Flourishing chancelloriids from the Cambrian Kaili Biota of South China

ABSTRACT

Diverse chancelloriids from two sections of the Kaili Biota (Cambrian Wuliuan Stage) in Guizhou Province, China, are systematically described. A total of 25 complete individuals were collected from calcareous silty mudstones of the Cambrian Kaili Formation and are assigned to 3 genera and 6 species, including Archiasterella anchoriformis sp. nov., Chancelloria zhaoi sp. nov., C. eros, Allonnia erjiensis, Al. phrixothrix, and Al. sp. The new species Ar. anchoriformis with sclerites characterised by a large angle between two marginal-lateral rays and an obvious apical tuft represents the first unambiguous Archiasterella scleritome in South China. The C. zhaoi is dominated by a series of bilaterally symmetrical, rosette-like sclerites that composed of five or six lateral rays and a central ray. Moreover, based on a careful survey of the spatial-temporal distribution of chancelloriids (including both scleritome and isolated sclerite fossils) in South China, two flourishing ages, though may be related to preservational bias, of this metazoan group are recognised: 1) a significant diversification in the upper Stage 2 to Stage 3 and 2) a thriving in the Wuliuan Stage.

KEYWORDS:

• Cambrian
• Kaili Biota
• Chancelloriida
• scleritomes
• South China

Introduction

Chancelloriids are a group of enigmatic animals that likely represent a stem group Eumetazoa (Janussen et al. 2002; Bengtson and Collins 2015; Yun et al. 2021). This animal is characterised by a radially symmetrical body, a unique orifice, a sessile lifestyle, and a flexible integument covered with dozens of composite hollow sclerites (Bengtson and Hou 2001; Janussen et al. 2002; Randell et al. 2005; Moore et al. 2014; Bengtson and Collins 2015; Yun et al. 2018, 2019a) that were formed with fibrous aragonites (James and Klappa 1983; Bengtson et al. 1990; Mehl 1996, 1998; Porter 2008; Yun et al. 2021).

The first appearance of chancelloriids may be traced back to Cambrian Precambrian–Cambrian transition (Zhu et al. 2017). From then on, they have experienced modest diversification and eventually went extinct in the late Cambrian (e.g. Mostler and Mosleh-Yazdi 1976; Bengtson and Hou 2001; Janussen et al. 2002; Paterson et al. 2016; Hong et al. 2016; Yun et al. 2019a). Isolated sclerites of chancelloriids are universally reported in early and middle Cambrian ‘SSFs’ (Small Shelly Fossils), while scleritomes and complete specimens have been recorded in the Lagerstätten due to extraordinary taphonomic events (e.g. Utah: Rigby 1978; Janussen et al. 2002; Canada: Randell et al. 2005; Harvey and Butterfield 2011; Bengtson and Collins 2015; South China: Chen and Zhou 1997; Hou et al. 1999; Bengtson and Hou 2001; Zhao et al. 2005; Zhao et al. 2018). However, the exact phylogenetic position of this animal group is still to be discovered.

Articulated chancelloriids had a cosmopolitan distribution in Konservat-Lagerstätten ranging from Cambrian Stage 3 to Drumian (Chen and Zhou 1997; Bengtson and Hou 2001; Bengtson and Collins 2015; Paterson et al. 2016; Cong et al. 2018; Yun et al. 2018; Zhao et al. 2018, p. 2019a). Based mainly on the structure, density, arrangement and symmetry of sclerites, 14 species and 5 genera have been distinguished. Almost all species of the genus Allonnia are characterised by sclerites composed of two lateral rays and one ascending ray. Whereas in the genus Archiasterella, the number of lateral rays is not less than three in sclerites. Chancelloria can be confidently discriminated from other genera by the presence of the central ray in a sclerite (Bengtson and Collins 2015; Yun et al. 2018, 2019a). In addition to the common types mentioned, the sclerites of Nidelric and Dimidia are formed with single ray and double rays, respectively (Hou et al. 2014; Zhao et al. 2018; Yun et al. 2019a; Yun et al. 2023).

In Kaili Biota, chancelloriids were only cursorily reported and not fully illustrated (Zhao et al. 2005, 2011). Based on new materials and restudy on the reported specimens, the underrated diversity and taxonomy of the Kaili chancelloriids are revealed. In addition to the previously reported C. eros and a poorly preserved Allonnia sp. (misinterpreted as Archiasterella sp.), there are four species of three genera discovered herein. With these Wuliuan taxa, the biogeographic and biostratigraphic distribution patterns of chancelloriids in South China are systematically summarised.

Geological setting, materials and methods

The Kaili Formation is a clastic shelf succession belongs to the narrow Jiangnan Slope basin of the south China during the Cambrian period (Yuan et al. 2002; Lin 2009; Gaines et al. 2011) and crops out in eastern Guizhou Province. Palaeogeographically, the Cambrian Kaili Biota is located along the southeastern margin of the Yangtze Platform (Figure 1A, 1B). It is perfectly exposed near the Balang village, Jianhe County of Guizhou, where the Kaili Formation at the Wuliu-Zengjiayan section (Figure 1C) was recently ratified as the GSSP (Global Stratotype Section and Point) for the Cambrian Miaolingian Series and Wuliuian Stage (Zhao et al. 2019).

