Preservation
The present study is based on two species, Selkirkia sinica (Figs. 1, 2, 3, 4 and 5) and S. transita sp. nov. (Figs. 6 and 7), both from the early Cambrian of China (see below systematic descriptions), that usually occur as isolated specimens and more rarely in relatively large concentration. Clusters of about four specimens (S. sinica) per cm2 have been found on single bedding planes (e.g. Ercaicun; Additional file 1: Figure S1) and usually show directional polarity, suggesting that individuals were re-oriented by currents. The preservation of Selkirkia is identical to that of other associated fossils. Elemental mapping indicates the presence of Al, Si, K, C, Fe, F, P and Mg in the matrix and infilled internal structures (here, intestinal; Figs 1i, 2f and 4g, k, m), suggesting the presence of a magnesium aluminosilicate clay minerals. By contrast, fossil structures reveal concentrations of Fe, as a component of either oxidized pyrite or metamorphic clay phases (Figs. 2f, 3g and 4g, k, m). Traces of C are found on the surface of fossilized tissues (here, gut and oocyte wall; Fig. 2f), as is expected from Burgess Shale-type (BST) preservation [30]. Most anatomical features are underlined by iron oxides which represent the weathered form of original pyrite microcrystals (pseudomorphosis [31]). For example, iron oxides faithfully replicate fine details of the introvert ornament (scalids, teeth) and occur in relatively large abundance over the external surface of the tube and within its wall structure (Fig. 3g).
Numerous fossil specimens show extensive eversion of the whole pharyngeal structure (see also Figs. 1a–c and 2a, d). This extreme state is not seen in living priapulids worms [3, 9] and may result from stress behaviour immediately after death. About 40% to 50% specimens of S. sinica and S. transita sp. nov. from the Chengjiang Lagerstätte have preserved soft parts. By comparison, S. columbia from the Burgess Shale (Canada) is overwhelmingly represented by vacant tubes [4].
Systematic palaeontology
Phylum Priapulida Delage and Hérouard, 1897
Order Selkirkiimorpha Adrianov & Malakhov, 1995
Family Selkirkiidae Conway Morris, 1977
Remark
This family currently accommodates two genera: Selkirkia [13] and Sullulika from the early Cambrian Sirius Passet Lagerstätte [27]. However, Sullulika is an ill-defined genus, based on empty annulated tubes comparable with those of Chinese selkirkiids and the supposed presence of convex lappets around the large aperture of the tube [27], that do not occur in any other selkirkiids. The lack of information on its soft anatomy and uncertainties concerning these lappets makes the tentative placement of Sullulika within Selkirkiidae uncertain. For these reasons, we find it premature to propose a diagnosis for this family.
Type genus: Selkirkia Walcott, 1911.
Genus included: Sullulika [27].
Genus Selkirkia Walcott, 1911
History of research
Selkirkia was first erected by Walcott [13] but it was not until 1977 that its anatomy and phylogenetic position were analysed in detail based on the extensive revision of a large number of exceptionally preserved specimens of S. columbia from the Burgess Shale [4]. Two decades later, similar fossils were found in the Chengjiang Lagerstätte. Luo and colleagues assigned them to Selkirkia sinica [18] and Hou and colleagues to Paraselkirkia jinningensis the same year [17]. These tubicolous worms were later discovered in the Xiaoshiba Lagerstätte (Hongjingshao Formation, equivalent of Cambrian Series 2, Stage 3) and assigned to Selkirkia sinica [19]. Paraselkirkia was erected based on differences with Selkirkia columbia from the Burgess Shale (Additional file 1: Table S1), such as the relatively smaller size and more complex introvert structure of P. jinningensis [17] (Additional file 1: Text S2) [3, 4]. However, the present systematic revision reveals no major morphological differences between Selkirkia columbia and the Chengjiang selkirkiids that would justify maintaining two genera. To us, Paraselkirkia is a younger synonym of Selkirkia [32, 33] and S. sinica and Selkirkia transita sp. nov. both belong to Selkirkia.
Type species: Selkirkia columbia Conway Morris 1977.
Species included: S. spencei and S. willoughbyi Conway Morris and Robison, 1986, S. sinica Luo et al. 1999 and Hou et al. 1999, S. transita sp. nov.
