First report of Sphenothallus Hall, 1847 from the lower Cambrian of North China

Abstract

Sphenothallus is a tubular organism that is one of the most widely distributed and longest-ranging genera through the Palaeozoic. Despite its apparent cosmopolitan distribution, the genus has never been reported from North China. New specimens of Sphenothallus sp. have been discovered in the upper part of the Houjiashan and base of the Mantou formations (early to middle Age 4, Epoch 2, Cambrian) in Jiangsu Province, North China. The specimens are small tubes (up to 5 mm long) and have typical Sphenothallus characteristics, such as a multilayered lamellar structure, and subcircular to elliptical transverse cross-section with a pair of longitudinal thickenings situated at the widest diameter. Our material shows that both the rate of apertural expansion and the curvature of the tubes are significantly larger in early growth stages than in the later growth stages. As the diameter of the aperture increases, the transverse cross-section of the Sphenothallus sp. tube changes from subcircular at the proximal end to elliptical or lenticular at the distal end, and its wall thickness changes from uniform to thickening longitudinally. The discovery of Sphenothallus sp. from the North China Platform represents an extension of its palaeogeographic range during the Cambrian.

Rao Fu [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China; Yazhou Hu [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China; Timothy P. Topper* [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China; Fan Liu [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China; Yue liang [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China; Zhifei Zhang corresponding author [[email protected]], State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life and Environments and Department of Geology, Northwest University, Xi’an 710069, China. *Also affiliated with Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, Stockholm 10405, Sweden.

Keywords:

• Sphenothallus
• Houjiashan and Mantou formations
• North China
• palaeogeography

THE TUBULAR genus Sphenothallus, named by Hall (1847), remains controversial. It has been suggested that Sphenothallus should be assigned to plants (Hall 1847), worms (Mason & Yochelson 1985, Kolar-Jurkovšek & Jurkovšek 1997) or cnidarians (Muscente & Xiao 2015, Landing et al. 2018, Chang et al. 2018). Recent studies have suggested that the presence of a holdfast, laminar shell structure and multiple branches indicate that the genus is best placed within Cnidaria (Zhu et al. 2000, Van Iten et al. 2013, Landing et al. 2018, Chang et al. 2018, Van Iten et al. 2023). Sphenothallus ruedemanni Kobayashi, 1934 from the early Ordovician of southern China, with eight longitudinal septa-like structures in the basal part of the conical tube, has similarities with coronate scyphozoan polyps (Dzik et al. 2017, Han et al. 2020). Similar microstructures were also reported in conulariids (Muscente & Xiao 2015); however, the characteristic triangular plicated closure of conulariids has not been found in Sphenothallus (Han et al. 2020).

Sphenothallus ranges in age from the early Cambrian to the late Triassic, and occurs worldwide (see Muscente & Xiao 2015, Van Iten et al. 2023 and references therein). In China, six species of Sphenothallus have been reported and described: Sphenothallus sp. (Li et al. 2004, Van Iten et al. 2013, Chang et al. 2018, Fang et al. 2022), S. taijiangensis Zhu et al., 2000, S. songlinensis Peng et al., 2005, S. kozaki Fatka et al., 2012 (Chang et al. 2018), S. ziguiensis Liu et al., 2022 and S. angustifolius Hall, 1847 (Yi et al. 2003). In China, Sphenothallus ranges from the lower Cambrian to the lower Silurian and is distributed throughout southern and central parts of the country. However, Sphenothallus has not been reported from the North China Platform. Lower Cambrian small skeletal fossils are abundant and diverse in North China with most studies focusing on the Xinji Formation and the lower part of the Houjiashan Formation (corresponding to upper Stage 3 to lower Stage 4 of the lower Cambrian: Pan et al. 2015, Li et al. 2016, Skovsted et al. 2016, Yun et al. 2016, Pan et al. 2018). Faunal assemblages above the aforementioned units are poorly known (Hu et al. 2021).

Here, we report the first occurrence of Sphenothallus from the upper Houjiashan and lower Mantou formations (corresponding to the middle to upper Stage 4 of the lower Cambrian), that are well exposed northwest of Xuzhou in Jiangsu Province of China. In addition, the morphological changes of Sphenothallus sp. from the proximal to distal end are discussed.