Figure 1. Location and geological setting of the studied area. A, map showing the location of the research area (red star) in the Yangtze Platform of South China. B, lithofacies reconstruction of South China during the Cambrian Miaolingian (after Feng et al. 2004; Zhang et al. 2008) with the studied area indicated by a red star. C, geological map of the Balang area and location of the Wuliu-Zengjiayan, Miaobanpo sections and the Jinyinshan quarry (modified from Zhao et al. 2019). D, stratigraphical column of the Kaili Formation at the Miaobanpo section, with fossil horizons indicated (after Yang et al. 2016).

The Kaili Formation in the Wuliu-Zengjiayan section and the adjacent Miaobanpo section conformably overlies the Tsinghsutung Formation (a succession of thinly bedded dolostones) and is overlain by the Jialao Formation (a mixture of siliciclastic and carbonate rocks). The thickness of the formation varies between sections, 214 m in the Wuliu-Zengjiayan section and 156 m in the Miaobanpo section (Figure 1C, 1D) where its lowermost part is cut by a small fault (Zhao et al. 2005). Lithologically, the Kaili Formation is subdivided into three parts, the lower part comprises thinly bedded limestones with shale interbeds and belongs to Cambrian Stage 4, the middle part consists of calcareous silty mudstones and shales , belonging to the early of the Cambrian Wuliuian (within the trilobite Oryctocephalus indicus zone; Zhao et al. 2011, Zhao et al. 2019), and the upper part consists of bedded limestone layers grading into grainy dolostones (Figure 1C). The middle part of the Kaili Formation contains a Burgess Shale-type Lagerstätte that known as the Kaili Biota. The biota occurs slightly above the FAD of Oryctocephalus indicus, its species diversity and abundance are dominated by panarthropods, other major groups include algae, sponges, chancelloriids, cnidarians, ‘worms’, medusiform fossils, brachiopods, molluscs, echinoderms and various problematic fossils (Zhao et al. 2005, 2011).

A total of 25 chancelloriid fossils in this study were collected from the calcareous silty mudstone beds in the middle part of Kaili Formation at the Miaobanpo section and the nearby Jinyinshan quarry (Figure 1C), corresponding to the lower part of the trilobite Oryctocephalus indicus Zone (Figure 1D). There are 20 specimens from the Miaobanpo section, and the rest of 5 were from the Jinyinshan quarry.

Most chancelloriid is preserved as pyritised scleritomes (Figures 3-6, 7B, 7D, 7E; 8) and only one specimen is as carbonaceous compression (Figure 7C). Some fossils sometimes occur in small clusters (Figures 4A, 6A, 8A) in which individual vary from size and shape, possibly suggesting in situ preservation. Some individuals, in addition, are attached onto brachiopods (Figures 3B, 4B), spherical structures (Figure 7B, 7D; less than 1 mm in diameter, spherical or sub-spherical, black, irregularly arranged in string, hollow in the centre and containing miniature quartz crystals and iron-stained powders (Lin et al. 2006).), hyolithoids (Figure 7C), and even other chancelloriids (Figures 6A, 8A). This is not the only case that chancelloriids attached to other benthos, some Burgess specimens, were also revealed as attaching to sponges and brachiopods (Bengtson and Collins 2015).

Figure 2. Terminology of chancelloriid sclerites. The lower right corner shows the schematic diagram of angle measurement between two marginal-lateral rays (modified from Bengtson and Collins 2015 and Yun 2019).

Figure 3. Archiasterella anchoriformis sp. nov. from the Kaili Biota. A, the fossil sketch drawing of B, dotted lines represent the indistinctive sclerites. B, the holotype scleritome (MBP-38); the white arrow points to an ascending ray of the sclerites. C, the sclerites of B (white arrow) and the proximal part of sclerites slightly swells. D, overall morphology, specimen no. MBP-43. E, overall morphology, specimen no. MBP-34. F, middle part of the body, specimen no. MBP-44. G, the apical tuft of MBP-43, is the countpart of D. H, J-K, the sclerites of D, E and F respectively, white arrow points to the ascending rays. I, the collapse of rays of D. Scale bars: A-B, D-F = 2 mm; C, G-H, J-K = 0.5 mm; I = 0.2 mm.

Figure 4. Archiasterella anchoriformis sp. nov. from the Kaili Biota. A, specimen no. JYS-1074. B, JYS-1074-1 of A (the lower left), 2 + 1A sclerites is present (red frame). C, JYS-1074-2 of A (the upper right). D, the specimen preserved from upper direction of A (the lower right), uncertain its type. E, detail of B (position marked by white frame). Scale bars: A = 5 mm; B-C = 2 mm; D = 1 mm; E = 0.5 mm.

All specimens illustrated here are housed in College of Resources and Environmental Engineering, Guizhou University, Guiyang, China. Some were prepared using a needle under a binocular microscope, for revealing parts covered by sediments. Digital photographs were taken by using a Canon 5D Mark Ⅳ camera with auxiliary light to increase the colour contrast of the morphological structures. Images and line drawings were made by using Coreldraw 19, Adobe Photoshop 2015.5 and image J software.

The morphological terms of the chancelloriid scleritome in Bengtson and Collins (2015) and sclerite formula in Yun et al. (2019b) are followed when describing materials. Structure of the sclerite is formulated as m+ nC and m+ nA, where m is the number of lateral rays, n is the number of C (central rays, only in genus Chancelloria) or A (ascending rays, applying to genera Archiasterella and Allonnia) (Yun et al. 2019b). MBP and JYS in specimen catalogue number indicate that the fossils came from the Miaobanpo section and the Jinyinshan quarry, respectively; and A and B denote part and counterpart of the fossil.