Diagnosis (emended from Conway Morris, 1977). Selkirkiid worm (body length between 3 mm and 75 mm). Body divided into a spinose introvert and a trunk entirely covered with a tube. Conical introvert with three distinct zones (I to III from proximal to distal). Zone I with two ornamented subzones (Ia and Ic) separated by a smooth ring-like area (Subzone Ib). Fully everted specimens showing more or less developed, smooth area (Zone II) followed by elongated pharynx (Zone III) bearing multispinose or spinose teeth. Finely-annulated tube with low-angle conical shape, and openings at both ends.
Selkirkia sinica Lou et al., 1999
1999 Selkirkia sinica, Luo et al., p. 81–82, Pl. 20, Figs. 4–6; Text-Fig. 30.
1999 Paraselkirkia jinningensis, Hou et al., p. 63–64, Figs. 73–76.
2002 Selkirkia sinica, Chen et al., p. 195, Plate 18, Figs. 4-5.
2004 Paraselkirkia jinningensis, Hou et al., p. 71, Figs. 12.4–12.5.
2004 Paraselkirkia sinica, in Chen 2004., p. 180-181, Figs. 271–272.
2015 Selkirkia sinica, Lan et al., p. 125–132, Figs. 1-5.
2017 Paraselkirkia sinica, in Hou et al. 2017, p.120–121, Fig. 17.5
2021 Paraselkirkia sinica, in Yang et al. 2021, p.3–4, Figs. 1, 2.
Stratigraphy and locality. Yu’anshan Formation (equivalent of Cambrian Series 2 Stage 3), Eoredlichia-Wudingaspis zone, Chengjiang Lagerstätte, Yunnan Province, China; Hongjingshao Formation (equivalent of Cambrian Series 2 Stage 3), Yunnanocephalus–Chengjiangaspis-Hongshiyanaspis zone, Xiaoshiba Lagerstätte, Yunnan Province, China.
Diagnosis (emended from Luo et al., 1999 and Hou et al., 1999). Selkirkia with a relatively small size (< 20 mm long on average). Subzone Ia with spines arranged in irregular quincunxes. Subzone Ib well-developed. Subzone Ic with spines arranged in dense and regular quincunxes. Pharynx bearing numerous tiny, evenly spaced spinose teeth. Trunk possibly extending posteriorly into two lobe-like caudal appendages. Oocytes are concentrated within the posterior half of the body cavity on either side of the gut. Tube bearing evenly spaced annulations (8 to 14 per mm).
Description
Introvert
Fully everted introverts show three distinct spinose (Ia, Ic and III) and two smooth areas (Ib and II) (Figs. 1a, b and 2a). The length of the best-preserved fully everted introvert is about 25% of total body length. Zone I is subdivided into three subzones termed Ia, Ib and Ic from proximal to distal. The boundary between the Zone I and Zone III (pharynx) is marked by a smooth, slightly constricted area (Zone II).
Subzone Ia (Figs. 1a, b and 2a). Its diameter is equal or slightly smaller than that of the anterior opening of the tube from which it protrudes. Its numerous stout spines are distributed in discrete quincunx, directed backwards and decrease in size posteriorly. They form six circles and 12~13 longitudinal rows in one side.
Subzone Ib (Figs. 1a, b and 2a). This smooth area marks the boundary between Subzone Ia and Subzone Ic. Its length is one-third to one-fourth that of Subzone Ia.
Subzone Ic (Figs. 1a, b and 2a). Characterized by relatively closely spaced, slender and long spines (N=ca 250) arranged in quincunx and directed backwards.
Zone II (Figs. 1a, b and 2a): This smooth area lies between Zone I and Zone III.
Zone III (Figs. 1a, b and 2a). This ornamented zone represents the pharynx. Its morphology and length vary depending on the degree of eversion (Figs. 1a–c and 2a, d). In fully everted specimens, the pharynx appears as a slender conical structure bearing a large number (N > 200) of tiny teeth, all of virtually the same size (Additional file 1: Fig. S2b). They arrange quincunxially as seen in the best-preserved specimens. Their morphology is simple (Additional file 1: Fig. S2c, f) compared with that of the multi-cuspidate teeth seen in S. columbia ([4, 21]; Additional file 1: Fig. S2i, j). Some of our specimens show a bulbous structure at the distal end of the pharynx (Figs. 1a and 2a).