Geological setting

All specimens in this study were collected from the upper part of the Houjiashan and base of the Mantou formations in the Dabeiwang section, located in Xuzhou City, northern Jiangsu Province (Fig. 1A, B). Xuzhou is situated on the southeastern margin of the North China Platform (Fig. 1A, B). The Houjiashan, Mantou, Maozhuang, Xuzhuang and Zhangxia formations (in ascending order) are well exposed in the Dabeiwang section, where the Terreneuvian and lower Stage 3 are absent (Fig. 1C). The Houjiashan Formation, about 30 m thick, is composed mainly of limestone beds of varied thickness, and the strata below the Houjiashan Formation at this locality are not exposed (Fig. 1C). The upper part of the Houjiashan Formation contains medium- to thick-bedded limestones, the intercalations of which contain abundant trilobite fragments (Xu & Lu 1984, Feng 2014, Chen 2020). The Mantou Formation is about 210 m thick and conformably overlies the Houjiashan Formation. The Mantou Formation is composed mainly of purple shale in the lower part, grey medium- to thick-bedded mottled limestone in the middle and purple shale and thin-bedded argillaceous limestone in the upper part (Xu & Lu 1984, Chen 2020, Feng 2014). The base of the Mantou Formation consists of purple shale and medium- to thick-bedded limestone, which contain abundant trilobites, hyoliths, brachiopods and algae (Feng 2014, Chen 2020). The upper part of the Houjiashan Formation yields Megapalaeolenus fengyangensis (Chu, 1962), and the base of the Mantou Formation yields Redlichia chinensis Walcott, 1905 (Xu & Lu 1984, Fang 1991, Feng 2014); these beds can be correlated with Stage 4 of the lower Cambrian (Zhu et al. 2019).

Fig. 1. Section locality and stratigraphic column of the early Cambrian Houjiashan and Mantou formations at the Dabeiwang section, Xuzhou, northern Jiangsu Province (modified from Zhang et al. 2018). A, Map showing the location of the Xuzhou area (box) on the southeastern margin of the North China Platform; B, Geographic map showing the location of the Dabeiwang section in Jiangsu Province, North China; C, Stratigraphic column of the lower Cambrian at the Dabeiwang section, D1–D4 represent the stratigraphic levels hosting Sphenothallus sp.

Materials and methods

Four sets of numbered limestone samples broken into small pieces were collected in bags of approximately 10 kg each, with over 100 kg collected in total. Samples D1 and D2 are from the limestone beds of the upper part of the Houjiashan Formation, and samples D3 and D4 are from the limestone beds near the base of the Mantou Formation (Fig. 1C). Limestone samples were treated with 5–10% acetic acid and organophosphatic tubes and their fragments were picked manually from the residues at the Early Life Institute (ELI), Northwest University, Xi’an. Some specimens with exquisite preservation were examined using a FEI Quanta 450-FEG SEM equipped with energy dispersive spectroscopic X-ray analysis (EDS) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. All specimens are currently deposited in the Early Life Institute, Northwest University, Xi’an of China.

Systematic palaeontology

PHYLUM CNIDARIA Verrill, 1865

SUBPHYLUM MEDUSOZOA Petersen, 1979

CLASS, ORDER, and FAMILY uncertain

Sphenothallus Hall, 1847

Type species

Sphenothallus angustifolius Hall, 1847, New York, Middle Ordovician.

Emended diagnosis

Theca slender, elongate (up to 90 mm or greater in length), single or branched, with a small, subconical holdfast and a pair of robust, longitudinal thickenings that extend from the holdfast to the aperture. Theca commonly curved in the apical region, elsewhere more or less straight, with a low rate of apertural expansion (range approximately 2°–26°), and there may be constriction on the tube. Apertural end open, with a smooth margin that may arch beyond the ends of the longitudinal thickenings. Transverse cross-section subcircular in the immediate vicinity of the basal attachment disk, elsewhere elliptical. Basal attachment disk subconical and consisting of a thick, cone-like upper part and an extremely thin, flat base. Longitudinal thickenings centered on the end points of the theca’s widest diameter and extending almost its entire length. Interior of the theca may contain a thin, finely lamellar, transverse wall (schott) that is convex toward the apical end and extends along the inner surface of the theca proper, toward the aperture (emended from Zhu et al. 2000, Chang et al. 2018).

