Balang Formation

The Balang Formation is exposed in eastern Guizhou and western Hunan, China, and represents part of the Jiangnan Slope, which represents the transitional slope belt between the Yangtze Platform and the Jiangnan Basin (Fig. 1.1; Peng and Babcock, Reference Peng and Babcock2001; Peng et al., Reference Peng, Zhao, Wu, Yuan and Tai2005; Liu and Lei, Reference Liu and Lei2013; Liu et al., Reference Liu, Wen, Peng, Liang and Wang2017). The lower part of this formation is composed of gray-greenish to yellow-greenish silty mudstone; while the upper part consists mostly of light gray to gray-yellow mudstone interbedded with light gray calcareous mudstone. In the Guizhou area, the Balang Formation is in conformable contact with the overlying Tsinghsutung Formation (also published as Qingxudong Formation) and the underlying Bianmachong Formation (Bureau of Geology and Mineral Resources of Guizhou, 1987; Yin, Reference Yin1996). The Tsinghsutung Formation is dominated by gray-greenish thin-bedded limestone; while the Bianmachong Formation is mainly composed of gray-black mudstone (Fig. 1.2; Bureau of Geology and Mineral Resources of Guizhou, 1987; Peng et al., Reference Peng, Zhao, Wu, Yuan and Tai2005; Peng, Reference Peng2009; Liu, Reference Liu2018). No isotopic dating is available for the Balang Formation, therefore its age is regarded as Stage 4 according to a series of trilobite-based correlations of Cambrian strata between South China and other continents (Zhou et al., Reference Zhou, Yuan, Zhang, Wu and Yin1979, Reference Zhou, Yuan, Zhang, Wu and Yin1980; Zhou and Yuan, Reference Zhou and Yuan1980; Peng and Babcock, Reference Peng and Babcock2001; Yuan et al., Reference Yuan, Zhao and Li2001, Reference Yuan, Zhao and Yang2006, Reference Yuan, Lin, Zhao and Peng2011; Qin et al., Reference Qin, Peng, Fu and Da2010; Yan et al., Reference Yan, Peng, Zhao, Wen and Sun2014; Yuan and Ng, Reference Yuan and Ng2014; Shen et al., Reference Shen, Peng, Wen and Liu2016; Y.F. Wang et al., Reference Wang, Peng, Wang, Wen, Zhang, Du and Shao2020), corresponding to the Qiandongian early Duyunian of the Cambrian chronostratigraphic system of China in slope facies (Peng and Babcock, Reference Peng and Babcock2001).

Figure 1.

(1) Map showing the distribution of the Balang Formation (gray areas) in Guizhou Province, the location of the fossil locality (star); and (2) the lithological column of the Lazizhai section.

Biostratigraphy

Zhou and Yuan (Reference Zhou and Yuan1980) first provided a biostratigraphic framework for the Balang Formation and recognized two trilobite zones: the Arthricocephalites-Changaspis Zone and the Arthricocephalus Zone. Subsequently, trilobite biostratigraphy has been revised on several occasions (Zhou et al., Reference Zhou, Yuan, Zhang, Wu and Yin1980; Yuan et al., Reference Yuan, Zhao and Li2001, Reference Yuan, Zhao and Yang2006; Peng, Reference Peng2009; Qin et al., Reference Qin, Peng, Fu and Da2010). Recently, Yan et al (Reference Yan, Peng, Zhao, Wen and Sun2014) included the entire Balang Formation in the Arthricocephalus chauveaui Zone based on trilobite biostratigraphy.

