Chancelloriid sclerites from the lowermost Cambrian of North China and discussion of sclerite taxonomy

Geobios 53 (2019) 65–75

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Research paper

Chancelloriid sclerites from the lowermost Cambrian of North China and discussion of sclerite taxonomy§ Hao Yun a, Xingliang Zhang a,*, Luoyang Li a, Bing Pan b, Guoxiang Li b, Glenn A. Brock c a State Key Laboratory of Continental Dynamics and Shaanxi Key Laboratory of Early Life and Environments, Department of Geology, Northwest University, 710069 Xi’an, China b State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, 210008 Nanjing, China c Department of Biological Sciences, Macquarie University, 2109 Sydney, NSW, Australia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 May 2018 Accepted 13 February 2019 Available online 18 February 2019

Chancelloriid sclerites from the lowermost Cambrian Xinji Formation (Series 2, Stage 3), southern margin of the North China platform, are systematically described. Thousands of isolated sclerites from three sections are assigned to three genera and four species, including Chancelloria cf. eros, Allonnia tripodophora, Archiasterella pentactina, and Ar. tetraspina. To accurately document the taxonomic significance of the sclerite structure, modified formulas (m+nC, m+nA, and m+0) are put forward to represent the full series and variation of sclerite forms. Based on the sclerite construction, statistical analysis on the proportions of different sclerite forms in the rock samples and the composition of sclerites in previously described chancelloriid scleritomes, a new scheme for identification and classification of isolated sclerites reconciled within the framework of the complete scleritome, is proposed.

C 2019 Elsevier Masson SAS. All rights reserved.

Keywords: Chancelloriids Taxonomy Xinji Formation Cambrian Series 2 North China

1. Introduction Chancelloriids are an extinct group of Cambrian animals with undetermined phylogenetic position. The sac-like body of this animal was covered by a flexible integument and a series of usually composite sclerites (Walcott, 1920; Bengtson and Hou, 2001; Janussen et al., 2002; Randell et al., 2005; Bengtson and Collins, 2015; Yun et al., 2018, 2019, 2019). Most chancelloriid sclerites have a rosette-like structure, composed of several hollow rays (filled with organic material in life; Rigby, 1978; Butterfield and Nicholas, 1996); each ray has a slender tapering shape and an originally calcareous (presumably aragonitic) wall composition (Bengtson and Missarzhevsky, 1981; Bengtson et al., 1990; Mehl, 1996; Bengtson, 2005; Porter, 2008). Chancelloriid sclerites are often remarkably abundant, making up a significant proportion of small shelly fossil (SSF) assemblages. They have a cosmopolitan distribution during the Cambrian (Terreneuvian to Miaolingian), including East Gondwana (Bengtson et al., 1990; Brock and Copper 1993; Demidenko, 2000; Wrona, 2004), West Gondwana (Ferna´ndez-Remolar, 2001; Clausen and A´lvaro, 2006; Elicki, 2007),

Laurentia (e.g., Rigby, 1986; Skovsted and Peel, 2007), Siberia (e.g., Vassiljeva and Sayutina, 1988; Khomentovsky et al., 1990; Kouchinsky et al., 2011, 2015, 2017; Zhu et al., 2017), South China (Luo et al., 1982; Duan, 1984; Qian and Bengtson, 1989; Li, 1999; Parkhaev and Demidenko, 2010; Moore et al., 2014), Tarim (Qian and Xiao, 1984; Xiao and Duan, 1992) and so on. Abundant chancelloriid sclerites, associated with extensive and diverse mollusks, hyoliths, trilobites, brachiopods and other SSFs have been previously reported from the Cambrian Series 2 Xinji Fm. of North China (He et al., 1984; Pei and Feng, 2005; Li et al., 2016; Yun et al., 2016). However, unlike other skeletal fossils, chancelloriid sclerites have received minimal attention and a systematic investigation of these disarticulated components is long overdue. Herein, the chancelloriid sclerites recently collected from the Xinji Fm. along the southwestern margin of the North China platform are taxonomically described. The taxonomy of isolated chancelloriid sclerites is carefully discussed in the context of not only individual sclerite form and variability but also their composition and arrangement within the scleritome. 2. Geologic setting

§

Corresponding editor: Catherine Girard.

* Corresponding author. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.geobios.2019.02.001 C 2019 Elsevier Masson SAS. All rights reserved. 0016-6995/

The Xinji Fm. represents a lowermost Cambrian rock succession in North China, disconformably overlying the late Ediacaran Dongpo Fm.