Remarks

Sphenothallus has rarely been cited in the literature and its definition has been unclear (Mason & Yochelson 1985, Zhu et al. 2000). The first revision of conical fossils that could be considered “modern” was made by Sysoev (1958), followed by Fisher (1962) and Howell (1962) in the Treatise Part W, without any mention of Sphenothallus. Even the diagnosis of Sphenothallus in the Treatise on Invertebrate Paleontology (Zhu et al. 2000) is referred to Glyptoconularia gracilis (Hall). Valvasoria carniolica Kolar-Jurkovšek & Jurkovšek, 1997, originally described as a tubicolous worm (Hitij et al. 2019), has been interpreted as Sphenothallus carniolica (Kolar-Jurkovšek & Jurkovšek, 1997) from the Mesozoic (Van Iten et al., 2023). Many genera have been created to accommodate Palaeozoic phosphatic tubes, e.g., Tubulelloides Howell, 1949, Serpulites Sowerby in Murchison 1839, Campylites Eichwald 1856 and Enchostoma Miller & Gurley 1896 and with a lack of diagnostic characters, it is not inconceivable that all of these genera are synonyms of Sphenothallus (Mason & Yochelson 1985, Zhu et al. 2000). In the Treatise on Invertebrate Paleontology, Campylites was accepted, without any mention of Sphenothallus (Howell 1962, Mason & Yochelson 1985, Zhu et al. 2000). Early researchers may have used synonyms of Sphenothallus and were unaware of this genus, thus Sphenothallus has been ignored in major publications.

Sphenothallus and Torellella Holm, 1893 are both apatitic conical tubes (Skovsted & Holmer 2006, Skovsted & Peel 2011, Landing et al. 2018, Tinn et al. 2020, Vinn 2022), and are very similar in general morphology (Skovsted & Holmer 2006, Vinn & Kirsimäe 2014, Tinn et al. 2020, Vinn & Mironenko 2020, Van Iten et al. 2023). The taxonomy of these genera is confusing (Zhu et al. 2000, Landing et al. 2018) and they have been considered synonymous by some authors (Schmidt & Teichmüller 1956, Zhu et al. 2000, Vinn 2006, Fatka et al. 2012, Stewart et al. 2015, Vinn 2022). Torellella laevigata Linnarson, 1871 has a relatively less robust thickening compared to species of Sphenothallus, and in some species of Torellella the longitudinal thickenings are absent (Zhu et al. 2000). Zhu et al. (2000) mentioned that Torellella has ‘marginal keels’ located at the end points where the diameter is at its widest. This can, for example, be seen in Torellella sp. from northern Siberia (Kouchinsky 2013) and Torellella cf. mutila from France (Devaere et al. 2014). Vinn (2006) argued that Torellella can be distinguished from Sphenothallus by the presence of a lateral furrow, but this feature is not present in every species of Torellella (Peel 2021). Both Sphenothallus and Torellella are palaeogeographically widespread in early Palaeozoic marine deposits, but Torellella has a shorter stratigraphic range (lower Cambrian to Ordovician) than Sphenothallus (upper lower Cambrian to Upper Triassic) (see Zhu et al. 2000, Landing et al. 2018, Van Iten et al. 2023 and references therein). Holdfasts are regarded as a key character of Sphenothallus and they have been reported in many species, e.g., Sphenothallus sp. (Li et al. 2004), S. ruedemanni (Dzik et al. 2017) and S. carniolica Kolar-Jurkovšek & Jurkovšek, 1997 (Van Iten et al. 2023). Holdfasts have never been associated with Torellella (Kouchinsky 2013, Skovsted et al. 2016, Landing et al. 2018, Peel 2021) and, as shown by Dzik et al. (2017), the unbroken apical end of Torellella lacks a holdfast (Landing et al. 2018). Another important difference between Sphenothallus and Torellella is that the latter lacks branching in specimens older than late Cambrian (Landing et al. 2018).

Stratigraphic and geographic range

Lower Cambrian to Upper Triassic of China, Denmark, USA, Canada, Czech Republic, Korea, Morocco, Estonia, Greenland, Germany, Brazil, Russia, Belgium, France, Netherlands, Slovenia, Ukraine (see Dernov 2023, Van Iten et al. 2023 and references therein).

