Terminology

Anatomical terminology used herein mostly follows precedents found in Conway Morris and Chen (Reference Conway Morris and Chen1992), Van Iten (Reference Van Iten1992), and Han et al. (Reference Han, Li, Wang, Yang, Guo, Sasaki and Komiya2018).

Included genera

Arthrochites Chen, Reference Chen1982; Hexaconularia He and Yang, Reference He and Yang1986; Septuconularia n. gen.

Emended diagnosis

Rapidly expanding, laterally compressed, biradially symmetrical periderm having a blunt apex and prominent transverse ribs. Each of the two major sides of the periderm bear three, five, or seven faces bounded by longitudinal grooves or ridges.

Type species

Septuconularia yanjiaheensis n. gen. n. sp., from the Aldanella attleborensis Assemblage Zone of the Yanjiahe Formation (Bed 5; Cambrian Stage 2) in the Yanjiahe section near the city of Yichang, Hubei Province, China.

Diagnosis

As for the species by monotypy.

Occurrence

The A. attleborensis Assemblage Zone in Bed 5 of the Cambrian Yanjiahe Formation (Terreneuvian, Stage 2) in the Yanjiahe section near the city of Yichang, Hubei Province, China.

Etymology

Septu, Latin, referring to the seven faces on each of the two major sides of the periderm; conul-, Latin, cone-shaped periderm.

Remarks

The new genus currently comprises the type species only. Septuconularia n. gen. differs from Hexaconularia and Arthrochites in having 14 faces.

Holotype

CUBar90-4 (Fig. 2.1); the A. attleborensis Assemblage Zone in Bed 5 of the Cambrian Yanjiahe Formation (Terreneuvian, Stage 2) in the Yanjiahe section near Yichang, Hubei Province, China.

Paratypes

A total of 21 relatively well-preserved specimens (CUBar2-1, CUBar9-1, CUBar9-2, CUBar9-6, CUBar9-7, CUBar10-23, CUBar38-1, CUBar38-2, CUBar38-7, CUBar38-8, CUBar60-4, CUBar79-1, CUBar90-6, CUBar99-1, CUBar99-2, CUBar99-8, CUBar99-9, CUBar115-31, CUBar115-38, CUBar119-12, CURBar5) from the same level and locality as the holotype.

Diagnosis

Fan-shaped, biradially symmetrical periderm exhibiting a narrowly lenticular transverse cross section and consisting of a short apical region and a long, tapered, strongly compressed abapical region. Apical region relatively smooth; abapical region with short, irregularly spaced, trochoidal (long.) transverse ribs arrayed on each of the two major sides of the periderm in seven, adapically tapering faces bounded by six, adaperturally diverging longitudinal ridges. Transverse ribs on each face variably offset along a subdued median (midline) groove.

Occurrence

As for the type species.

Description

Fan-shaped tubular periderm with a narrowly lenticular transverse cross section. Available specimens range from 1.7 to 6.3 mm in width and from 1.3 to 5.4 mm in length (Table 1). Periderm strongly tapered, with the apical angle measuring approximately 80°–100° in the plane of greatest width (Table 1). External ornament of the periderm defines a small apical region (apr) and a much larger abapical region (abr) terminating along a slit-like apertural margin that may be incomplete (Figs. 2, 3).

Table 1.

Measurements of S. yanjiaheensis as for the species by monotypy.

Both sides of the abapical region exhibit seven adapically tapering faces, each bounded by prominent longitudinal ridges (lr; Fig. 2.1, 2.5, 2.10) and crossed by short, irregularly spaced transverse ribs (tr; Fig. 2.3, 2.7, 2.10, 2.11)). Longitudinal ridges measure approximately 40–50 μm in width and extend from the boundary between the apical and abapical regions (aab; Fig. 2.1, 2.2) to the apertural margin (am; Fig. 2.3, 2.5, 2.7). Beginning at a distance of approximately 4–5 ribs from the apertural margin, each longitudinal ridge is divided by a single, narrow, longitudinal furrow (lf) into two, mutually parallel narrow ridges (Fig. 2.4–2.6, 2.12–2.14). Seven faces constituting each side of the periderm collectively consist of two marginal faces (mf), four lateral faces (lfs), and one central face (cf; Fig. 2.1). Faces nearly equal in width, with the apical angle measuring ~11° –17° (Table 1). Along the apertural margin, each face is curved slightly toward the opposite side of the periderm (Fig. 2.4). Faces further subdivided into two equal halves by a single subdued, discontinuous, longitudinal median (midline) groove (lmg; Fig. 2.7, 2.11).