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H. Yun et al. / Geobios 53 (2019) 65–75

and conformably underlying the Cambrian Stage 4 Zhushadong Fm. Chancelloriid sclerites investigated herein were collected from phosphatic bioclastic limestones of the Xinji Fm. at three stratigraphic sections including the Chaijiawa section in Longxian area (abbreviated to LXC), Zhoujiaqu section in Longxian area (LXZ) and Shangzhangwan section in Luonan area (LNS), located along the southwestern margin of North China platform (Fig. 1). Detailed stratigraphy, sedimentology and related fossil assemblages have previously been summarized by Li et al. (2014), Yun et al. (2016), and Pan et al. (2018). 3. Material and methods 3.1. Sample processing Carbonate rock samples were chemically dissolved using dilute (10%) acetic acid. The samples were washed, dried and phosphatized chancelloriid sclerites were then picked out from the residues using a binocular stereoscope microscope. Selected

specimens were mounted on stubs, gold coated and imaged using a LEO 1530VP Scanning Electron Microscope (SEM) at the Department of Geology, Northwest University (Xi’an, China). Dozens of thin sections were also prepared to investigate wall ultrastructure of the sclerites using a petrological light microscope. Sclerites from the Longxian area are reposited in the Shaanxi Key Laboratory of Early Life and Environments (LELE), Northwest University; specimens from Luonan are housed in the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS). Eight groups of samples from three sections (LXC, LXZ and LNS) were carefully examined to reveal the diversity and distribution of the sclerites (Table 1). Samples B1, B2, B3, B4 and B5 are respectively from beds 1 to 5 of the bioclastic limestone unit, and sample C is from the calcareous siltstone unit in the LXC section. Sample Z is from the bioclastic limestone in the LXZ section. A total of 538 isolated sclerites were picked out from these seven samples, including 218 fragmented and 320 relatively intact (composite) sclerites. Samples from the LNS section, derived from a thin layer (20–40 cm thick) of phosphoric silty

Fig. 1. Geologic setting of the study areas. A. Outline of mainland China and three ancient tectonic blocks. B. Simplified traffic map of the studied localities. C. Rock successions in the three studied sections. Abbreviations: DF: Dongpo Fm.; Ed: Ediacaran; LF: Luoquan Fm.; LNS: Shangzhangwan section in Luonan; LXC: Chaijiawa section in Longxian; LXZ: Zhoujiaqu section in Longxian; ZF: Zhushadong Fm.

H. Yun et al. / Geobios 53 (2019) 65–75 Table 1 Distribution of chancelloriid sclerites in different samples from the Xinji Formation of North China. Pairwise x2 tests and linear regression analyses were performed on the numbers of m+nC sclerites from samples B1, B2 and B3. Sclerite form 3+0 4+0 6+0 2+1A 3+1A 4+1A 5+1C 6+1C 7+1C 8+1C 9+1C 10+1C Fragments Total

B1 7 1 2 1 2 9 13 7 1 1 21 65

B2

B3

6

1

5

2

4 14 72 74 21 1 2 114 313

6 13 10 3

B4

B5

C

Z

1

1 1 4 3

1 6 4

LN

Total

19 4

19 19 1 1247 155 20 98 188 144 48 3 3 218 2163

1

2

1 1

7 6 2

1238 152 14 74 76 33 15

2 5

27 44

– 1625

1 27 62

21 31

6 18

limestone in the lower Xinji Fm., are collectively named sample LN. A total of 1,625 composite specimens were picked out from the LNS section. 3.2. Terminology Terminology of isolated chancelloriid sclerites was standardized by Moore et al. (2014), while terms depicting whole-body

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chancelloriid specimens proposed by Bengtson and Collins (2015) are also significant in differentiating the sclerite structure. Thus, it is necessary to combine these previous schemes to develop a more coherent system in a biological context. The main components of each sclerite, i.e., the hollow rays, are distinguished herein as lateral rays, central rays and ascending rays. The sclerite formula (modified after Sdzuy, 1969 and Qian and Bengtson, 1989) is expressed as m+nC and m+nA, where m is the number of lateral rays and n the number of central rays (C) or ascending rays (A). The abaxial, adaxial, abapical and adapical orientations (Moore et al., 2014: fig. 2) are suggested to represent the relative position of different rays in a sclerite. Steinkerns of sclerite rays are easily disarticulated from each other in transportation or during laboratory treatment. The disarticulated rays usually retain intact outlines, possessing a foramen on the basal facet and an apparent opening on the distal tip of the ray caused by breakage (Fig. 2). Around the basal inflation of each ray, the original calcareous walls and organic matter are often replaced by phosphate and clay materials, defining the sutures of the sclerite (Fig. 2(A, E, F)). Isolated lateral rays have an elongated cone-shaped distal part and a compressed polygonal proximal part (Fig. 2(B)). Central rays usually have a flask-like shape, with a long tapered distal ‘neck’ and an inflated proximal base (Fig. 2(C)) with a polygonal cross section. Ascending rays possess a strongly curved distal part and a slightly elongated proximal part (Fig. 2(D)). In general, the basal facet of lateral rays usually parallels to, or forms a small angle to the long axis of the ray, while in the central and ascending rays, the basal facet is relatively perpendicular to (at least the proximal part of) the long axis of the ray.