Sphenothallus sp.

(Figs 2, 3)

Fig. 2. Sphenothallus sp. from the early Cambrian Houjiashan and Mantou formations at the Dabeiwang section, Xuzhou County, northern Jiangsu Province. A1–A4, ELI-DBW_003; A1, A2, general view, constriction indicated by white arrows; A3, lateral view, constriction indicated by white arrows; A4, enlarged general view of apertural region of A2; B, ELI-DBW_018, lateral view, constriction indicated by white arrows; C, ELI-DBW_036, general view; D1, D2, ELI-DBW_034; D1, general view, white box showing circular hole; D2, enlarged general view of circular hole; E, ELI-DBW_057, interior view, showing the internal surface of the constriction; F, ELI-DBW_033, general view, showing low transverse ridges in the outer surface; G, ELI-DBW_006, general view, showing constriction indicated by white arrows; H, ELI-DBW_041, general view, growth displacement indicated by white arrows; I, ELI-DBW_019, general view, constriction indicated by white arrows separated from the thin wall; J1–J3, ELI-DBW_002; J1, transverse cross-section; J2, enlarged transverse cross-section of J1, showing lamellar structure of the thin wall; J3, enlarged transverse cross-section of J1, showing lamellar structure of the longitudinal thickenings; K, transverse cross-section of the longitudinal thickenings fragment; L, lateral view of the longitudinal thickenings. Scale bars represent: 500 μm (A1–A3, B–D1, E–I, L); 300 μm (A4, M1, N1); 200 μm (J1, K); 100 μm (D2); 50 μm (J3, M2); 30 μm (N2); 20 μm (J2).

Fig. 3. Sphenothallus sp. from the early Cambrian Houjiashan and Mantou formations at the Dabeiwang section, Xuzhou County, northern Jiangsu Province. A, ELI-DBW_046, general view, white arrows show constriction; B1, B2, ELI-DBW_024; B1, general view; B2, transverse cross-section of the distal end; C1, C2, ELI-DBW_042; C1, general view; C2, transverse cross-section of the distal end; D1, D2, ELI-DBW_061; D1, general view; D2, transverse cross-section of the distal end; E, ELI-DBW_001, general view; F1, F2, ELI-DBW_025; F1, general view; F2, transverse cross-section of the distal end; G, ELI-DBW_012, general view; H, ELI-DBW_059, general view; I1–I3, ELI-DBW_015; I1, general view; I2, transverse cross-section of distal end; I3, transverse cross-section of the proximal end; J1–J3, ELI-DBW_017; J1, general view; J2, transverse cross-section of the distal end; J3, transverse cross-section of the proximal end; K1–K3, ELI-DBW_011; K1, general view; K2, transverse cross-section of the proximal end; K3, transverse cross-section of the distal end; L1, L2, ELI-DBW_009; L1, general view; L2, transverse cross-section of the proximal end. Scale bars represent: 500 μm (A, B1, C1, D1, E, F1, G, H, I1, J1, K1, L1); 300 μm (B2, C2, F2, K3); 200 μm (D2, I2, J2, L2); 100 μm (I3, J3, K2).

Referred material

Eighty-two Sphenothallus specimens were included in our study; 44 specimens were broken owing to the thin wall and are preserved as left or right sides of fragments of longitudinal thickenings (Fig. 2E, K, L, M1, N1); 38 specimens were broken perpendicular to the long axis yet retained their tubular shape (Figs 2A1–D1, F–J3, 3). All specimens are black or brown and constructed of phosphatic lamellae (Fig. 4).

Fig. 4. Peak values of elements in the tube walls of specimen ELI-DBW_003, which yielded strong spectral peaks for Ca and P.