All 14 faces bear numerous, irregularly spaced, trochoidal, adaperturally arching transverse ribs. Height of the transverse ribs variable but generally lower than that of the longitudinal ridges. Most ribs extend across the entire width of the face; others terminate along the facial midline (Figs. 2.7, 3.7, 3.10, 3.13). Some ribs appear as irregular folds that bifurcate toward the longitudinal ridges or longitudinal median grooves (Fig. 2.8, 2.11, 2.13); such ribs curve slightly toward the aperture, numbering approximately 12–13 ribs per 1 mm near the aperture. Transverse ribs in the upper part of the abapical region do not cross the longitudinal ridges. Lower part of the abapical region extends up to 7–8 transverse ribs away from the boundary with the apical region; transverse ribs in the lower part of the abapical region number approximately 20 per 1 mm, in some cases crossing the longitudinal ridges and reaching the midline of the adjacent face or faces, the longitudinal ridges not being as prominent as those in the upper part of the abapical region (Fig. 2.1, 2.2, 2.15–2.17). Viewed from above, apertural margin lenticular in outline, paralleling the transverse ribs and thus gently arched in side view. Apertural margin truncates some of the transverse ribs, suggesting incomplete preservation of the aperture (Fig. 2.4, 2.12).

Apical region preserved in two specimens (Figs. 2.1, 2.2, 3.13), measuring 0.4 mm long and 0.6 mm wide in the holotype, and 0.5 mm long and 1.5 mm wide in CUBar99-1 (Fig. 3.13); apical region cone-shaped, with apical angle measuring 125°. Surface of the apical region mostly smooth but with 4–5 long, continuous, concentric arc-like folds near the boundary with the abapical region (Fig. 2.2). Six longitudinal ridges terminate at the apical-abapical boundary, though with their lines of extension possibly meeting at the apical end (Fig. 2.1, 2.2).

Micro-CT observations reveal that within the plane containing the minimum width of the periderm and the oral-aboral axis, the greatest width of the tube is located at the boundary between the embryonic region and the abapical region, with the width decreasing abapically from that level (Fig. 4; see supplemental data Movie 1). Specimens are partially filled with secondarily phosphatized, irregular debris, with no evidence of relic soft parts. As indicated by the positions of the longitudinal ridges, arrangement of the 14 faces on the two opposite sides of the periderm is not exactly biradial (Fig. 2.4).

Figure 4.

Micro-CT tomography of the holotype specimen (CUBar90-4) of Septuconularia yanjiaheensis n. gen. n. sp. (1) 3D volume rendering of the periderm; (2) virtual intersection along the maximum width and the oral–aboral axis of the periderm; (3) virtual intersection close to the minimum width along the oral–aboral axis; (4) virtual horizontal intersection at the level indicated by 4' in (1); (5) virtual horizontal intersection at the level of the boundary between the apical and abapical regions. Scale bars = 100 μm.

Etymology

Yanjiahe, a fossil locality in the Three Gorges area near Yichang, Hupei Province, China.

Preservation

All 64 examined specimens of S. yanjiaheensis show three-dimensional preservation but have undergone secondary phosphatization. Their biradial symmetry probably is a primary anatomical feature, as the specimens occurred together with undeformed cylindrical tubes of other taxa (e.g., Conotheca Missarzhevsky in Rozanov et al., Reference Rozanov1969 and Turcutheca Missarzhevsky in Rozanov et al., Reference Petersen, Larwood and Rosen1969; see Guo et al., Reference Guo, Li and Li2014). The internal cavity of S. yanjiaheensis is generally filled with diagenetic phosphatic debris (Fig. 2.1, 2.4, 2.9, 2.12), and the edges of most specimens are incomplete, indicating that they may have been subjected to transport and abrasion prior to final burial. As in conulariids and other hexangulaconulariids (He, Reference He1987; Han et al., Reference Han, Li, Wang, Yang, Guo, Sasaki and Komiya2018), the apical region is rarely preserved (CUBar90-4 and CUBar38-2), suggesting that this portion of the periderm was relatively thin and fragile. None of the examined specimens preserves the entire aperture, and thus the tube wall in this area probably was very delicate.