Fig. 2. A–F. Chancelloriid sclerite rays. A: fragmental sclerite showing the articulatory structure, LZ04054; B: lateral ray of the sclerite LC0307045 in basal view; C: central ray of the sclerite LC0300026; D: ascending ray of the sclerite LC0201153; E: central ray with the articulatory structure in the basal part, LC0300012; F: possible ascending ray with the articulatory structure in the basal part, LC0307062. G, H. Chancelloriid sclerites in thin sections. G: adjacent mineralized walls pointed by white arrows, lc05-02-02; H: dark wedges at the end of the walls pointed by black arrows, lc06-06-04. Scale bars: 500 mm (A–F), 400 mm (G, H).

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4. Results 4.1. Sclerite structure and construction Most chancelloriid sclerites are in compound structures, composed of several mineralized hollow rays (Bengtson et al., 1990; Porter, 2008). Since each sclerite ray has only one original opening (the basal foramen), the cavity in each ray of a composite sclerite is separated by adjacent biomineralized walls and filled with organic material. The interspace between the walls of adjacent rays is visible in thin sections (Fig. 2(G); Bengtson and Hou, 2001: fig. 13A; Bengtson et al., 1990: fig. 37) and demonstrates the independence of the ray walls which were possibly formed separately by mineralization of organic precursors housed within the cavity of each ray. It is also noted that the mineralized wall between two adjacent rays is a thin single layer (20 mm) in a number of thin sections (Fig. 2(H)). This could be attributed to the ray boundary being obscured by fusion of the walls during the original mineralization or diagenetic remineralization (Bengtson and Hou, 2001), since the dark wedges (the remnant of the interspace) are present at the ends of the wall. All composite chancelloriid sclerites can be divided into three morphological series based on the number and type of the rays present (Fig. 3). Series 1 sclerites are characterized by having central rays surrounded by sutured lateral rays (denoted as m+nC). Series 2 sclerites are composed of ascending rays and several lateral rays (m+nA). Series 3 sclerites possess only lateral rays that are all approximately parallel to, or curved away (abaxially) from the basal plane (m+0). 4.2. Statistical analysis A total of 2163 chancelloriid sclerites from the Xinji Fm. in Longxian (sections LXC and LXZ) and Luonan (section LNS) areas were used for statistical analysis. Composite sclerites included all

three series of forms (m+nC, m+nA, and m+0; Table 1). In the LXC and LXZ sections, 85% of composite sclerites are from samples B1, B2 and B3, which are mostly 5+1C, 6+1C, 7+1C, 8+1C and 4+0 forms (over 90%), and rare 2+1A, 4+1A, 3+1A, 9+1C, 10+1C, and 6+0 forms. In the LNS section, sclerites of 2+1A forms are in overwhelming majority (70%) (Fig. 4), while 3+1A, 5+1C, 6+1C, 7+1C, 8+1C, 3+0, 4+1A, and 4+0 forms are less common. Sclerites integrated within known chancelloriid scleritomes (see below, Section 6.1.), tend to have Series 1 m+nC (where m is usually 5–10) sclerites as common components of Chancelloria scleritomes (e.g., Janussen et al., 2002; Zhao et al., 2011; Bengtson and Collins, 2015); Series 2 m+nA sclerites (dominated by 2 4+1A forms) as characteristic components of scleritomes belonging to Allonnia (specifically the 3+1A form) and Archiasterella (e.g., Bengtson and Hou, 2001; Randell et al., 2005; Yun et al., 2018); Series 3 m+0 sclerites are occasionally distributed in the scleritomes of all three genera, especially Chancelloria and Allonnia (Bengtson and Collins, 2015). Therefore, the isolated sclerites recovered from sections LXC and LXZ are most likely to belong to Chancelloria scleritomes while composite sclerites from section LNS are mainly derived from scleritomes of Allonnia. The genus Archiasterella is not abundant in any of the sampled sections. As noted by Moore et al. (2014), isolated sclerite forms originally associated within the scleritome of a single species are often present in rock samples in the same relative proportion as in the scleritome. As such, recovery of large numbers of isolated composite sclerites in acid etched samples can provide statistically robust relative proportions that allow scleritome identification. The relative proportions of m+nC sclerites in samples B1, B2 and B3 were statistically compared in order to test whether the sclerites are associated with the scleritome of one or multiple species. Results of pairwise x2 tests on samples B2 vs. B1, and B2 vs. B3 indicate no significant difference between them (B2–B1: x2 = 5.874, d.f. = 5, p > 0.25; B2–B3: x2 = 4.857, d.f. = 5, p > 0.40). Therefore, it is inadvisable to reject the null hypothesis