Many fossil assemblages have been discovered from the middle to upper part of the Balang Formation at various localities (Peng et al., Reference Peng, Zhao, Wu, Yuan and Tai2005, Reference Peng, Sun, Zhao and Tai2012a, Reference Peng, Sun, Zhao and Yanb; Peng, Reference Peng2009; Da et al., Reference Da, Peng, Zhao, Ma and Gu2011; Ma et al., Reference Ma, Peng, Zhao, Da and Sun2011; Liu and Lei, Reference Liu and Lei2013; Zhao et al., Reference Zhao, Peng, Wu, Luo, Wen and Liu2015; Y.F. Wang et al., Reference Wang, Peng, Wang, Wen, Zhang, Du and Shao2020), and the Balang Biota comprises at least 80 species of 63 genera (Zhao et al., Reference Zhao, Parsley and Peng2007; Peng et al., Reference Peng, Sun, Zhao and Tai2010a, Reference Peng, Zhao and Sun2012c, Reference Peng, Huang, Zhao and Sun2016; Sun et al., Reference Sun, Babcock, Peng and Zhao2015, Reference Sun, Babcock, Peng and Kastigar2017; Liu, Reference Liu2018; Liu et al., Reference Liu, Peng, Wen and Liang2018a; Wen, Reference Wen2019; Wen et al., Reference Wen, Babcock, Peng, Liu and Liang2019). In Guizhou, collections were conducted mainly in three localities: Xiangshuidong Village in Songtao County; Tongren City and Jiaobang and Lazizhai villages in Jianhe County; and Kaili City. In recent years, abundant fossils have been collected from the middle-upper part of the Lazizhai Section at Lazizhai Village (Fig. 1.1). This fossil-bearing horizon contains a greater diversity of fossils (Liu et al., Reference Liu, Wen, Peng, Liang and Wang2017, Reference Liu, Peng, Wen, Wang and Du2018b), including algae (Morania globosa Walcott, Reference Walcott1919; Sinocylindra sp.; Yuknessia sp.), chancelloriids, cnidarians (Sphenothallus sp.; Byronia sp.), sponges (e.g., Jianhella obconica Wang and Peng in Wang et al., Reference Wang, Peng, Wen, Du, Zhang, Wang and Wang2019; Leptomitus teretiusculus Chen, Hou, and Lu, Reference Chen, Hou and Lu1989; Hamptonia jianhensis Wang and Peng in Q.J. Wang et al., Reference Wang, Peng, Wang, Wen, Zhang, Du and Shao2020), worms (Wronascolex geyiensis Peng et al., Reference Peng, Huang, Zhao and Sun2016), brachiopods (Diandongia cirularis Peng et al., Reference Peng, Zhao, Qin, Yan and Ma2010b; Lingulellotreta malongensis [Rong, Reference Rong1974]), echinoderms (Guizhoueocrinus yui Zhao, Parsley, and Peng, Reference Zhao, Parsley and Peng2007), mollusks, hyoliths (Haplophrentis reesei Babcock and Robison, Reference Babcock and Robison1988; Ambrolinevitus maximus Jiang, Reference Jiang, Luo, Jiang, Wu, Song and Ouyang1982; ?Galicornus seeneus Val'kov, Reference Val'kov1975; Linevitus guizhouensis Sun et al., Reference Sun, Babcock, Peng and Kastigar2017; Meitanovitus guanyindongensis Qian, Reference Qian1978), trilobites (Redlichia [Pteroredlichia] chinensis [Walcott, Reference Walcott1905] Chang, Reference Chang1966; Probowmania [Probowmania] balangensis Yuan and Zhao in Yuan et al., Reference Yuan, Zhao, Wang, Zhou and Chen1997; Arthricocephalus chauveaui Bergeron, Reference Bergeron1899; Arthricocephalites xinzhaiheensis Chien and Lin in Lu et al., Reference Lu, Zhang, Qian, Zhu, Lin, Zhou, Qian, Zhang and Wu1974; A. intermedius Zhou in Lu et al., Reference Lu, Zhang, Qian, Zhu, Lin, Zhou, Qian, Zhang and Wu1974; Duyunnaspis duyunensis Zhang and Qian in Zhou et al., Reference Zhou, Liu, Meng and Sun1977; Balangia balangensis Chien, Reference Chien1961; Changaspis elongata Lee in Chien, Reference Chien1961) and other arthropods (Tuzoia retifera Walcott, Reference Walcott1912; Occacaris lazizhaiensis Wen and Peng in Wen et al., Reference Wen, Peng, Liu and Liang2017; Isoxys acutangulus [Walcott, Reference Walcott1908] Simonetta and Delle Cave, Reference Simonetta and Delle Cave1975; I. auratus [Jiang, Reference Jiang, Luo, Jiang, Wu, Song and Ouyang1982] Hou, Reference Hou1987; I. jianheensis Liu et al., Reference Liu, Peng, Wen and Liang2018a; Comptaluta kailiensis Peng et al., Reference Peng, Sun, Zhao and Tai2010a, C. inflata [Zhang, Reference Zhang1974] Hou et al., Reference Hou, Siveter, Williams and Feng2002; Alutella elongta Peng et al., Reference Peng, Sun, Zhao and Tai2010a; Anomalocaris sp.), as well as many trace fossils.