Trunk
A single specimen shows the body of the animal almost completely pulled out from its tube (Fig. 5o, p). The body is almost as long as the tube. Although faint traces of the gut are discernible, the external boundary of the trunk remains relatively featureless but with weak annulations (Additional file 1: Fig. S3b, c, e, f). It does not seem to have suffered from severe decay as indicated by its consistent cylindrical shape and a relatively well-preserved introvert. In other specimens, the distal part of the trunk protrudes outside the large opening of the tube in different lengths allowing the trunk cuticle to be observed (Figs. 1a, 2a, 3e, 4a and 5; Additional file 1: Fig. S3). It is represented by a 20~30 μm thick reddish layer (Figs. 3f and 5b, d, l, n) approximately one third of the tube wall (ca. 100 μm; measured in empty tubes; Fig. 3b, d, g). Small, irregularly spaced protuberances are present around the protruding trunk (Fig. 5f) and recall the papillae seen in the middle part of the trunk of S. columbia [4]. However, they do not distribute in rows and may not have a biological origin. Some specimens show a tiny gap between the tube and the external surface of the trunk (Fig. 5d, f, h, k), which suggests that the body was free from the tube and could possibly move within it. In the vast majority of individuals, the trunk is some distance away from the internal surface of the tube (e.g. posteriorly) although in contact with it locally (Fig. 1d, e). The exact morphology of the trunk end is unclear but may have born a pair of relatively short caudal appendages that can be seen protruding outside the small opening of the tube (Fig. 1g, h) or in a more retracted position (Fig. 1e, f).
Gut
The gut generally appears as a dark strip running from the everted pharynx to the trunk end (Figs. 1, 2, 3 and 4). The anus seems to open close to the posterior opening of the tube (Fig. 1f) and within the axial plane of the animal. The gut has an axial position and a straight outline although loops can be seen in some specimens (Fig. 2a, b). It is often filled with pellet-like elements aligned in rows or more irregularly distributed (Fig. 2c–e). These gut contents have a consistent ovoid shape (0.5~0.6 mm and 0.15–0.20 mm in length and width, respectively) and present a slight relief. Their relatively large number and local concentration suggest that the gut wall had the capacity to expand in order to accommodate food or undigested residues [19, 34]. Scanning electron microscope (SEM) observations revealed no internal elements that may help characterize the worm diet. Two ovoid features of equal size occur near the posterior end of one specimen (Fig. 2c) and seem to be aligned within the gut tract. However, their size (1.5 mm long, 0.9 mm wide) is larger than that of the pellet-like elements usually found in a more anterior location within the gut. Their width is almost three times the gut diameter (0.5 mm in width). They may represent undigested shells (e.g. brachiopod), or unusually large pellets transiting through the digestive tract (Fig. 2e). Possible faeces seen in a few specimens consist of shapeless coloured clusters of minerals such as mica and quartz grains (Fig. 4n, o).
Tube
The tube forms a conical structure with a very low opening angle (ca 20°) and opens at both ends. Its length ranges from 4 to 16 mm with the proximal opening being 1 to 2 mm wide (N = 120, see Additional file 2: Excel S1). The external surface of the tube is regularly annulated with 8 to 14 tiny transverse ridges per millimetre (Fig. 1b) that faithfully correspond to comparable small furrows along the internal surface. The tube wall (ca. 100 μm; Fig. 3b, d, g) is preserved in iron oxide (Fig. 3g) and does not show any ultrastructural details (e.g. cuticle layers). The posterior opening of the tube is relatively narrow and often broken (irregular margins) due to possible interactions with sediment or transportation (Fig. 4a). Tiny ovoid, relatively featureless objects (size between 1 and 1.5 mm; Fig. 4b–n) occur in about 810 specimens (ca. 45%), that seem to be attached to the external wall of the tube, close to its posterior end. A comparable association recently described in Selkirkia from the slightly younger Xiaoshiba Lagerstätte [35], provides clear evidence that these rounded objects are actually brachiopod epibionts.
