Figs. 2 and 6E, F

Materials. Four specimens.

Description. The cuticle displays a vermiform morphology, appearing either C-shaped (Figs. 2A, B, and 6E, F) or cylindrical (Fig. 2C and D) in cross-section depending on preservation state. Two types of ornamentation can be found on these specimens. The first ornamentation consists of broad, sinuous ridges measuring 52–279 µm in width (mean 105.7 µm, n=13), oriented obliquely at approximately 30° (Fig. 2E and F, arrows) to 90° (Fig. 6E and F, arrows) to the long axis. These ridges show irregular patterns, occasionally bifurcating or merging. The second ornamentation is composed of finer, more uniform striations (Fig. 2G–H and K, arrows) ranging from 4.8 to 6.2 µm in width (mean 5.2  µm, n=20), which generally follow the orientation of the ridges but display subtle undulations, bifurcations (Fig. 2G–H and K, arrows), and localized transitions to weak polygonal patterns (Fig. 2L, arrow) with long axes measuring 4.4–9.8 µm (mean 6.5 µm, n=8). A distinctive feature of Form A is the presence of smaller-diameter tubular branches extending laterally from the main cylindrical body (Fig. 2I and J). The external surface shows minimal taphonomic deformation, with ridge patterns faintly visible on the internal surface while striations remain restricted to the exterior. All of these branches are incompletely preserved, measuring 279 µm in length and 118 µm in width for the specimen in Fig. 2A and B, which has only one branch on the left, and 72 and 132 µm in length and 133 and 116 µm in width for the specimen in Fig. 2C and D, which displays two branches. Form A is notably characterized by the complete absence of spines, tubercles, or sclerites, distinguishing it from most other cuticular types described in this study. Furthermore, some cuticle specimens show no fine striations or branching patterns. Nevertheless, they can be readily attributed to this form based on their distinctive C-shaped contours and broad, undulating ridges, spaced 552–654 µm apart (mean 594.4 µm, n=4), which also run obliquely across the surface of the cuticles. The maximal size of this form is about 3.1 mm (Fig. 7; Table S2).

Comparisons. While the broad ridges superficially resemble the transverse annulations seen in some worm-like organisms, their oblique orientation (approximately 30° to the long axis) and irregular patterns argue against their interpretation as true annulation or segmentation. Similar ridge patterns have been described in other cuticular fossils (see figs. 3A and 5B in Zhang et al., 2021). The finer striations, however, likely represent primary biological structures. The surface ornamentation, characterized by fine parallel wavy striations, roughly resembles cuticular structures observed in Yicaris dianensis (Zhang et al., 2007), the earliest reported crown-group crustacean discovered in the Yu'anshan Formation (Cambrian Series 2, Stage 3) of Yunnan, China, although its drop-shaped pores (Zhang et al., 2007, 2021) are absent in our specimens. Comparable micro-ornamentation also occurs in cnidarian periderms and some earliest ecdysozoan cuticles, including the W-shaped striae on the cuticle of Saccorhytus. The localized development of the polygonal pattern in our specimens may tentatively support an ecdysozoan affinity, although this interpretation requires further evidence. The laterally positioned tubular structures resemble appendages of some panarthropods (see Zhang et al., 2007). More detailed discussion can be found in Sect. 6.2.

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.

Form B

Fig. 3

Materials. One specimen.