This fossil assemblage includes benthic, nektobenthic, and nektonic to planktonic organisms. The taphonomic and ecological characteristics of this fossil assemblage indicate an outer-shelf deep setting, and those of the fossil-bearing horizon represent a low-energy depositional environment (Liu et al., Reference Liu, Wen, Peng, Liang and Wang2017, Reference Liu, Peng, Wen, Wang and Du2018b).

Type species

Saetaspongia densa Mehl and Reitner in Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993 from the Niutitang Formation at Sansha, Dayong County, Hunan, China, by original designation.

Other species

Saetaspongia cf. S. densa Mehl and Reitner in Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993; Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010, p. 352, pl. 2, figs. 3, 4; Botting and Peel, Reference Botting and Peel2016, p. 473, fig. 6A, B. Saetaspongia procera Botting and Peel, Reference Botting and Peel2016 (p. 475, fig. 6C, E).

Diagnosis

Moderately small to large, thin-walled, circular, prolate ovoid, or elongated to cylindrical sponges. Primary skeleton composed of fine or large spicules (monaxons), and these spicules densely arranged. Hexactines and hexactinellid-like spicules absent. Body wall possessing (at least) two major plumose arrays in the outer part of the body wall. Body surface with irregularly positioned, large or fine spicules (either monaxons or irregular stauractines).

Occurrence

This genus has three previous records reported from the Cambrian (Stages 2–3) of South China (Saetaspongia densa from the Niutitang Formation of Dayong County, Hunan Province, South China and S. cf. S. densa from the Niutitang Formation of Danzhai County, Guizhou Province, South China) and the Cambrian (Stage 3) of North Greenland (S. procera and S. cf. S. densa from the Buen Formation).

Remarks

The presented diagnosis is emended from that of Steiner et al. (Reference Steiner, Mehl, Reitner and Erdtmann1993) and Botting and Peel (Reference Botting and Peel2016), for the reasons discussed below. To date, the accurate phylogenetic position of Saetaspongia is unclear. It is an enigmatic genus that so far cannot be definitely assigned to any class-level crown group (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993; Chen et al., Reference Chen, Müller, Hou and Xiao2015; Botting and Peel, Reference Botting and Peel2016). Because a competing scenario that involves the diagnostic features of this genus has emerged, the interpretations of Saetaspongia have fallen into two distinct schools: one based on the presence of plumose arrays (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993; Botting and Peel, Reference Botting and Peel2016), and the other based on the (possible) presence of hexactines (Rigby and Hou, Reference Rigby and Hou1995; Yang and Zhao, Reference Yang and Zhao2000; Zhao et al., Reference Zhao, Yang, Yang and Mao2006; Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010; Hou et al., Reference Hou, Siveter, Siveter, Aldridge, Cong, Gabbott, Ma, Purnell and Williams2017; Wang et al., Reference Wang, Yang, Zhao and Duan2017). The diagnosis proposed by Steiner et al. (Reference Steiner, Mehl, Reitner and Erdtmann1993) emphasized only two critical features: (1) the primary skeleton is dominated by dense accumulations of fine diactines arranged in plumose-appearing bundles, and (2) spicules do not protrude beyond the outer surface of the sponge body. Due to rock breakage, the outer margin of some articulated specimens may be damaged and broken. Therefore, diagnostic feature (2) cannot be observed on all relevant specimens and is not demonstrably useful in their classification. However, diagnostic feature (1) is a reliable feature to use in classification and can be effectively applied to fully articulated specimens and even to partly articulated specimens (Botting and Peel, Reference Botting and Peel2016, fig. 6C–E). It should be noted that although some spicules from the specimens described by Steiner et al. (Reference Steiner, Mehl, Reitner and Erdtmann1993) might be tri-, tetr- or even hexactines, these spicules were not considered to belong to those specimens by the authors (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993). Especially regarding the critical features provided by the material from the Sirius Passet Biota (Botting and Peel, Reference Botting and Peel2016), specimens lacking plumose structures but possessing hexactines should not be assigned to Saetaspongia. In this context, the presence of hexactine spicules is not considered to be a diagnostic feature of the genus, and the assignment of specific specimens to Saetaspongia based on the (possible) presence of hexactines is problematic. It has been clear and unambiguous that Saetaspongia does not have hexactine-based spicules and should not be classified as a hexactinellid.