Description

All specimens are incomplete and unbranched, and no holdfasts have been preserved. The most complete specimens are long conical tubes with an oval cross-section (Fig. 2A4, J1). The tubes gradually expand and thicken from the apex to the aperture, with an incomplete and unclear marginal morphology. The tubes range from 1.01 mm to 5.04 mm (mean 2.56 mm, N = 24) in length and 0.24 mm to 1.17 mm (mean 0.58 mm, N = 23) in diameter (Table 1). Constriction (Figs 2A1–B, E, G–I, 3B1) is preserved in some of the most complete specimens. The smaller (< 500 μm diameter) region of the tube (Fig. 2C, F, 3G, I1, J1, 154 K1, L1), in the apical area, is generally more curved and has a great rate of apertural expansion (6°–8°) (Table 1). By comparison, the region with the larger diameter (more than 500 μm: Figs 2D1, G, I, 3B1, C1, D1, E, F1), towards the apertural end is relatively straight and has a lower rate of expansion (2°–6°) (Table 1). At the aperture, the tube has an obviously elliptical cross-section and bears significant longitudinal thickenings (Figs 2A4, 3B2, C2, F2, K3); whereas the apex of the tube is nearly circular and uniform in thickness (Fig. 3I3, J3, K2, L2). The majority of specimens are easily broken from the thin wall and only the longitudinal thickening region is retained (Fig. 2E, K, L, M1, N1). The longitudinal thickenings in the apertural area are crescentic in transverse cross-section and are 55.28–237.16 μm (mean 137.39 μm, N = 22) in thickness (Table 1; Fig. 2K). The tube has a multilayered, lamellar wall structure (Fig. 2J1–J3, M1–N2), and the longitudinal thickenings have more lamellae and more space between the lamellae than the thin wall (Fig. 2J2–J3). Some tubes preserve indistinct and low transverse ridges on the outer surface of both longitudinal thickenings and the thin wall; approximately 20 ridges are developed within 1 mm of length (Fig. 2F). One specimen bears five circular holes with diameters of 20–80 μm on the surface of the thin wall (possibly a result of boring: Fig. 2D1–D2).

Table 1. Average dimensions and ratios of Sphenothallus sp. from the Cambrian (Series 2) Houjiashan and Mantou formations of the Jiangsu Provinces, North China.

	Length (μm)	Diameter (μm)	Aae (°)	Lt (μm)
N	24	42	21	21
Mean	2560	491	5.60	137.79
Min	1005	191	2.62	55.28
Max	5040	1171	8.78	237.16
Median	2406	461	5.59	126.54
SD	998	214	1.82	57.71

Abbreviations: A_ae = angle of apertural expansion; L_t = longitudinal thickenings.

Remarks

The specimens described above suffered relatively little compaction and distortion, which has aided their identification. The specimens have some morphological characteristics in common with Sphenothallus and Torellella, including a straight or curved conical tube, either high or low rate of apertural expansion, and a pair of longitudinal crescentic thickenings and possessing a lamellar microstructure. However, the differences discussed above indicate that our specimens more likely represent Sphenothallus based on the apparent longitudinal thickening, absence of the ‘marginal keel’ and lack of a lateral furrow.

Sphenothallus taijiangensis described by Zhu et al. (2000) from the lower part of the Kaili Formation (corresponding to Cambrian Series 2, Stage 4) (Table 2) has an apertural diameter and density of striae similar to Sphenothallus sp. described herein, but with a maximum length nearly four times greater. Constriction has also not been documented from S. taijiangensis. Sphenothallus sp. and S. angustifolius from the lower part of the early Floian Tonggao Formation and the lower Silurian Lungmachi Formation have a conical tube nearly ten times longer than Sphenothallus sp. documented herein and a greater apertural diameter (Yi et al. 2003, Van Iten et al. 2013). Peel (2021) reported Sphenothallus sp. from the early Silurian of western North Greenland having a circular holdfast and, most importantly, incorporating a constriction that is similar to that of Sphenothallus sp. herein. Sphenothallus sp. of Peel (2021) has a similar conical tube size to Sphenothallus sp. illustrated herein, but differs from the latter in that the former has angulations on both sides of the conical tube.

Table 2. Selected documented occurrences and their characteristics among the lower Cambrian records of Sphenothallus.