Additionally, the angle between lateral rays of the sclerites is an important feature in the classification of chancelloriids. In order to avoid unreliable results, three criteria are suggested in the measurement (Figure 2): 1) the site for measurement should be close to the proximal part of lateral rays; 2) measuring angles between the central axis of the two lateral rays; 3) taking the mean values of the angles.

Figure 5. Chancelloria zhaoi sp. nov. from Kaili Biota. A, the fossil sketch drawing of B, dotted lines represent the uncertainty of sclerites. B, the holotype scleritome (MBP-42-1). C, apical tuft and orifice of B, the white arrow point to a stoutly composite sclerites. D, the paratype scleritome (MBP-46-3). E, 6 + 1C and 5 + 1C sclerites of C. zhaoi sp. nov., the length of ‘adr’ (adapical lateral ray) is longer than ‘abr’ (abapical lateral ray). Scale bars: A-B, D = 1 mm; C = 0.5 mm.

Systematic palaeontology

Phylum and Class uncertain

Order Chancelloriida Walcott 1920

Family Chancelloriidae Walcott 1920

Genus Archiasterella Sdzuy 1969

Type Species

Archiasterella pentactina Sdzuy 1969 (based on isolated sclerites).

Other representative species

Ar. tetraspina Vassiljeva and Sayutina 1993; Ar. fletchergryllus Randell et al. 2005; Ar. coriacea Bengtson and Collins 2015.

Archiasterella anchoriformis sp. nov.

(Figures 3–4)http://zoobank.org/BB46368F-C974-43E2-AFA1-BB40EDA1E3D4

Derivation of name

From the Latin ‘anchora’ (anchor) and ‘formis’ (shape), in reference to the shape of sclerites closely resembles an anchor.

Holotype

MBP-38A, B, from the Wuliuan Stage of Cambrian, Kaili Formation, Miaobanpo section, Kaili, Guizhou Province, China, housed in Institute of Palaeontology, College of Resources and Environmental Engineering, Guizhou University.

Paratypes

MBP-43A, B, from the same fossil locality as above.

Other Materials

MBP-34, , MBP-44A, B, and MBP-50, from the Miaobanpo section; JYS-1072A, B, JYS-1074 A, B-1, and JYS-1074 A, B-2, from the Jinyinshan quarry of the Kaili Biota.

Diagnosis

Species of Archiasterella having a spindle-shaped body with 3 + 1A sclerites and rare 2 + 1A sclerites, which are characterised by a bilateral symmetry, and the angle between two marginal-lateral rays is usually more than 120°. The apical orifice is surrounded by densely distributed slender, spine-shaped sclerites that form a teepee-like apical tuft.

Description

The overall shape of the holotype (Figure 3A, 3B) is spindle-shaped, ca 15.4 mm high, with a flat apical part (ca 2.6 mm wide), an inflated middle part (5.7 mm maximum width), and a narrow basal end (ca 1 mm wide). The apical orifice is surrounded by a dozen of sclerites consisting of modified spine-shaped rays (0.5–1.7 mm long, ca 0.025 mm wide), and the contracted basal end anchors to a brachiopod fragment. The sclerite formula of the specimen is 3 + 1A, including three lateral rays and one ascending rays (as indicated by the white arrow in Figure 3B). All rays apically directed, and the lateral rays close the body surface while ascending rays curved away from the body surface. Remains of the basal part of the ascending ray are preserved. The proximal part of sclerites slightly swells and the distal part keeps original size (Figure 3C, ca 0.09 mm width). The middle-lateral rays are straight, while the two marginal-lateral rays curve towards each other within the plane formed by lateral rays. Therefore, the sclerites is bilaterally symmetrical and look like an anchor. At least 35 sclerites are irregularly arranged on the body. The separation of sclerites is inhomogeneous, ranging from 0.6 to 1.4 mm; the orientation of some sclerites in different positions of the body are a little deviate from the apical part. There is no obvious size variation of the sclerites throughout the scleritome.

The paratype MBP-43 (Figure 3D) is 36.5 mm high, showing a ‘lamp bulb-shaped’ outline that includes a widest upper part (ca 16.5 mm wide) and a cylindrical lower part (ca 11.2 mm) with a rounded basal end. The counterpart of this specimen reveals a complete apical tuft composed of densest spine-shaped sclerites (Figure 3G, ca 6.1 mm high and 7.6 mm wide) when compared to other chancelloriids. The distal parts of these spine-shaped sclerites (about 0.1 mm in diameter) are closely converged to the central axis of the body and firmly shelter the apical orifice. The sclerites near the apical area (at least 3.4 mm in length and 0.3 mm in width of middle lateral ray) are bigger than the sclerites in the lower part (at least 1.4 mm length and 0.2 mm width of middle lateral ray). The marginal-lateral rays in this specimen form a 137° angle between each other (Figure 3H).

The specimen MBP-34 is of medium size, 28 mm high and 13.8 mm wide, (Figure 3E). The density of spine-shaped sclerites of apical tuft is between MBP-43 and MBP-38 (holotype). The basal part is incomplete and covered by sediments. The size and irregular arrangement pattern of sclerites is similar to that of the holotype. At the proximal part of the composite sclerites (the contact position of adjacent rays), four rays (three lateral rays and one broken ascending ray) in the plane can be obviously observed (Figure 3J).