Holotype

JLSHM130 (Fig. 2.1), one nearly complete specimen with a fully articulated skeleton and a well-defined outer margin of sponge body; collected from the Balang Formation at Lazizhai Village, Jianhe County, Kaili City, Guizhou, China.

Diagnosis

Moderately small, probably circular sponges. Skeleton consists of loose to dense accumulations of fine spicules arranged in plumose-appearing bundles. These fine spicules of primary arrays are monaxons. There is one major plumose array with an uncertain number (perhaps two) of small plumose domains. The primary skeleton is interspersed with some irregularly oriented individual spicules that probably are monaxons.

Occurrence

The only known occurrence is from the Cambrian (Stage 4) Balang Formation in Lazizhai Village, Jianhe County, Kaili City, Guizhou, China.

Figure 2.

Saetaspongia jianhensis n. sp. (1) the holotype, specimen JLSHM130; (2) an overall image of the sponge showing the positions of the other close-up images (Fig. 2.3–2.6) (dotted and solid boxes); (3) few spicules beyond the margin (arrow); (4) the angle of divergence increasing to 65° at the apical of the large plume (box); (5) a small plumose array; (6) the other small plumose array (box) and this region shown in close-up in Figure 2.7; (7) two minor plumes are superimposed on the primary skeleton (boxes); (8) enlargement of the central region of the fossil specimen, showing that spicules radiate from the center towards the periphery and are in sub-parallel arrangement; (9) the primary skeleton with some irregularly oriented individual spicules (arrows). All scale bars represent 2 mm.

Description

Material consists of a single nearly complete flattened specimen, 13.3 mm long and 9.7 mm wide. The outline of the sponge body is presumed to have been circular to ovoid before compaction. Oscular margin cannot be distinguished, but the outer margin of the sponge body is visible.

Skeletal architecture is distinctive, and spicules are inclined to be densely arranged so that they are in frequent contact (Fig. 2). Specimen shows that these spicules are restricted within the body wall and do not protrude beyond the outer surface of the sponge body (Fig. 2.1). Very few spicules extend up to 1 mm beyond the margin (Fig. 2.3); these slightly projecting spicules may have acted as anchoring spicules or resulted from a taphonomic artefact. Spicules are fine monaxons, probably oxeas. Limitations of preservation prevent clear indications of spicule terminations, and no distinctive or rounded terminations are observed. Majority of visible spicules are 0.28–0.37 mm long and 0.04–0.06 mm wide, but the full length is obscured by adjacent spicules. Many spicules >2 mm long in the outermost part of the specimen (Fig. 2.3), and their maximum observed thickness is ~0.1 mm.