Remarks
Our specimens do not show noticeable differences with the type specimens of S. sinica figured by Luo et al. (1999; plate 20, text-figs 4–6) and Hou et al. (1999; text-figs 73–76) and those more recently described from the Xiaoshiba Lagerstätte [19]. All display the same type of everted spinose introvert divided into two parts and elongated pharynx bearing teeth. Detailed observations of the arrangement, density and morphology of cuticular elements (scalids, pharyngeal teeth) did not reveal small-scale differences either (Additional file 1: Table S1).
Selkirkia transita sp. nov.
Etymology. From transita (Latin), alluding to resemblances with both S. sinica and S. columbia.
Holotype: ELI-0000601 (Fig. 6a)
Paratype: ELI-0000602, ELI-0000603 (Fig. 6f, i)
Stratigraphy and locality. Yu’anshan Formation (equivalent of Cambrian Series 2 Stage 3), Eoredlichia-Wudingaspis zone, Chengjiang Lagerstätte, Yunnan Province, China.
Diagnosis. Selkirkia with a relatively large size (> 20 mm on average). Subzone Ia with spines arranged in discrete quincunxes. Subzone Ib very narrow to virtually invisible. Subzone Ic with spines in dense and regular quincunxes. Pharynx bearing hundreds of multispinose teeth arranged quincunxially, pointing forwards and decreasing in size distally. Oocytes in the posterior half of the body cavity and possibly arranged in two longitudinal rows. Tube bearing evenly spaced external annulations (5 to 9 per mm).
Description
Introvert
Subzone Ia (Fig. 6a–d). It is the widest part of introvert. Spines distribute in discrete quincunxes with 6 circlets of about 25 spines arranged longitudinally. The distance between adjacent spines in diagonal is about 0.15 mm. Spines increase in size from proximal to distal part. The shortest and longest spines are 0.12 mm and 0.27 mm long (basis 0.10 mm and 0.12 mm), respectively.
Subzone Ib (Fig. 6d). Relatively narrow (ca 0.2 mm in longitudinal length), smooth and marks the boundary between Subzone Ia and Subzone Ic.
Subzone Ic (Fig. 6d). Slightly narrower than Subzone Ia and varies in length from 1.0 mm to 2.3 mm. Subzone Ic bears closely packed spines arranged in quincunx. Spines increase in size gradually from proximal to distal (the shortest and longest ones are 0.10 mm and 0.34 mm, respectively) and all point backwards. The basal width of spines is about 0.07 mm. Distal spines are three times longer than proximal ones.
Zone II (Fig. 6e). In specimens with a fully everted introvert, its length is 25% that of Subzone Ic and its diameter decreases distally.
Zone III (Fig. 6e). Corresponds to the pharynx and has a tapering shape especially well-marked in its terminal region. Bears numerous teeth arranged quincunxially and regularly decreasing in size distally. The most proximal ones (basal width between 0.05 and 0.11 mm) are clearly multispinose with one long central tip flanked with two smaller ones (Fig. 6e; Additional file 1: Fig. S2d, e, g). The remaining spines appear as densely coloured dots of ca 0.02 mm in diameter with no visible details and point forwards.
Trunk
In one specimen, the broken part of the tube reveals details of the relation between trunk and tube (Fig. 6f) with a gap between the cuticle and the internal surface of the tube (Fig. 6f). Possibly paired retractor muscles (Fig. 6h) seem to be attached to the middle part of the trunk.
Tube
The conical tube of Selkirkia transita sp. nov. is 10 to 38 mm long (see Additional file 2: Excel S1) and opens at both ends (Fig. 7a, d). As in Selkirkia sinica, it bears tiny low-elevated transverse ridges along its external surface that correspond to furrows along its internal wall (Fig. 7a–c). Most ridges are regularly spaced (commonly 5 per mm) although local variations (from 5 to 9 per mm) may occur. Some ridges seem to fuse or bifurcate (Fig. 7d, e). Although often broken and damaged the posterior end of the tube seems to have a sharp ovoid opening with no additional cuticular features.
Gut
As seen in S. sinica, it appears as a narrow tube running from the tip of introvert to the anus and often contains pellets (Fig. 6f).