A similar characteristic of sclerites can be seen in the incomplete specimen (MBP-44, Figure 3F), which is 22.5 mm high and 8.4 mm wide. The angle between marginal-lateral rays of the sclerites is about 120°(Figure 3K), larger than that of the holotype. Size and arrangement of sclerites are also irregular.

The specimen JYS-1074-1 (Figure 4B) is preserved in the large slab JYS-1074 (Figure 4A, 54 × 35 mm²), which was buried together with other individuals, including the specimen JYS-1074-2 (Figure 4C, see below) and a specimen originally compressed from top (Figure 4D).

The specimen JYS-1074-1 is of the smallest size of Ar. anchoriformis sp. nov., which has a baglike body 10.1 mm high and 5.5 mm wide (at the upper part of the body). The widths of the apex and the contractive basal end are about 4.0 mm and 1.4 mm, respectively. Like other specimens, it has a conspicuous apical tuft, a series of 3 + 1A sclerites with large angle between the marginal-lateral rays (ca 130°, Figure 4E). The basal end is attached to a brachiopod fragment. The sclerites at the lower part of the body are smaller, more densely arranged than the upper part (from the lower to upper part, the ray length is from 0.8 mm to 1.3 mm and the intervals of sclerites from 0.5 mm to 1.4 mm). A few small 2 + 1A sclerites are scattered on the body margin (the lateral rays form an angle of 34°–53°).

The specimen JYS-1074-2 (Figure 4C) has a cylindrical body with a slightly contracted middle part. The height is 17.9 mm, the maximum width is 5.2 mm, the minimum width 2.98 mm (at the middle part). Dozens of sclerites are irregularly arranged in the scleritome. The sclerites are discerned as two forms: 3 + 1A and rare 2 + 1A.

Comparison. In accordance with the typical sclerite formula (lateral rays + one ascending ray, the number of lateral rays is mostly three or more), this species belongs to the genus Archiasterella (Yun et al. 2019b). There are at least five other species characterised by 3 + 1A sclerites, including Ar. tetraspina (= ?Ar. tetractina Duan 1984), Ar. cometensis Moore et al., 2019, Ar. uncinata Moore et al., 2019, Ar. hirundo Bengtson in Bengtson et al. 1990, and Ar. coriacea Bengtson and Collins 2015. Although this genus is widely distributed all over the world, most of them are represented by the isolated sclerites instead of complete individuals. To date, except for the Ar. anchoriformis described herein, only Ar. coriacea was established based on exceptionally preserved scleritome materials, and it has peculiar integument with a scaly surface pattern and an inconspicuous apical tuft (Bengtson and Collins 2015). As mentioned above, the average angle between the marginal-lateral rays in the sclerites of Ar. anchoriformis is is generally greater than that of Ar. coriacea (see Annex Figure A1, Figure A2, Figure A3, Figure A4). Moreover, the apical tuft formed by dozens of single-rayed sclerites is more conspicuous in Ar. anchoriformis.

Genus Chancelloria Walcott 1920

Type Species

Chancelloria eros Walcott 1920 from the Burgess Shale (based on scleritome).

Other representative species

C. cruceana Rusconi 1954; C. pentacta Rigby 1978; C. irregularis Qian 1989; C. australilonga Yun et al. 2019a.

Chancelloria zhaoi sp. nov.

(Figure 5)

http://zoobank.org/FD0F2F20-E3BE-474C-8329-BD25E6D5ACEC

Derivation of name

To commemorate professor Zhao Yuanlong of Guizhou University for discovering and extraordinary contributions on the Kaili Biota.

Holotype

MBP-42, from the Wuliuan Stage of Cambrian, Kaili Formation, Miaobanpo section, Kaili, Guizhou Province, China, housed in the Institute of Palaeontology, College of Resources and Environmental Engineering, Guizhou University.

Paratype

MBP-46, from the Miaobanpo section of the Kaili Biota.

Diagnosis

Saclike body mainly composed of 5 + 1C and 6 + 1C sclerites with typical bilateral symmetry. The lengths of adapical and abapical lateral rays are significantly different in the same sclerite. Generally, the size and density of sclerites are almost unchanged throughout the scleritome, and the arrangement is irregular. The apical tuft is conspicuous.

Description

The holotype (MBP-42, Figure 5A, 5B) has a saclike body about 11.2 mm high. The abapical part tapers down to a blunt end and the apex is encircled by a series of sparsely distributed slender single-rayed sclerites (Figure 5C). The rays of these sclerites are around 0.04 mm in diameter while the thickest one is up to 0.2 mm (Figure 5C, marked by a white arrow). The width of both ends of the body is similar, and the widest part is at the upper part (at least 4.2 mm wide). The sclerites of C. zhaoi sp. nov. are bilaterally symmetrical and can be classified into two structures: 6 + 1C and 5 + 1C (Figure 5E), in which the central rays are mostly broken off and not preserved. In the 6 + 1C sclerites, abapical lateral rays and adapical lateral rays are both three in number. The shortest ray in a sclerite is the abapical middle-lateral ray. The structure of the 5 + 1C sclerites is similar to that of the 6 + 1C sclerites in a lacking of a abapical middle-lateral ray. The density and size of sclerites are almost unchanged throughout the whole body.

The paratype MBP-46 (Figure 5D) is incomplete, with a hemispherical body that is 3.6–6.5 mm wide. In this scleritome, a small number of composite sclerites are recognisable, which reveal a 5 + 1C structure and a bilateral symmetry.