Description. This fragmented specimen represents a compressed trunk segment, with its original three-dimensional morphology significantly altered during preservation (Fig. 3A and B). Notably, however, the internal body cavity maintains sufficient structural integrity to allow clear observation (Fig. 3A). Segmentation of the trunk is evident, allowing for the identification of three individual annuli or zonites (Fig. 3A and B). The segments are subequal in length (ca. 1.13 mm). Pronounced constrictions occur at the segment boundaries, affecting both the external and internal morphology (Fig. 3A and B). The specimen exhibits a uniform morphology across its circumference, with no discernible differentiation between dorsal and ventral sides (Fig. 3A and B). Dense ornamentation of spines, ranging in length from approximately 100 to 190 µm (mean 138 µm, n=12), adorns the trunk surface. The spines are conical to blade-like in shape, with their apices curved slightly outward. Furthermore, they exhibit a weak preferential orientation along the trunk's longitudinal axis, possibly aligned toward the inferred posterior direction (Fig. 3C–I, arrows). Spine morphology is consistent across both sides of the fossil, with no discernible difference in form. The cuticle exhibits a well-defined sculpturing of closely packed polygonal units. The maximal size of the polygonal ornaments is about 3.6 to 5.4  µm (mean 4.7 µm, n=17). Each polygon features a central concavity encircled by a faintly elevated rim, with adjacent units sharing these raised margins (Fig. 3G and H). The polygonal ornamentation of the cuticle is particularly well defined around the spine bases, creating a consistent ring that demarcates their points of attachment (Fig. 3G and H, arrows).

The specimen is characterized by lateral branch-like structures (Fig. 3A, B, E, F, I, and J). Although preserved on only one side, their morphology suggests an original bilateral symmetry. Each branch originates from the presumed posterior margin of its segment. The most completely preserved branch is about 1 mm in length, extends laterally at approximately 18° to the trunk axis, and appears oriented posteriorly. This orientation is consistent with the general alignment of the trunk spines (Fig. 3A, B). The branch is tubular, hollow, and circular in cross-section. It tapers distally from a broader proximal region near the trunk (Fig. 3E, F, I, and J, arrows). The incompletely preserved distal extremity suggests a truncate-conical termination, which may have borne at least one terminal spine, now represented only by its basal remnant (Fig. 3J, arrow). An examination reveals no apparent dorsoventral asymmetry on the branch as well. A second branch on the same side is preserved only at its base; its orientation appears consistent with the more complete branch (Fig. 3E and F, arrows). Spines on the branch match those of the trunk in morphology; however, their distribution shows a pronounced proximal-to-distal gradient, with density gradually declining until they are absent near the tip (Fig. 3I and J). In addition to the spines, the branch cuticle shows a corresponding change in ornamentation. Proximally, it displays polygonal reticulation, which transitions distally into fine, closely spaced longitudinal striations that extend almost to the tip (Fig. 3J). The size of this form is about 2.7 mm (Fig. 7; Table S2).

Comparisons. Form A and Form B are morphologically distinct beyond their shared feature of symmetrical lateral branches. Form B displays clear, well-defined segmentation with boundaries perpendicular to the longitudinal axis, while segmentation in Form A is ambiguous and any divisions are irregularly oriented. The trunk surfaces differ fundamentally. Form A entirely lacks spines and shows a pattern of bifurcating ridges, whereas Form B is densely spinose with a polygonal reticulate ornament. Furthermore, the branches of Form B are more robust and bear longitudinal striations, a transition not seen in Form A. The tubular and lateral nature of these structures suggests a potential comparison to the lobopodous limbs of early panarthropods (Lerosey-Aubril and Ortega-Hernández, 2022; Yang et al., 2024). This interpretation is based on a shared, simplified tubular construction seen in fossils like phosphatized lobopodians (Maas et al., 2007). The comparison, however, is not straightforward. Unlike typical lobopods, these structures lack diagnostic features such as transverse annulations, constrictions, or any form of internal subdivision. Interpreting these structures as annelid parapodia is unlikely because they lack the flattening or lobation typical of such appendages. A comparison to the setae of palaeoscolecids is also difficult, as our specimen is both more robust and lacks the defining annular rings of that group (Xian et al., 2024a). The segmentation and lateral branches of Form B invite comparison with extant kinorhynchs, where the trunk segments could be analogous to zonites and the branches to spines. If valid, this interpretation would suggest the specimen is phylogenetically closer to the kinorhynch crown-group than stem taxa like Eokinorhynchus rarus (Zhang et al., 2015). However, key differences challenge this view, as the branches are more robust and integrated, resembling flexible body extensions rather than simple spines. Given these unresolved questions, determining its precise taxonomic placement and evolutionary role will require additional fossil evidence.