Species	Age	Formation(s)	Locality	Length	Width	Branches	Holdfast	Reference
Sphenothallus? sp.	Stage 4	Ringstranda	Scandinavia	15.51	0.67	Present	Unreported	Høyberget et al. (2023)
Sphenothallus sp.	Stage 3	Guojiaba	Shaanxi, South China	1.00–3.00	≤0.8	Unreported	Unreported	Luo et al. (2023)
Sphenothallus sp.	Stage 3	Shuijingtuo	Hebei, South China	≤17.5	≤4.28	Unreported	Unreported	Muscente & Xiao (2015)
Sphenothallus sp.	Stage 3–4	Xiannvdong	Shaanxi, South China	≤3	≤0.7	Unreported	Present	Li et al. (2004)
?Sphenothallus sp.	Stage 4	Shipai	Three Gorges area, South China	6–12	1.1–2.0	Unreported	Unreported	Chang et al. (2018)
Sphenothallus sp.	Stage 4	Houjiashan and Mantou	North China	1.01–5.04	0.24–1.17	Unreported	Unreported	This study
Sphenothallus sp.	Stage 3–4	Harkless	Nevada, America	>0.57	>0.36	Unreported	Unreported	Skovsted and Holmer (2006)
S. taijiangensis	Stage 4	Kaili	Guizhou, South China	3–23	0.3–1.5	Unreported	Unreported	Zhu et al. (2000)
S. cf. taijiangensis	Stage 3–4	Tsinghsutung	Guizhou, South China	6.49–20.72	0.39–1.32	Unreported	Present	Zhang et al. (2013)
S. songlinensis	Stage 3	Niutitang	Guizhou, South China	≤17.5	≤5.0	Present	Unreported	Peng et al. (2005)
S. kozaki	Stage 4	Shipai	Three Gorges area, South China	7–57	0.7–1.5	Unreported	Unreported	Chang et al. (2018)
S. ziguiensis	Stage 3–4	Shuijingtuo	Hubei, South China	≤21.8	≤6.3	Unreported	Unreported	Liu et al. (2022)

Sphenothallus sp. from the upper Furongian of Mount Kinnekulle, south-central Sweden, has a high degree of branching (Stewart et al. 2015), a feature absent in the material studied herein. Sphenothallus sp. from Xuzhou also lacks a preserved holdfast, unlike Sphenothallus sp. of Li et al. (2004), which possesses a subconical, relatively complete holdfast (Table 2). Sphenothallus sp. of Li et al. (2004) differs from Sphenothallus sp. herein in having a greater rate of apertural expansion and a generally shorter tube. Sphenothallus songlinensis erected by Peng et al. (2005) has an angle of apertural expansion of 15°–26° and a length of more than 17 mm (Table 2), whereas Sphenothallus sp. herein has an angle of apertural expansion of 2.62°–8.78° (Table 1) and an estimated length of no more than 10 mm. Sphenothallus kozaki (Table 2) can be differentiated from the Sphenothallus sp. herein by the circular outline of the conical tube and weak internal thickenings (Fatka et al. 2012, Chang et al. 2018). The outer surface of S. ziguiensis of Liu et al. (2022) is characterised by longitudinal striae, a feature that distinctly differs from the transverse ridges of Sphenothallus sp. herein. Many species from the Ordovician to Triassic are markedly larger (Zhang et al. 2013, Chang et al. 2018, Landing et al. 2018) than Sphenothallus sp. from Xuzhou, e.g., aff. S. longissimus with a maximum length of 8.8 mm and maximum width of 13.0 mm (Vinn and Kirsimäe 2014), Sphenothallus ruedemanni with a length of 108.0 mm (Dzik et al. 2017), S. sica Salter, 1856 (Van Iten et al. 2019) has a maximum length of 145 mm, cf. S. angustifolius is approximately 42 mm long and 6 mm wide (Vinn and Mironenko 2020), and S. carniolica ranges up to 129.5 mm in length and 5.5 mm in width (Van Iten et al. 2023).

Discussion

The incomplete nature of Sphenothallus specimens has hindered our understanding of their taxonomy and biological affinity. Sphenothallus sp. identified in our material has similarities to several Sphenothallus species that have been documented from around the globe. For example, Sphenothallus sp. is of similar size with strong proximal curvature to juvenile specimens of S. ruedemanni from Hubei (mature specimens reach up to 108 mm in length; Dzik et al. 2017). Sphenothallus sp. herein also closely resembles the daughter tube of the branched S. sica documented from southern Brazil (Van Iten et al. 2019). Branched Sphenothallus forms have been described from the Cambrian (Table 2; Peng et al. 2005, Høyberget et al. 2023) and without the presence of a holdfast it is reasonable to conclude that Sphenothallus sp. may represent discrete daughter tubes.