Fig. 3. Sketches of different sclerite series. In each series, the left one is the lateral view (l.v.), the right one is the basal view (b.v.). Orientations of the sclerites are shown in the lateral view of the Series 2 sclerites (upper right).

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Fig. 4. Distribution curves of different sclerite forms in four representative samples, and linear relationships observed between samples B2 and B1 (left) and B2 and B3 (right).

that the m+nC sclerites come from a same species. The further linear regression analyses show coefficients of determination (R2) higher than 0.85 in both pairs, suggesting an approximate linear relation between the number of m+nC sclerites in samples B1 vs. B2 and B2 vs. B3 (Fig. 4), reflecting stable relative proportions of these sclerite forms in the samples. Thus, the 5+1C, 6+1C, 7+1C and 8+1C sclerites from the samples likely represent isolated components from a single species of Chancelloria, probably C. eros Walcott, 1920 due to the similar composition of sclerite forms (Janussen et al., 2002; Zhao et al., 2011; Bengtson and Collins, 2015). In addition, sclerites with 6+1C and 7+1C formulae are the dominant forms of m+nC sclerites in sections LXC and LXZ, while in the sample LN from the LNS section the 6+1C and 5+1C sclerites are the dominant sclerite forms (Fig. 4). Despite variation in the relative abundance and dominance of sclerite forms in different samples, all are known to be associated with C. eros scleritomes.

5. Systematic paleontology Phylum and Class uncertain Order Chancelloriida Walcott, 1920 Family Chancelloriidae Walcott, 1920 Genus Chancelloria Walcott, 1920 Type species: Chancelloria eros Walcott, 1920, from the Burgess Shale. Emended Diagnosis (after Bengtson and Collins, 2015): Chancelloriids with scleritome dominated by rosette-like m+nC (m usually 4–11, n usually 1) sclerites and rare m+0 (m usually 3– 6) sclerites with all rays approximately parallel to the basal plane. Remarks: The genus Chancelloria was initially defined based on scleritome specimens rather than isolated sclerites (Walcott,

1920). A series of nominal sclerite-based genera, such as Fangxianites Duan, 1984, Stellispinella Vassiljeva et Sayutina, 1993, and Chancelloriella Demidenko, 2000, are all likely components of the Chancelloria scleritome, and their basic form falls within the known sclerite variation of the genus Chancelloria (see below, Section 6.2.). Chancelloria cf. eros Walcott, 1920 Fig. 5 cf. 1920. Chancelloria eros nov. sp. - Walcott, pp. 329–331, pl. 86, fig. 2, pl. 87, figs. 1, 2. ?1969. Chancelloria maroccana nov. sp. - Sdzuy, pp. 129–131, pl. 14, figs. 1–10, 12–14, 16–30. 1990. Chancelloria spp. - Bengtson et al., p. 51, figs. A–I. ?1992. Archiasterella quadratina Lee - Lee et al., p. 157, pl. 3, fig. 3, 4. 1993. Chancelloria sp. - Brock and Cooper, p. 764, fig. 6.7, 6.8. 1998. Chancelloria eros Walcott - Mehl, p. 1175, pl. 7, figs. 1, 2, 7, 8, 12, 14, 15. 2001. Chancelloria sp. Form C - Ferna´ndez-Remolar, pp. 46, 48, fig. 5d, e. 2004. Chancelloria sp. A - Wrona, p. 25, fig. 6A, B, G–K. ?2007. Chancelloria sp. - Skovsted and Peel, p. 741, fig. 6A, B. 2011. Chancelloria sp. - Elicki, pp. 161–162, fig. 5V–Z. cf. 2013. Chancelloria eros Walcott - Cuen et al., pp. 586–587, fig. 5. 2015. Allonnia tetrathallis Jiang – Yang et al., p. 1550, fig. 7I. 2015. Chancelloria spp. - Kouchinsky et al., pp. 456–457, fig. 34A–E, G–I. 2016. Chancelloria sp. - Yun et al., fig. 5C. 2016. Archiasterella quadratina Lee - Yun et al., fig. 5E. Description: The 5+1C, 6+1C, 7+1C and 8+1C sclerites are generally 1–2 mm in maximal length, with radial or nearly-bilateral