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.

Figs. 4 and 5A–C, F–J

Material. Seventeen specimens.

Description. This is the most abundant form in our materials (Fig. 8). These fragments also display two distinct preservation modes: molded counterparts showing inverted topographical relationships (Fig. 4A and B) and secondary permineralized specimens retaining original surface relief (Fig. 5C). At least four morphologically distinct spine types can be recognized on the relatively large cuticular fragments. The majority of spines are fractured, making it difficult to obtain accurate measurements of their key morphological parameters. Larger elements comprise tubular spines (likely longer than 397.6 µm in height, n=6) with dome-shaped bases (not less than 164.8 µm in maximal width, n=2; Fig. 4A–C and F–H, arrows a; Fig. 4K, arrows) and coniform spines (likely longer than 221.2 µm in height, n=4) featuring depressed funnel-shaped bases (not less than 197.5 µm in maximal width, n=3; Fig. 4A, B, and F–I, arrow b; Fig. 5I, arrows). Smaller elements include coniform spines (not less than 111.2 µm in height, n=7) with flared trumpet-shaped bases (not less than 179.1 µm in maximal width, n=7; Fig. 4D, E, and M, arrow) and blade- or hook-like spines (not less than 326.3 µm in height, n=2, Fig. 5A and G, arrow) exhibiting characteristic “cowboy hat” (Fig. 5A and F) or discoid basal expansions (not less than 471 µm in maximal width, n=1; Fig. 5B and H). All spines are hollow (Fig. 4F–I and M). Relatively regularly distributed perforations can be observed on the inner side of some coniform spines (Fig. 4M, arrow). Their spatial distribution shows no discernible pattern at this stage. In molded specimens, continuous polygonal ornamentation (2.4–9.7 µm in maximal width, mean 5.3 µm, n=173) extending from spine shafts to funnel bases can be found on the inner side of the spines (Fig. 4F, G, and I, arrows b). Generally, the morphological characteristics and maximum of width of the polygons vary across different regions, ranging from 4.45 to 7.54 µm (Fig. 4F, G, and I, arrows b). The transitional zones between spines and cuticle exhibit plicated longitudinal striations (inter-striation spacing: 3.5–8.7 µm, mean 5.3 µm, n=18) that display pronounced folding deformation (Fig. 5F–H). However, this feature could also relate to deformation caused by a change in the angle of the spine during the preservation process, which pulled the cuticle, as not all spine bases exhibit similar plicated structures at their connection points with the cuticles. Dome-shaped bases display relatively less distortion compared to their funnel-shaped counterparts (Fig. 5H, arrows). Interspinal cuticular regions appear exceptionally flexible, displaying a hierarchical reticulated pattern consisting of smaller irregular ridge-defined polygons (between 3.6 and 19.7 µm in maximal width, mean 9.7 µm, n=32) superimposed with larger orbit-like meshes (25–40 µm in maximal width, mean 30.2 µm, n=29; Fig. 4J and L). One particularly instructive inverted specimen shows circular depressions at the center of each orbit-like structure, likely representing attachment points for the smaller coniform spines (Fig. 4L, arrows). More regular polygonal structures (Fig. 5J and K) occur on the cuticle of some foliated specimens (Fig. 5C and J). The maximum of size of this form is about 4.3 mm (Fig. 7; Table S2).

Comparisons. The polygonal ornamentation suggests possible ecdysozoan affinities (Mussini and Butterfield, 2025), with particular morphological parallels worth noting. The larger-sized, blade-like spines resemble some SSFs such as the Protohertzina unguliformis and Mongolodus longispinus, and chaetognath grasping spines (Vannier et al., 2007). The smaller-sized, blade- or hook-like spines with distinctive basal expansions are conversely reminiscent of hooks found on the introvert or trunk of putative Cambrian stem-group scalidophorans and kinorhynchs from the early Cambrian (e.g., Eokinorhynchus rarus; Zhang et al., 2015; Shao et al., 2020a, b) and on the introvert of fossil and living priapulids (Mussini and Butterfield, 2025). In particular, large conical sclerites associated with polygonal cuticular patterns are known in Scalidodendron (SCFs, e.g., Scalidodendron crypticum; Mussini and Butterfield, 2025). Comparable falcate, hook-like sclerites were likewise present in Cambrian palaeoscolecids, located either on the introvert or trunk (Mussini and Butterfield, 2025). The dome-shaped bases of some coniform spines bear striking similarity to the basal structures of Saccorhytus-type protrusions (e.g., the large spine sclerites on the body of Saccorhytus coronarius; see Han et al., 2017, and Liu et al., 2022).