Because of the inherently fragmented nature of the tubes, the growth of Sphenothallus has been well studied (Zhu et al. 2000, Yi et al. 2003, Li et al. 2004, Dzik et al. 2017, Peel 2021, Van Iten et al. 2023). At the proximal end of Sphenothallus (here interpreted as the oldest part of the tube) the transverse cross-section in both the inner and outer layer of the tube is circular with no apparently longitudinal thickenings (Fig. 3I3, J3, K2, L2) (Li et al. 2004, Dzik et al. 2017, Peel 2021). The documentation of circular holdfasts supports the idea that the first-formed tube of Sphenothallus is circular (Yi et al. 2003, Peel 2021). During growth, the diameter of the aperture expands and the transverse cross-section of the tube becomes elliptical (Li et al. 2004, Dzik et al. 2017, Peel 2021). The longitudinal thickenings generally occur when the cross-section of the tube becomes elliptical (Li et al. 2004, Dzik et al. 2017, Peel 2021; Fig. 3B2, C2, D2, F2, I2, J2, K3). This region of the tube becomes thicker with the addition of more lamellae (Yi et al. 2003) and increased space between the lamellae (Fig. 2J1–J3). As a consequence of this thickening, the transverse cross-section of the tube becomes very elliptical or lenticular (Figs 2A4, 4B2, C2, F2). The rate of apertural expansion at the small diameter is relatively greater than at the large diameter, suggesting that the early growth stages had a higher rate of apertural expansion (Zhu et al. 2000). No significant change in the morphology of the transverse cross-section of the tube having the larger diameter has been observed. The tube can be strongly curved at the smaller diameter (Yi et al. 2003, Van Iten et al. 2023) and relatively straight at the larger diameter. A morphologically similar form of the daughter tube is found in Sphenothallus sica from the Early Devonian Ponta Grossa Formation, Paraná State, southern Brazil (Landing et al. 2018, Van Iten et al. 2019). As discussed above, Sphenothallus sp. herein may represent discrete daughter tubes, and the curvature of the tubes (Figs 2A1, A2, A3, 3A, I1, J1, K1, L1) may be due to the tendency of the daughter tubes to maintain upward growth relatively parallel to their parent tube.

Six species of Sphenothallus have been reported and described from the early Cambrian: Sphenothallus sp. (Li et al. 2004, Skovsted & Holmer 2006, Luo et al. 2023), S. taijiangensis (Zhao et al. 1999, Zhu et al. 2000), S. songlinensis (Peng et al. 2005), S. kozaki (Chang et al. 2018), S. ziguiensis (Liu et al. 2022) and S. angustifolius (Yi et al. 2003; Fig. 5; Table 2). Early Cambrian Sphenothallus species occur in southern and central China, southern Nevada in the United States, Warnemünde of Denmark and Scandinavia (Fig. 5) (Table 2), but there have been no previous reports of Sphenothallus from North China. Thus, Sphenothallus sp. herein in the lower Cambrian from the Dabeiwang section in the Xuzhou area of Jiangsu Province is the first report of Sphenothallus from the North China Platform.

Fig. 5. Palaeogeographical reconstruction of the early Cambrian showing the geographical distribution of Sphenothallus. The palaeogeographic reconstruction is modified from Yang et al. (2015), Zhang et al. (2016) and Pan et al. (2018).

Acknowledgments

We express our sincere thanks to Juanping Zhai, Baopeng Song (from Northwest University) for their kind help in the field. We would like to thank the anonymous referees and Alcheringa Associate Editor Stephen McLoughlin for constructive suggestions, which have greatly improved the manuscript.

Data availability statement

All the specimens dealt with in this paper are deposited in the Early Life Institute and Department of Geology (http://geology.nwu.edu.cn/). Correspondence and requests for materials should be addressed to ZZF ([email protected]).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by the National Natural Science Foundation of China [NSFC Nos. 41890844, 41720104002 and 41621003 to Zhang Zhifei]. We also thank the Department of Science and Technology of Shaanxi Province [2022TD–11] and the 111th project [D17013] at Northwest University.