Comparison

To date, the species of Chancelloria based on scleritome materials include C. eros Walcott 1920, C. cruceana Rusconi 1954, C. pentacta Rigby 1978, and C. australilonga Yun et al. 2019a (Walcott 1920; Rusconi 1954; Rigby 1978; Beresi and Rigby 2013; Bengtson and Collins 2015; Yun et al. 2019a). These species, and C. zhaoi described herein, all have 5 + 1C and 6 + 1C sclerites while the proportion and detailed structures of these sclerites vary. As described above, the sclerites of C. zhaoi have two groups of lateral rays (three adapical and two or three abapical lateral rays) with nearly opposite orientations. The conspicuous apical tuft and sclerites formed by three groups of lateral rays in different directions (including two subparallel, remarkably elongate adapical rays) and a bilateral symmetry characterise C. australilonga (Yun et al. 2019a). The C. pentacta has two adapical lateral rays, two horizontal lateral rays and an abapical lateral ray, in which the horizontal lateral rays extend outward with ray tips curving slightly upwards (Rigby 1978). There are various types of sclerites in C. eros (including 5–8 + 1C and 3–4 + 0), most of which are asymmetrical or radial symmetrical (Walcott 1920; Bengtson and Collins 2015). With regard to C. cruceana, the general structure of the sclerites is similar to that of C. zhaoi, however, the complete scleritome of this species is not preserved and there were only 6 + 1C sclerites ever discovered. In addition, the bilateral C. irregularis Qian 1989 based on isolated sclerites is similar to some of the sclerites in C. zhaoi, however, it is impractical to reconstruct the arrangement pattern of the former.

Chancelloria eros Walcott 1920

(Figure 6)

1920 Chancelloria eros; Walcott, partim; pls 86:2, 2a, 2b, ?2c, 88:1 f, non pl. 88:1,1a-e.

1973 Chancelloria eros; Goryansky, p. 43.

1978 Chancelloria eros; Rigby, pl. 2, 3.

2002 Chancelloria eros; Janussen et al., Figures 3, 4.

2005 ?Chancelloria cf. eros; Randell et al., Figure 6.

2015 Chancelloria eros; Bengtson and Collins, Figures 1, 5–11, 13–14.

Materials

A slab MBP-41 (36 × 13 mm², Figure 6A) from the Miaobanpo section of the Kaili Biota, containing an association of 4 specimens (MBP-41-1, MBP-41-2, MBP-41-3, and MBP-41-4).

Description

The specimen MBP-41-1 (Bottom of Figure 6A) has a cylindrical body that is about 16.7 mm high and 6.4 mm wide. Both ends are rounded, and the minimum width is 2.5 mm. More than 50 sclerites are irregularly arranged, while the density is generally uniform on the body wall. Sclerites in the studied specimens are mainly 6 + 1C in structure (Figure 6E), with a few 5 + 1C (Figure 6F) and 7+1C (Figure 6G), and others poorly-preserved. They are almost all asymmetrical and less common in radial symmetry, and all rays are slightly curved.

The specimen MBP-41-2 (Figure 6B) is adjacent to the upper right margin of MBP-41-1, and the size of body is about a third of the latter. The sclerites are mainly 6 + 1C forms, and their sizes gradually decrease downward the body (except for the sclerite pointed by the white arrow whose length is up to 1.7 mm).

Figure 6. Chancelloria eros from the Kaili Biota. A, including four specimens of Chancelloria eros, specimen no. MBP-41. The specimen with the largest size is MBP-41-1. B, the second specimen (MBP-41-2) from the bottom of A, the white arrow indicates that the sclerite has a longer lateral ray. C, the third specimen (MBP-41-3) from the bottom of A, the white arrow stands for the possibility of stalk structure, and the red arrow indicates that the basal part of MBP-41-3 probably anchored to the top of MBP-41-2. D, the fourth specimen (MBP-41-4) from the bottom of A. E-G, 6 + 1C, 5 + 1C and 7 + 1C sclerites of the first C. eros from the bottom of A. Scale bars: A = 5 mm; B-D = 2 mm; E-G = 0.5 mm.

Figure 7. Allonnia from Kaili Biota. A-D, F, Allonnia erjiensis. A, the fossil sketch drawing of B, dotted lines represent the uncertainty of sclerites. B, MBP-35. C, MBP-45. D, MBP-37. E, G, Allonnia phrixothrix, specimen no. MBP-40. F, detail of D (position marked by white frame). G, integument detail of E; ‘lr’, lateral ray; ‘asr’, ascending ray. Scale bars: A-E = 2 mm; F, G = 0.1 mm.

The scleritome (MBP-41-3) (Figure 6C) shows a mushroom-shaped body that is the smallest (2.48 mm high and 0.95 mm wide at its broadest area) when compared to other specimens in the slab MBP-41 (Figure 6A). The body is divided into two parts: 1) a narrow basal part (no more than 0.15 mm in width) and probably representing a contracted stalk (indicated by a white arrow in Figure 6C) probably anchored to the top of another specimen (MBP-41-2, indicated by a red arrow in Figure 6C), and 2) a bulged upper part with a series of composite sclerites.

The specimen MBP-41-4 (Figure 6D) at the upper end of the slab also has a small body size and a similar association of 6 + 1C, 7 + 1C and 5 + 1C sclerites. But the length of the rays is relatively longer (up to 3.1 mm long) than that of the MBP-41-2.

Comparison

With indistinctive apical tuft and diverse forms of sclerites that are dominated by asymmetrical or radial symmetrical of structures, C. eros is distinctive from other Chancelloria species.