Reproductive system
Clusters of spherical elements (Nmax = 14, diameter between 270 and 480 μm) found in the posterior part of the body cavity of Selkirkia transita sp. nov. and are highlighted by reddish iron oxides and small black patches (Fig. 7f, g). They form either a single cluster close to the inner wall of the body or irregular paired longitudinal rows on either side of the gut (Fig. 7g). Similar features (less than 30; diameter between 300 and 450 μm) occur in a few specimens of Selkirkia sinica from the Xiaoshiba Lagerstätte and were recently interpreted as oocytes based on comparisons with extant priapulid worms [35].
Remarks
S. transita sp. nov. has an introvert and a stiff tube, that closely resemble those of Selkirkia columbia [4] but shows differences with other Selkirkia formerly described in the literature. In general, the size of S. transita sp. nov. is larger than that of S. sinica but smaller than that of S. columbia [4, 17, 18]. Subzone Ib of the introvert is poorly developed in S. transita sp. nov., absent in S. columbia and instead well-marked in S. sinica. The most anterior spines of Subzone Ic point either outwards or backwards in S. transita sp. nov. and S. sinica, but always forwards in S. columbia. Concerning the pharyngeal region, only one type of multispinose teeth part, is present in S. transita sp. nov., whereas two types occur in S. columbia [21]. In contrast, S. sinica seems only to bear simple teeth. The tube bears about 40 annulations per mm in S. columbia, 8 to 14 in S. sinica and only 5 to 9 in S. transita sp. nov. The morphological differences between S. sinica and S. transita sp. nov. were all observed in specimens of approximately the same size, which rules out the possibility that both forms represent ontogenetic stages of a single species. Altogether, this whole set of morphological divergence justifies the erection of a new species (Additional file 1: Table S1).
Relation of tube to body
In both species, the tube wall (ca. 100 μm; Fig. 3b, d, g) is preserved in iron oxide (Fig. 3g) and does not reveal any ultrastructural details (e.g. cuticle layers). It is about three times thicker than the cuticle that covers other parts of the worm’s body (ca. 20 to 30 μm; Fig. 3f). The introvert is frequently the only part of the worm that protrudes outside the anterior opening of the tube and displays varying degrees of extension (from contracted to fully everted state showing pharynx). However, in numerous specimens a substantial portion of the trunk stands exposed outside the trunk (Figs. 1a, 2a, 3e, 4a and 5; Additional file 1: Fig. S3) suggesting that the worm could move freely within its tube. The lack of a large gap between the trunk and the inner surface of the tube indicates that the worm could probably slide along its protective encasement.
In one specimen of S. sinica the worm’s body is almost completely pulled out from its tube (Fig. 5o, p). Although extremely slim, its trunk displays regular outlines. This configuration may represent the worm in the process of extricating itself from the tube (Fig. 5o, p).
Phylogenetic relation of Selkirkia to other scalidophoran worms
Our phylogenetic results differ considerably from those that were published before [10], owing to our extensive recoding and implementation of neomorphic/sovereign characters (see the ‘Methods’ section).
Heuristic search with Tree bisection and reconnection (TBR) only and Tree with new technology analyses (TNT) retrieve topologies with basal Loricifera and Kinorhyncha forming a sister clade to other scalidophorans, most fossils being part of the priapulid stem group, although internal branching is mostly polytomous (Additional file 1: Fig. S4a, b). By contrast, the unconstrained Bayesian runs converge on a basal Selkirkia sister group to Palaeoscolecida and Scalidophora (Additional file 1: Fig. S5). In the unweighted TreeSearch analysis (Additional file 1: Fig. S4c), Loricifera and Kinorhyncha are also resolved basally, but Selkirkia is retrieved in the closest sister clade to crown Priapulida, along an extensive fossil priapulid stem. When using implied weighting in TreeSearch, consensus trees for individual concavity constants are well resolved, but they significantly differ from one another, leading to a polytomy if one combines these results (Additional file 1: Fig. S4d). These topological discrepancies between concavity constants [36, 37] constitute an example that the method of implied weighting may indeed be too inconsistent despite good performances with simulated data [38].
