Palaeoscolecidomorpha using WinClada for equal weights parsimony. 1 = posterior terminal hooks. 2

= cuticle surface with ornament of tessellating polygons. 3 = Zone 1 armature comprises fewer than 3 rings. 4 = two pairs of posterior hooks. 5 = triplet of posterior hooks. 6 = unarmed posterior introvert. 7

= absence of coronal spines. 8 = three pairs of posterior hooks. 9 = Serially repeated epidermal specializations (sclerites). 10 = Zone II armed. 11MANUSCRIPT = 3:1 relationship of sclerites to ventral projections.

12 = acute distal termination to sclerites. 13 = sclerites in complete transverse rings. 14 = sclerites taller than wide (i.e., spines).

Figure 8. Reconstructions of Cambrian ecdysozoans with seriated trunk structures. Seriated ventral/ventrolateral structures are shown in red, and seriated lateral structures in blue. (a)-(d) show animals in lateral view. (a) Mafangscolex, exhibiting paired spine-like ventral projections perpendicular to the body wall, located in the middle of every second annulation, with annulations demarcated by rows of tessellating plates/microplates. (b) Cricocosmia, exhibiting paired curved ventral projections facing ACCEPTED posteriorly, located on every annulation – corresponding to a pair of lateral sclerites. (c)

Tabelliscolex, exhibiting paired curved ventral projections facing posteriorly, corresponding to the annulation between every third set of sclerites. (d) Microdictyon, exhibiting nine ventrolateral lobopod pairs corresponding to nine lateral sclerite pairs, with an additional 10th posterior pair of lobopods not associated with a sclerite pair. (e) shows the same four animals, respectively from top to bottom, in transverse cross section. Downloaded from http://jgs.lyellcollection.org/ by guest on September 28, 2021

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