Genus Allonnia Doré and Reid 1965

Type Species

Allonnia tripodophora Doré and Reid 1965 (based on isolated sclerites).

Other representative species

Al. phrixothrix Bengtson and Hou 2001; Al. tintinopsis Bengtson and Collins 2015; Al. erjiensis Yun et al. 2018; Al. nuda Cong et al. 2018; Al. tenuis Zhao et al. 2018.

Allonnia erjiensis Yun et al. 2018

Figure 7, A-D, F

2018 Allonnia erjiensis sp. nov; Yun et al., figs. 1

2019 Allonnia erjiensis; Yun, figs. 4.2D, 4.3E, 4.4 H-J, 9.6

Materials

MBP-35, MBP-45, MBP-37A, B, and MBP-48, from the Miaobanpo section; JYS-874, from the Jinyinshan quarry of the Kaili Biota.

Description

The specimen MBP-35 (Figure 7A, 7B) is slim rod-shaped, 19.9 mm high. The body was divided into equal two parts. The lower part forms a cylindrical stalk, 10.1 mm long and 1.4 mm wide, with dense and short sclerites. The upper part is spindle-shaped, 9.8 mm long and 4.8 mm broad at the widest area, with sclerites not denser than the lower part but composed of long-conical rays. The apical tuft is not preserved. The base is attached to some spherical structures (whether they are organism remains or sedimentary structures is still to be discussed). The 2 + 1A sclerites are predominated on the body surface, with ascending rays visible only in the marginal parts of the body due to their outward-pointing position (Yun et al. 2018) (Figure 7A, 7B). The length of the rays increases upward from 1.4 mm to 6.9 mm.

The specimen MBP-45 (Figure 7C) shows a slightly twisted and club-shaped body, about 10.6 mm high. The body width gradually increases from the base (0.6 mm wide) to the apex (ca 2.8 mm wide). The basal end anchored to a hyolithid conch, yet the upper part is incompletely preserved. The size change of sclerites is less evident than that of MBP-35, with ray length between 0.6 mm and 1.8 mm. Moreover, this specimen is dark grey in colour, indicating the preservation of carbonaceous films. The sclerites are 2 + 1A in structure, with ascending rays remarkably protruding out the body surface.

The specimen MBP-37 (Figure 7D) shows a narrow, incomplete body, which is 10.8 mm high and 1.8 mm wide. The basal end attaches to a dark spherical structure (similar to MBP-35). The sclerites of the upper part are longer than those of the lower part, and the length of rays ranges from 3.7 mm to 7.3 mm. In consideration of the poor preservation, the sclerite structure is not well revealed, however, some ascending rays are discernable.

Comparison

The sclerites of Al. erjiensis is the densest of all known Allonnia species, and covers the whole body surface. Other species, such as Al. nuda, Al. tenuis, Al. phrixothrix, Al. tintinopsis, usually have seldom sclerites in the basal area (Bengtson and Hou 2001; Bengtson and Collins 2015; Cong et al. 2018; Zhao et al. 2018). Compared to the holotype, specimens described herein have a distinct basal end attaching to a hyolithid conch, and the body width is much narrower though the general heights are similar (ca 10 mm −15 mm) (Yun et al. 2018; Yun 2019).

Allonnia phrixothrix Bengtson and Hou 2001

(Figure 7E, 7G)

2001 Allonnia phrixothrix sp. nov.; Bengtson and Hou, figs. 1, 2, 3, 4, 5, 7, 8, 11

2002 Allonnia junyuani sp. nov.; Janussen, Steiner and Zhu, figs. 5, 6.2

2019 Allonnia phrixothrix; Yun, figs. 4.2A, C, 4.3A–D, F, G, 4.4 A–G, 6.1, 8.1, 9.5

Material

MBP-40 from the Miaobanpo section of the Kaili Biota.

Description

Figure 7E shows an incomplete specimen, with a cylindrical, short body, 4.9 cm in high and 2.1 cm wide in the middle section. The upper part is ringed by sclerites composed of long rays and the apical tuft is not revealed. The lower part is flat. The soft integument is well preserved in reddish colour. There are obvious granular ornamentations (about 0.06 mm in diameter) on the integument (Figure 7G). Dozens of sclerites with a uniform size, shape and density are irregularly arranged in the scleritome. Each of sclerites has two lateral and one ascending rays (2 + 1A), and the rays are about 1.5 to 2.2 mm long and 0.08 mm wide. The lateral rays form a mutual angle of about 40°-50° and curve slightly towards each other, and the ascending ray leaves a pit or a tiny proximal part that were broken off due to the taphonomic processes.

Comparison

The sclerites of MBP-40 are strictly limited to the 2 + 1A form. The size, density, and structure of these sclerites are all in accordance with the holotype, except for the apical tuft.

Allonnia sp.

(Figure 8)

Material

MBP-47-1, MBP-47-2, MBP-47-3, MBP-49, MBP-39, from the Miaobanpo section of the Kaili Biota.

Description

Three incomplete specimens are preserved on the slab MBP-47 (25 × 19 mm², Figure 8A). The specimen MBP-47-1 (at the bottom right corner of Figure 8A) shows a short, ellipsoidal body, 7.7 mm high and 3.3–4.1 mm wide. The sclerite structure is discerned as 2 + 1A and (possibly) 3 + 1A, with lateral rays about 0.7–1.6 mm long (the mutual angle of lateral rays is 35–60°). Some single-rayed sclerites are densely arranged parallel to each other, forming an apical tuft (about 1.6 mm long and 1.3 mm wide) (Figure 8B). The lower part of the body is not well preserved.