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.

Figs. 4 and 5A–C

Materials. Two specimens.

Description. These fragments are preserved in their molds; their replica casts reveal that Form D is defined by parallel ridge-like bulges spaced 133–217 µm apart (mean = 172.6 µm, n=5), creating at least six distinct regions (Fig. 5D, arrows; Fig. 5K, arrow). Its surface ornamentation consists of a polygonal reticulation of 7–15 µm (mean = 11.2 µm, n=51; Fig. 4L) units and is further marked by numerous depressed pores, some displaying concentric marginal rings (Fig. 4L, arrows). The largest specimen measures about 1.8 mm across (Fig. 7; Table S2).

Comparisons. The cuticular morphology suggests a possible affinity with vermiform ecdysozoans. The regularly spaced transverse ridges likely represent trunk annulations, while the pores or pits may correspond to the setae of some ecdysozoans (e.g., Xian et al., 2024a). Although contemporaneous Scalidophora fossils also display fine annulations (e.g., Eopriapulites sphinx; Liu et al., 2014), the broader annulations here may instead represent macroannuli. This pattern aligns more closely with certain palaeoscolecids, such as Paleoscolex, which possess both prominent annulations and setal structures (Xian et al., 2024a). However, the present specimens lack the characteristic sclerites or spines of most palaeoscolecids, which precludes a definitive assignment.

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.

Form E

Figs. 5E, M and 6A, G, H

Materials. Two specimens.

Description. These cuticular fragments are characterized by relatively uniformly distributed papillate knobs (Figs. 5E, 5M and 6A, G, and H). Two subtypes can be recognized in our material. In subtype 1, each knob consists of a hemispherical apex and a broad base, with adjacent papillate knob separated by pentagonal or hexagonal boundaries (Fig. 5E and M). Polygonal fields are irregular and defined by thin grooves (Fig. 5M, arrow). The papillate knobs measure 21–32 µm in diameter (mean 25.2 µm, n=30) with spacings of 30–65 µm (mean 44.9 µm, n=49). The subtype displays comparable papillate knob structures but differs from subtype 1 in several aspects (Fig. 6A, G, and H). First, the papillate knobs are larger, measuring 38–60 µm in diameter (mean 45 µm, n=45) and more closely spaced at 51–107 µm intervals (mean 69.7 µm, n=47). Second, subtype 2 lacks broad bases and polygonal boundaries (Fig. 6A). Third, the broken papillate knobs of subtype 2 exhibit a thin and hollow structure (Fig. 6G, arrow b), which seems to be absent in the apparently solid papillate knobs of subtype 1. Lastly, the apices of the papillate knobs of subtype 2 exhibit irregular morphologies, some of them tapering into pointed spines (Fig. 6H, arrow a), while others develop shallow depressions (Fig. 6G, arrow a; Fig. 6H, arrow b). Both subtypes lack additional cuticular features, and their minimal soft deformation suggests originally thick, rigid textures. The maximal size of this form is about 4.9 mm (Fig. 7; Table S2).

