Zhang et al.—Cambrian Fortunian scalidophoran worms in South China
densely spaced annuli, and each annulus is about 42 μm long. The annuli are counted to be about 72 for this fragment. There are no other ornaments or structures on the trunk part.
Materials.—One specimen, NIGP160493.
Measurements.—Specimen NIGP160493 is about 2.3 mm long and 830 μm wide. Each annulus is about 42 μm long (anterior– posterior).
Remarks.—Form C might be the trunk part of Eopriapulites sphinx Liu and Xiao in Liu et al., 2014, because they share a similarly unornamented trunk with densely spaced trunk annuli.
Form D Figure 7.1, 7.2
Occurrence.—Xinli section, Nanjiang County, northern Sichuan Province, SouthChina (Yang et al., 1983; Zhang et al., 2015). Small shelly fossils, Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone, Xinli Member, Dengying Formation, Cambrian Fortunian Stage (Steiner et al., 2007; Peng et al., 2012).
Description.—Part of the trunk is preserved as external mold, judged by the morphology of the only preserved large spinose sclerite and the preserved trunk annuli (Fig. 7.2) that have clear boundaries in between. The base of the large spinose sclerite is nearly circular in shape. Its outer edge is continuous into the annuli of the cuticle. The cuticle is irregularly annulated with a large number of annuli.
Materials.—One specimen, NIGP160443.
Measurements.—Specimen NIGP160443 is about 1.3mmlong, with the only large spinose sclerite about 200 μm long.
Remarks.—It is possible that FormDrepresents the trunk part of Form A because they share some similarities; for example, the trunk cuticle is densely annulated with a large number of annuli, and the large spinose sclerite is sparsely distributed. Due to the lack of completely preserved specimens, we tentatively adopt open nomenclature and treat them as different forms. Deduced from the size of the large spinose sclerite, Form D might have reached a body length of more than 2 mm.
Affinities
Scalidophoran worms possess, plesiomorphically, a sub- ectodermal double-layered body-wall musculature, i.e., made of longitudinal and circular layers (e.g., Brusca and Brusca, 2003; Nielsen, 2012). During locomotion, the contraction of the cir- cular muscles would result in trunk annuli; thus, the scalido- phoran worms might be superficially annulated. In the absence of cilia along the epidermal cell layer, locomotion is mainly achieved using the extrudable introvert and backwardly oriented scalids (as anchoring devices) to pull the trunk forward. From extant taxa, it is known that these scalids are internally hollow, with the cuticle limited to a thin outer covering (Merriman, 1981; Neuhaus et al., 1996, 1997; Neuhaus and Higgins, 2002).
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Nematoids autapomorphically lack circular muscles, and their ‘scalids,’ if developed (e.g., in larval nematomorphs and some nematodes), are solid and composed exclusively of cuticle (Schmidt-Rhaesa, 1998; Nielsen, 2012). Eokinorhynchus rarus has cuticular annulation and, most likely, internally hollow scalids, indicating a close relationship with Scalidophora. Eokinorhynchus rarus has heterogenous annulation
embodied by the bilaterally symmetrical arrangement of large spinose sclerites on different parts of the trunk and the randomly distributed small spines especially on the posteroventral side of the trunk. Moreover, E. rarus has 20 annuli, each annulus being comparatively long relative to trunk length. This type of annulation differs from the homonomous annulation of other Cambrian putative scalidophorans, for example, the long, macroscopic worm-like forms from the Chengjiang biota Chen, 2004), the likewise worm-like species of Markuelia, known only from individuals close to hatching (Dong et al., 2004; Haug et al., 2009) that might have as many as 60 or more annuli (Cheng et al., 2011), or Eopriapulites sphinx, which has a long vermiform trunk with more than 140 annuli (Shao et al., 2016). The annulation of the middle Cambrian Shergoldana australiensis Maas et al., 2007a is restricted to the neck region, while the hind body is characterized by hexagonal plates that are arranged in rings with each plate extending into a posteriorly oriented spine (Maas et al., 2007a). The annulation type of
E. rarus coincides with the macroannuli (= zonites) exempli- fied by kinorhynch worms (Budd, 2001; Neuhaus, 2013). E. rarus also has some other features that are similar to modern kinorhynchs, for example, each trunk zonite ornamented with densely sutured plates, trunk with small spines, and trunk end with caudal spines. Despite these similarities, E. rarus differs from modern kinorhynchs in many aspects. For example, E. rarus has a trunk with 20 macroannuli, whereas modern kinorhynchs have a trunk with a consistent 11 zonites. Each zonite of modern kinorhynchs has only two to four cuticular plates (dorsally and ventrally, no rings), while each annulus of E. rarus has as many as 10 to 40 small plates. The modern kinorhynchs have one pair of lateral terminal spines (relatively long) and one pair of lateral terminal accessory spines (relatively short), and all these spines are positioned lateral to the anus; some taxa bear one single midterminal spine that is 150%longer than the lateral terminal spines (Sørensen and Pardos, 2008), whereas the caudal spines of E. rarus are positioned ventrally to the anus and relatively short. This may indicate that E. rarus represents a stem member of the taxon Kinorhyncha, with the macroannuli as a key synapomorphy. Since the Kinorhyncha is represented by about 240 extant species, with no fossil species reported yet (Neuhaus, 2013), E. rarus would be the first and only fossil record of this cycloneuralian taxon. It has previously been argued that a long vermiform trunk
with a large number of annuli might be a plesiomorphic state inherited from the last common ancestor of Scalidophora, or even Cycloneuralia; thus, most Cambrian so-called priapulid- like worms might be assigned to total-group Cycloneuralia or total-group Scalidophora as a whole, but a further assignment to an exact taxon within Scalidophora is currently not possible (e.g., Maas et al., 2007b; Harvey et al., 2010;Wills et al., 2012). In addition, Eopriapulites sphinx from the lower Cambrian Zhangjiagou section was described as ‘priapulid-like’; however,
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