Spicules are inclined to be loosely to densely arranged into plumose arrays; the skeleton is composed primarily of plumose bundles. The plumose region covers approximately two-thirds of the total area of the fossil on the specimen. It seems that there are one major plumose array (Fig. 2.1, 2.2) and at least two small plumose arrays (Fig. 2.5, 2.6). Spicules within the large plume diverge outwards from a well-defined, curved axis; the initial angle of divergence is very shallow, especially in the central area of the plume (Fig. 2.1, 2.4); angle of divergence increases to 65° at the apical part of the large plume (Fig. 2.4). In the other area, there are two additional small plumose spicule arrays (Fig. 2.5, 2.6), the spicules of which are slightly arranged and are parallel to the plumose axis (Fig. 2.5) or parallel to part of the diverging plume spicules (Fig. 2.6). In addition, minor plumose structures also can be found to be superimposed on the primary skeleton and do not affect it (Fig. 2.7). The positioning of these minor plumes relative to body orientation is unknown. Additionally, some spicules generally radiate from the center towards the periphery and are in sub-parallel arrangement (Fig. 2.8). Central region of the fossil specimen is slightly confused, with extremely fine spicules distributed. Dense arrangement of these fine spicules makes full recognition of these individual spicules impossible.

Primary skeleton is occasionally interspersed with some irregularly oriented individual spicules (Fig. 2.9). There is no consistent pattern of ray overlap, which may be only a reflection of the suppression of two separated spicules. Some individual spicules cross the primary skeletal spicules at high angles. These individual spicules are usually 0.1 mm wide, with a few reaching up to 0.75 mm long and 0.5 mm in diameter.

Etymology

The species name jianhensis refers to the fossil discovered from Jianhe County.

Remarks

Saetaspongia fossils have been reported from the Niutitang Formation of South China (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993; Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010, pl. II, fig. 4) and the Buen Formation of North Greenland (Botting and Peel, Reference Botting and Peel2016). These specimens can fully reveal a consistent organization to the skeleton that contains some local plumose arrangements of spicules (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993, text-fig. 13; Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010, pl. II, fig. 3; Botting and Peel, Reference Botting and Peel2016, fig. 6A, B). The plumose architecture is a critical diagnostic feature of Saetaspongia. The fundamental skeletal architecture of the present specimen shows plumose structures and broadly conforms to that of previously reported Saetaspongia fossils (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993; Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010; Botting and Peel, Reference Botting and Peel2016). Therefore, there seems to be little doubt that the material here should be assigned to this genus.

Comparison should be made between the present specimen and the Sansha and Sirius Passet specimens. The Sansha specimens were referred to Saetaspongia densa by Steiner et al. (Reference Steiner, Mehl, Reitner and Erdtmann1993) and somewhat resemble S. jianhensis n. sp. in body form and spicule features, including a circular outline, the plumose structure, and irregular fine spicules. However, the Sansha specimens differ dramatically in the following features (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993): skeleton composed primarily of a dense, radiating array of fine monaxons; spicules generally radiate towards the periphery; spicules line the outer margins as dense, tangential bundles.

Three specimens from the Sirius Passet Biota have been assigned to Saetaspongia cf. S. densa (Botting and Peel, Reference Botting and Peel2016) and share some similarities in terms of their skeletal architecture with S. jianhensis n. sp., including irregularly oriented individual spicules and several spicule bundles. The Sirius Passet material represents prolate ovoid sponges, with spicules becoming dominantly transverse at the lowest part of the body (Botting and Peel, Reference Botting and Peel2016). However, the irregularly (sub-)transverse to oblique spicules are not dominant in the Balang material.

Saetaspongia cf. S. densa Mehl and Reitner in Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993
Figures 3, 4

Reference Yang, Zhao, Zhu, Cui and Yang2010

Saetaspongia densa; Yang et al., p. 352, pl. 2, figs. 3, 4.

Reference Botting and Peel2016

Saetaspongia cf. S. densa; Botting and Peel, p. 473, fig. 6A, B.

Figure 3.

Saetaspongia cf. S. densa. (1, 2) Specimen JLSHM406 and its counterpart, showing a large sponge body, the major plumose array (two large boxes), two small plumose arrays (indicated by a small box and this region shown in close-up in Fig. 3.3) and sub-parallel large spicules frequently crossed or overlapped by many irregularly oriented large spicules (indicated by a small box in Fig. 3.1; this region shown in close-up in Fig. 4.4); (3) two small plumose arrays (boxes). (1, 2) scale bars = 10 mm; (3) scale bar = 2 mm.

Figure 4.