The specimen MBP-47-2 in the middle part of Figure 8A has a conical shape, a height of 5.6 mm and a width (upper part of the body) of 1.9 mm. The scleritome is poorly preserved, but the lateral rays of 2 + 1A sclerites are discernable. The length of the rays dramatically increase from the lower to upper part (1.2 mm to 5.1 mm). The lower part contracts abapically to a basal end that probably attaches to the upper part of another specimen (MBP-47-3) (Figure 8A). The specimen MBP-47 in the upper left part of Figure 8A is even more poorly preserved that only a few of 2 + 1A sclerites with remains of the ascending rays can be observed (Figure 8C).

Other specimens with probable 2 + 1A sclerites are shown in Figure 8D (MBP-39) and Figure 8E (MBP-49). They have similar body shape and size (about 7–8 mm). The sclerite rays on the upper part of the body are slender than that in the three specimens described above.

Discussion

A diverse chancelloriid assemblage in the Kaili Biota

Well-preserved fossil specimens reveal a relatively high taxonomic diversity of chancelloriid assemblage in the Kaili Biota (Figure 9). Totally three genera and six species are recognised, including Archiasterella anchoriformis sp. nov., Chancelloria zhaoi sp. nov., C. eros, Allonnia erjiensis, Al. phrixothrix, and Al. sp. At the genus level of abundance, the assemblage is dominated by Allonnia (44% of the total), of which Al. erjiensis and Al. sp. occupy 45.45% of the whole genus, followed by Al. phrixothrix. In addition, Archiasterella, which accounts for a small proportion of the three genera and probably has a narrow geographic distribution, has become the most abundant type in the Kaili Biota at the species level (32% of the total). The abundance of Chancelloria is modest by comparison (24% of the total), and C. eros and C. zhaoi account for 66.7% of 33.3% of the whole genus, respectively.

Figure 8. Allonnia sp. from Kaili Biota. A, three specimens in different direction, specimen no. MBP-47. The white box indicates the basal part of MBP-47-2 probably anchored to the top of MBP-47-3. B, apical tuft of the lower right corner of A. C, sclerites detail of the upper left corner of A. D, MBP-39. E, MBP-49; ‘lr’, lateral ray; ‘asr’, ascending ray. Scale bars: A = 5 mm; B, C = 0.2 mm; D, E = 2 mm.

Figure 9. The reconstruction of chancelloriids in Kaili Biota (By Liu X). The labels 1 to 5 indicate Allonnia erjiensis, Chancelloria eros, C. zhaoi sp. nov., Archiasterella anchoriformis sp. nov., and Al. phrixothrix, respectively.

Previously, well-preserved complete scleritomes of Chancelloria were only discovered in the Burgess Shale (Walcott 1920; Bengtson and Collins 2015), Little Bear Biota (Butterfield and Nicholas 1996), and Wheeler Formation (Janussen et al. 2002) of Laurentia and Emu Bay Shale of East Gondwana (Yun et al. 2019a). The scleritomes of Archiasterella are even rarer and were only reported from the Burgess Shale (Bengtson and Collins 2015) and Sekwi Formation (Randell et al. 2005) of Laurentia. With regard to the Chengjiang and Guanshan biotas of South China, there are only Allonnia, Dimidia, and Nidelric ever discovered (Bengtson and Hou 2001; Hou et al. 2014; Cong et al. 2018; Zhao et al. 2018; Yun et al. 2018, Yun et al. 2023). Therefore, Ar. anchoriformis, C. zhaoi, and C. eros of the Kaili Biota represent the first occurrence of complete bodies of Chancelloria and Archiasterella in South China.

Spatial-temporal distribution of chancelloriids in South China

Chancelloriids were a globally distributed group which had lasted at least 45 million years (Cambrian Fortunian to Jiangshanian stages) (Yun 2019). In South China, almost all common chancelloriid genera were discovered, including Chancelloria, Allonnia, Archiasterella, Dimidia, Nidelric, and Cambrothyra (see the supplemental material. Yun 2019; Yun et al. 2019a). The earliest chancelloriids of South China were likely from the Kuanchuanpu Formation of the Fortunian Stage in southern Shaanxi Province, which are classified into Chancelloria and Cambrothyra (Fu 1983; Yang et al. 1983; Steiner et al. 2004). The sclerites of Chancelloria (associated with small shelly fossils) were discovered in a number of slightly younger localities such as Hubei (Qian et al. 1979; Qian 1989), Sichuan (Yin et al. 1980; Li 1999), and Yunnan (Luo et al. 1982; Moore et al. 2014) provinces. From late Stage 2 on, chancelloriids experienced a significant diversification, with abundant sclerites and scleritomes (of at least six genera) distributed across the Yangtze block (Qian 1989; Qian et al. 1999). While after the Wuliuan Stage, chancelloriids appeared to suffer a setback and were in serious decline with only few records from Huaqiao Formation of Hunan Province (Zhu et al. 2003), and they apparently became extinct after Cambrian the Guzhangian Stage.

In summary, two flourishing ages of chancelloriids in South China are distinguished, though they may be related to preservational bias. The first one (Flourishing Age I) is from late Stage 2 to middle stage 3, and the second one (Flourishing Age II) is the Wuliuan Stage (Figure 10).