Remarks

Together, pronounced biradial symmetry and large apical angles distinguish hexangulaconulariids from all conulariids except Conulariella Bouček, Reference Bouček1928 (Bouček, Reference Bouček1928), which, however, differs from hexangulaconulariids in having only four faces and a well-developed sulcus at each of the four corners. Septuconularia yanjiaheensis is most similar to Hexaconularia sichuanensis in having a strongly compressed conical periderm with more than four faces. However, S. yanjiaheensis differs from both H. sichuanensis and Arthrochites emeishanensis in having or showing: (1) substantially greater size (up to 6 mm in width); (2) 14 faces instead of six; (3) much larger apical angles (generally in excess of 90° [Table 1] as opposed to 75° on average in Hexaconularia [Table 2] and 35° on average in Arthrochites); and (4) irregularly spaced transverse ribs, with the most prominent ribs occurring in the upper part of the abapical region. By contrast, the transverse ribs of Hexaconularia and Arthrochites are much more regular in their spacing, and they may cross the longitudinal furrows (Conway Morris and Chen, Reference Conway Morris and Chen1992; Van Iten et al., Reference Van Iten, Zhu and Li2010). In addition, and as in the lower part of S. yanjiaheensis, the transverse ribs of Arthrochites are continuous and elliptical and are not interrupted along the longitudinal furrows. Additional differences between S. yanjiaheensis and both Hexaconularia and Arthrochites emeishanensis are: (5) the presence of 14 longitudinal ridges in the abapical region of S. yanjiaheensis instead of four prominent longitudinal furrows (though with the 14 longitudinal ridges becoming elevated longitudinal furrows near the aperture); (6) the absence of prominent longitudinal striations between the transverse ribs (present not only in Hexaconularia and Arthrochites but also in conulariids); (7) the presence of closely spaced transverse ribs and intercalated transverse grooves in S. yanjiaheensis versus widely spaced ribs in Hexaconularia and Arthrochites; (8) the convex apertural margin of each face of S. yanjiaheensis (indicated by the course of the apertural transverse ribs) versus the nearly rectilinear margin of Arthrochites; and (9) the central face of S. yanjiaheensis being nearly equal in width to the other faces as opposed to being much wider than the other faces. It should be noted, however, that in some specimens of Arthrochites (Conway Morris and Chen, Reference Conway Morris and Chen1992, fig. 4) and Hexaconularia (Van Iten et al., Reference Van Iten, Zhu and Li2010, text-fig. 1), the central face can be subdivided into three equal sectors, making it similar to the faces of S. yanjiaheensis. To some extent, then, Hexaconularia is morphologically intermediate between Arthrochites and S. yanjiaheensis, though the periderm of Hexaconularia appears to have been much more rigid than that of either Arthrochites or S. yanjiaheensis.

Table 2.

Measurements of Hexaconularia sichuanensis. NXC9-01 quoted from Steiner et al. (Reference Steiner, Li, Qian and Zhu2004, fig. 8.1); He22–99 from Steiner et al. (Reference Steiner, Qian, Li, Hagadorn and Zhu2014, fig. 7.20); GMPKU3132, GMPKU3134, and GMPKU3135 from Duan et al. (Reference Duan, Dong, Porras, Vargas, Cunningham and Donoghue2017, fig. 5(a), (f), (h)); NIGP160430 from Xian et al. (Reference Xian, Zhang, Liu and Zhang2018, fig. 3A); NIGP150159, NIGP150161, and NIGP150162 from Van Iten et al. (Reference Van Iten, Zhu and Li2010, text-fig. 1A, D, E). Other specimens from early Cambrian Kuanchuanpu Formation in Ningqiang and Hexi section (provided by J. Han).