Comparisons. Various papillate knob-bearing fossils have been reported from the Ediacaran to Cambrian, including acanthomorphic acritarchs (e.g., Bacatisphaera baokangensis in Zhou et al., 2001; and Megasphaera cymbala in Xiao et al., 2014), putative embryos (e.g., Archaeooides granulatus in Qian, 1977; and Archaeooides sp. in Pyle et al., 2006), algae-like forms (e.g., Shaanisphaera in Xian et al., 2024b; and Qinlingisphaera in Xian et al., 2024b), and tentative sponges (e.g., Aetholicopalla in Luzhnaya et al., 2023). These typically exhibit spherical morphologies with diverse nodules, although some superficial similarities (e.g., depressions on the nodules; see fig. 6G in Xian et al., 2024b) may reflect taphonomic artifacts. Among them, Megasphaera cymbala (Xiao et al., 2014) strongly resembles subtype 1 as both are characterized by polygonal sculpture with papillate knobs. However, all the above-mentioned taxa are markedly smaller (generally <1 mm) than the studied fragments (1.17–2.51 mm in axis length). Similar cones and polygonal patterns also occur on arthropod cuticles or compound eyes in fossil specimens from Cambrian Stage 3 or younger deposits (e.g., Zhang, 1987; Paterson et al., 2011; Lee et al., 2011; Zhang et al., 2021). This cuticular structure also shows some resemblance to putative mollusks reported from the upper Shuijingtuo Formation (Cambrian Stage 3) (see fig. 4C in Zhang et al., 2021), although major differences are evident. The Shuijingtuo specimens exhibit pocket mark-type protrusions that are considerably less regular in morphology and significantly larger in individual size compared to the well-defined nodules described here. Similar “ornamented plates” also occur in the Wernecke Mountains of eastern Yukon Territory, northwestern Canada (see figs. 8.11–8.19 in Pyle et al., 2006). Some undetermined specimens (e.g., fig. 8.18 in Pyle et al., 2006) may share the same characteristics as seen in Form E.

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.

Form F

Fig. 6B–D and I–N

Materials. Five specimens.

Description. These cuticular remains bear densely and uniformly distributed short coniform to hook-like spines (Fig. 6B–D and I–N). As for Form E, two subtypes can be distinguished. In subtype 1, spines have a consistent shape (Fig. 6B, C, and I–L), with base widths of 29–51 µm (mean 39.4 µm, n=79) and heights of 36–44 µm (mean 40.4 µm, n=7). Spacing ranges from 39 to 115 µm (mean 64 µm, n=60). All these cuticular elements exhibit wrinkling to varying degrees, with some specimens being intensely crumpled and folded (Fig. 6I–L). Specimen sizes are notably large, with axes measuring 1.92 and 4.32 mm. Some specimens are strongly crumpled (e.g., Fig. 6B), which suggests an original centimetric size. Subtype 2 is characterized by beak-shaped spines (Fig. 6D, M, and N). These spines are arranged in an orderly pattern, with interspinal spacing of 27–69 µm (mean 47.8 µm, n=50) and width of the spine base ranging from 31 to 59 µm (mean 46.6 µm, n=29). The spines are hook-like, resembling a bird's beak, but, unlike the former, their tips are blunt and rounded (Fig. 6M and N). The curved outline of the beak-shaped spines is largely consistent (Fig. 6M and N). No primary features other than spines are observed. The maximal size of this form is about 8.6 mm (Fig. 7; Table S2).

Comparisons. Various Ediacaran to Cambrian organisms possessed spine-like structures especially comparable to subtype 1 (e.g., acanthomorphic acritarchs, Sterocapsoides wenganensis in Xiao et al., 2014; and Mengeosphaera reticulata in Xiao et al., 2014). However, the pronounced wrinkling and crumpling state of the specimens suggests that this type of cuticle was less rigid than that of Form E. Notably, the spine morphology and distribution pattern show remarkable similarities to Saccorhytus coronarius (Han et al., 2017) from the Fortunian Kuanchuanpu Formation, although the spines of Saccorhytus are typically more slender. This similarity might suggest a close phylogenetic relationship, potentially placing Form F among basal ecdysozoans. However, the organisms represented by Form F may have been substantially larger (potentially close to centimeter-scale Fig. 7) than Saccorhytus (approximately 1 mm), implying that saccorhytids possibly had larger members.

Occurrence. Lower part of the Yanjiahe Formation, Muyangxi section, eastern Three Gorges area, Cambrian Fortunian.