Figure 10. Distribution of the chancelloriids in South China.The stratigraphical distribution, shows two flourishing ages of genus-level chancelloriids. References for different formation: Kuanchuanpu Fm. = Fu (1983), Yang et al. (1983) and Steiner et al. (2004); Huangshandong Fm. = Qian et al. (1979) and Qian (1989); Maidiping Fm. = Yin et al. (1980), Qian (1989), Li (1999), Feng et al. (2022); Zhongyicun to Dahai Mbs. of Zhujiaqing Fm. and Shiyantou Fm. = Luo et al. (1982), Parkhaev and Demidenko (2010), Moore et al. (2014); Middle Jiulaodong Fm. = Yin et al. (1980) and Qian (1989); Xihaoping Fm. = Qian and Zhang (1983), Duan (1984), Wang and Xu (1987), Qian (1989), Ding et al. (1990), Li et al. (2004), Steiner et al. (2004), Moore et al. (2010), Yang et al. (2015), Zhang et al. (2021) ; Lower Shuijingtuo Fm. = Li et al. (2004) and Yang et al. (2015); Qingjiang Biota = Fu et al. (2019); Chengjiang Biota = Bengtson and Hou (2001), Janussen et al. (2002), Hou et al. (2014), Yun et al. (2018), Cong et al. (2018), Sun et al. (2020), Yun et al. (2022); Upper Jiulaodong Fm. = Yin et al. (1980); Middle Guojiaba Fm. = Yun et al. (2021) ; Lower Mingxinsi Fm. = Qian (1989); Guanshan Biota = Hu et al. (2010, 2013), Chen et al. (2019), Zhao et al. (2018); Mufushan Fm. = Zhang (1995); Balang Fm. = Liu and Lei (2013); Kaili Biota = Zhao et al. (1999, 2005, 2011), Harvey et al. (2012), this study; Upper Damao Gp. = Jiang and Huang (1986), Damao Group is distributed in Hainan and likely near to the Yangtze Block in the Cambrian; Huaqiao Fm. = Zhu et al. (2003).

The Flourishing Age I is characterised by a rapid diversification of Archiasterella, Allonnia, Dimidia, and Nidelric following the widespread of Chancelloria. The genera Allonnia, Archiasterella, and Dimidia first appeared in the upper part of the Shiyantou Formation (eastern Yunnan) and middle member of Jiulaodong Formation (central Sichuan) of late Stage 2 (Luo et al. 1982; Qian 1989; Qian et al. 2002; Parkhaev and Demidenko 2010; Moore et al. 2014). As one of the globally distributed chancelloriids, Allonnia plays an important role in the diversity and abundance of chancelloriids from the well-known Chengjiang Biota. Dimidia, characterised by double-rayed sclerites, was previously regarded as a junior synonym of Allonnia, however, is discovered from scleritome fossils from the Chengjiang Biota (Yun et al. 2023). The genus Nidelric, dominated by single-rayed sclerites, is only discovered in the Chengjiang and Guanshan biotas of South China (Hou et al. 2014; Zhao et al. 2018).

The Flourishing Age II represents another thriving of diversity of the chancelloriid group. Although Cambrothyra and Nidelric likely went extinct after Stage 4, there were at least four genera, including Chancelloria, Allonnia, Dimidia, and Archasterella, flourished in the chancelloriid communities represented by the Kaili Biota (Zhao et al. 1999, Zhao et al. 2005, Zhao et al. 2011; Harvey et al. 2012; this study) and the upper Damao Group of Hainan Province (likely near to the Yangtze Block in the Cambrian Period; Jiang and Huang 1986).

Conclusions

New fossil materials from the Kaili Biota of South China have increased our knowledge on the diversity of chancelloriids during the Wuliuan Stage of Cambrian. Totally three genera and six species are recognised, in which the new species Archiasterella anchoriformis sp. nov. represents the only known complete body of the genus Archiasterella in South China. A careful survey on the spatial-temporal distribution of genus-level chancelloriids in South China reveals two flourishing ages of the group. The Flourishing Age I is late Stage 2 to Stage 3 that represented by the Chengjiang Biota and the Flourishing Age II is the Wuliuan Stage that represented by the Kaili Biota described herein. These flourishing ages can not only shed light on the biostratigraphic correlation of chancelloriid-bearing successions throughout South China, but also potentially reflect an evolutionary history.

Acknowledgments

We thank the constructive suggestions from reviewers and the editor, which conduces to improve the quality of papers.

Disclosure statement

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

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/08912963.2023.2212382

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [41890845, 42162003, 42002011, 41702022]; Guizhou Province Graduate Research Fund [YJSKYJJ (2021) 077]; Guizhou Bureau of Science and Technology [Gui. Sci. Tal. (2017) 5788]; Talent base project of Guizhou Province [RCJD2018-21]; State Key Laboratory of Palaeobiology and Stratigraphy [173130].

Annex

Figure A1. Archiasterella anchoriformis of Kaili Biota (MBP-38). The average angle of this specimen is 119.9°.

Figure A2. Archiasterella anchoriformis of Kaili Biota (MBP-43). The average angle of this specimen is 127.7°.

Figure A3. Archiasterella anchoriformis of Kaili Biota (JYS-1074-1). The average angle of this specimen is 130.8°.

Figure A4. Ar. coriacea of Burgess Shale (based on Bengtson and Collins, 2015). The average angle of this specimen is 105.1°.