Saetaspongia cf. S. densa. (1) Specimen JLSHM406, showing the positions of the other close-up images (Fig. 4.2, 4.3, 4.5) and many irregularly oriented large spicules (arrows) crossing or overlapping on sub-parallel large spicules; (2) enlargement of Fig. 4.1, showing two probable small plumose arrays; (3) enlargement of Fig. 4.1, showing sub-parallel large spicules crossed by an irregularly oriented large spicule, crossing each other at high angles (indicated by arrow); (4) sub-parallel large spicules are frequently crossed or overlapped by many irregularly oriented large spicules (arrows); this image is the enlargement of a region in Figure 3.1 (indicated by a small box in Fig. 3.1); (5) enlargement of Figure 4.1, showing the plumose array interspersed with some irregularly oriented individual spicules that cross each other at high angles (indicated by arrows). (1, 4, 5) scale bars = 10 mm; (2, 3) scale bars = 2 mm.

Holotype

An articulated specimen (San 102-L) from the Niutitang Formation, Hunan, China (Steiner et al., Reference Steiner, Mehl, Reitner and Erdtmann1993, pl. 3, fig. 1).

Description

A single large specimen (Fig. 3.1, 3.2) represents a partial sponge and shows the critical features that allow identification. Due to irregular rock breakage, no fragment shows the full size of the sponge. The small fragment is partly obscured by rock (Fig. 3.2). The large fragment is >108.46 mm long and 61.46 mm wide (Fig. 3.1). Orientation of these fragments is difficult to determine because the specimen represents a partial sponge.

Spicules are mostly large monaxons, while a few are coarse monaxons. Most spicules are 10.2–15.4 mm long and ~0.05–0.1 mm in diameter. Primary skeleton is dominated by an uncertain number of diverging plumes of large spicules, including one major plumose array (Fig. 3.1, 3.2) and perhaps at least two small plumose arrays (Fig. 3.2, 3.3). Initial angle of divergence is relatively shallow, gradually increasing towards the distal apex of plumose bundles and approaching 15–20° at the apical part (Fig. 3.1–3.3). Central spicules within the plume continue longitudinally towards the apex (Fig. 3.1–3.3). The positioning of these plumose regions relative to the body orientation is unknown. Plumose arrays are interspersed with some irregularly oriented individual spicules (Fig. 4.5). There is no consistent pattern of ray overlap, which may be only a reflection of the suppression of two separated spicules. Some spicules cross each other at high angles (Fig. 4.5). Other areas are composed largely of sub-parallel large spicules (Fig. 4.1, 4.4). These sub-parallel large spicules are frequently crossed or overlapped by many irregularly oriented large spicules (monaxons), with some rays crossing the primary skeletal spicules at variable angles (Fig. 4.3, 4.4).

A few irregularly oriented large spicules are much larger, up to 20.7 mm long and 0.1 mm in diameter. They are superimposed on the primary skeletal layer (Fig. 4.1).

Remarks

According to the specimens described by Steiner et al. (Reference Steiner, Mehl, Reitner and Erdtmann1993) and Botting and Peel (Reference Botting and Peel2016), the current understanding of Saetaspongia is that the primary skeleton consists of some local plumose arrangements of spicules. The plumose architecture is a critical diagnostic feature of Saetaspongia. The fundamental skeletal architecture of the present specimen conforms well with that of S. cf. S. densa described by Yang et al. (Reference Yang, Zhao, Zhu, Cui and Yang2010, pl. II, fig. 4) and Botting and Peel (Reference Botting and Peel2016, fig. 6B), including primary plumose arrays and overlying irregular, coarse monaxons. Additionally, the specimens described here show similar dimensions to those of the largest specimens from the Niutitang Formation of Guizhou (Yang et al., Reference Yang, Zhao, Zhu, Cui and Yang2010) and the Buen Formation (Botting and Peel, Reference Botting and Peel2016). For these reasons, and especially given the typical plumose structure, there seems to be little doubt that the material here should be assigned to the genus Saetaspongia, and therefore is placed tentatively in S. cf. S. densa.