A basal position of Loricifera and Kinorhyncha with a large total-group Priapulida being consistently recovered by parsimony analyses, we decided to test the strength of this model by enforcing a backbone on the Bayesian analysis. The resulting topology (Fig. 8) bears overall similarity with the unweighted TreeSearch topology in the basal placement of palaeoscolescids and yields on average slightly better harmonic means than the unconstrained model (− 1083.58 vs. − 1081.42, averaged each 5 searches; see Additional file 3: Excel S2). A better treatment of inapplicable states in TreeSearch therefore helped parsimony converge with Bayesian likelihood, assuming a model with basal Loricifera and Kinorhyncha. However, major scalidophoran nodes have abysmal posterior probabilities of 0.1 or below in the likelihood analysis (Fig. 8; Additional file 1: Fig. S5). We therefore consider the constrained Bayesian tree as the representative of the best evolutionary model for our data, but the stability of this model, and in particular the basal placement of palaeoscolecids in total-group Priapulida, will require further testing (Fig. 8). Selkirkia is otherwise consistently retrieved in both Bayesian analyses in a clade with Markuelia and Eokinorhynchus, suggesting an order-level grouping defined by the subdivisions of the Zone I of the introvert.
Selkirkia has long been assigned to priapulids based on overall similarities with extant priapulids, such as the introvert structure, pharyngeal teeth and trunk annulation [4, 11, 18, 39,40,41,42,43]. Other authors have considered the tube of Selkirkia has the equivalent and a possible homologue of the lorica of extant priapulid larvae and loriciferans [44], and advocated a position within scalidophorans without specifying to which group it may belong [28, 29, 44,45,46]. Selkirkia clearly differs from extant Kinorhyncha and Loricifera, which both are characterized by the presence of a mouth cone and oral stylets [47]. There are no such oral features in Selkirkia which instead has multispinose pharyngeal teeth arranged quincunxially as in numerous fossil and extant priapulid worms. Another major difference with these two groups is that the trunk of Selkirkia bears no external “segments” (the so-called zonites of kinorhynchs). Its finely annulated tube most probably secreted by the epidermal cells of the trunk (see below) has no equivalent in other groups.
Our favoured phylogenetic result (Fig. 8) implies that the Selkirkia clade (with Laojieela as its most basal member) might be part of the close stem of Priapulida, and could therefore document a morpho-anatomy lying close to the priapulid radiation. The priapulid affinities of Selkirkia are supported by the following set of morphological characters: (1) twenty-five longitudinal rows of scalids are found in Selkirkia and the vast majority of extant priapulids (except Meiopriapulus fijiensis); (2) the pharynx of Selkirkia is lined with multispinose teeth as in extant [26] and Cambrian [4] priapulids; and (3) the homology between the tube rings and the cuticular annulations seen in modern priapulids. However, the first trait is absent in most basal members of the putative Selkirkia clade (e.g. Eopriapulites, Laojieella) as well as Ottoia (Fig. 8), which would imply either broad convergences or that Selkirkia may lie even closer to crown priapulids.
In addition, the introvert of Selkirkia is subdivided in the same way as that of extant priapulids (i.e. Zone I, II, III) and clearly differs from that of kinorhynchs and loriciferans (Zone I, Zone II, and mouth cone). The possible paired caudal appendages of Selkirkia have counterparts in some extant (e.g. Priapulus [26]) and fossil (e.g. Paratubiluchus and see [26, 48]) priapulids.
In this scenario, the crown of Priapulida includes the Carboniferous Priapulites, the early Cambrian Paratubiluchus, Xiaoheiqingella, Sicyophorus, and Yunnanpriapulus. Ottoia and possibly Ancalagon are the closest priapulid stem taxa. Fieldia is here resolved at the basalmost position of total-group Priapulida.
Considering the weak support of higher nodes, the alternative topology based on an unconstrained Bayesian analysis which retrieves Selkirkia and palaeoscolecids as stem scalidophorans (Additional file 1: Fig. S5) should not be too hastily discarded. This scenario implies that many important priapulid apomorphies—as evidenced by Selkirkia—appeared very early in the evolution of the group, with kinorhynchs and loriciferans secondarily losing some of these traits (e.g. 25 longitudinal rows of scalids). This contrasts with the topology described above according to which the known Cambrian fossil radiation was mostly linked to the build-up of the priapulid body plan.