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H. Yun et al. / Geobios 53 (2019) 65–75

Fig. 5. Sclerites of Chancelloria cf. eros. A. 6+1C sclerite, LC0302083. B. 6+1C form sclerite, LC0305079. C. 7+1C sclerite, LC0300054. D. 6+1C sclerite, LC0405046. E. 7+1C sclerite, LC0601026. F. 7+1C sclerite, LC0604003. G. 8+1C sclerite, LC0301056. H. 5+1C sclerite, LC0602079. I. 4+0 sclerite, LC0202035, basal view. J. 4+0 sclerite, LC0202173, top view. K. 4+0 sclerite, NIGPAS 169628, basal view. L. 4+0 sclerite, NIGPAS 169627, basal view. Scale bars: 500 mm.

symmetry. Each sclerite is composed of 5 to 8 lateral rays and a central ray. All rays are hollow, tapered tangentially, up to 1 mm in length. Lateral rays occur parallel to the basal plane (Fig. 5(A–C)) or are slightly curved abaxially (Fig. 5(D–H)). Central ray usually vertical to basal plane, tapering drastically, forming a flask-shaped outline (Fig. 5(E)). Basal facets of sclerites are approximately flat except for round depressions or projections (Fig. 5(A, B)) representing foramina of each rays. Articulatory facets of rays either preserved as straight grooves between two adjacent rays or covered by probable clayey material. The 4+0 sclerites with approximately bilateral symmetry, composed of 4 lateral rays parallel to the basal plane, are rare elements of the C. cf. eros scleritome. All rays are compressed, conical-shaped and taper tangentially. In some sclerites, rays are

incurved slightly distally (Fig. 5(J, K)). Articulatory facets between rays are usually preserved as obvious grooves on the upper (Fig. 5(J)) and basal surfaces (Fig. 5(I, K)). Foramina of rays usually eccentric, located away from the center of sclerites. Remarks: The 4+0 sclerites composed of only lateral rays and with all rays parallel to the basal plane have previously been assigned to Archiasterella quadratina (Lee, 1987; Lee et al., 1992; Yun et al., 2016) or arbitrarily lumped with 3+1A sclerites into Ar. tetractina (Duan, 1984). Since this sclerite form is also known as a rare component in the scleritome of C. eros (Bengtson and Collins, 2015), taxonomic priority suggests it should be placed in the genus Chancelloria. Herein, the 4+0 sclerites are suggested to be minor components of the same C. cf. eros scleritome dominated by accompanied m+1C sclerites.

H. Yun et al. / Geobios 53 (2019) 65–75

Genus Allonnia Dore´ et Reid, 1965 Type species: Allonnia tripodophora Dore´ et Reid, 1965. Emended diagnosis (after Bengtson and Collins, 2015): Chancelloriids with scleritome characterized by sclerites of 2+1A and 3+0 (all rays curved abaxially away from the basal plane) forms; apical tuft of the scleritome usually conspicuous. Remarks: The genus Allonnia was originally defined based on sclerites with three rays (Dore´ and Reid, 1965). It was then expanded as m+0 (m ranges from 2 to 8) sclerites with all rays curved abaxially away from the basal plane (Qian and Bengtson, 1989; Moore et al.,

71

2014). However, further detailed investigation of complete Allonnia material revealed that the three-ray (2+1A and 3+0 forms) sclerites are dominant in the scleritome (Bengtson and Hou, 2001; Bengtson and Collins, 2015; Yun et al., 2018; Cong et al., 2018; Zhao et al., 2018). Thus, the 2+1A and 3+0 sclerites are largely confined to the scleritome of Allonnia and so can be confidently assigned to this genus. The emended diagnosis of Allonnia herein follows the recent scleritome investigation (Bengtson and Collins, 2015) and the initial definition of this genus. Other nominal sclerite-based Allonnia, such as two-ray (once assigned to Dimidia Jiang in Luo et al., 1982) and

Fig. 6. Sclerites of Allonnia and Archiasterella. A. 2+1A sclerite, NIGPAS 169625. B. Detail of the basal disc of A. C. 2+1A sclerite, NIGPAS 169629. D. Basal view of C. E. 2+1A sclerite, NIGPAS 169630, top view. F. Basal view of E. G. 2+1A sclerite, LN0602. H. 2+1A sclerite, NIGPAS 169632. I. Detail of the basal disc of H. J. 3+0 sclerite, NIGPAS 169633. K. 3+0 (or 2+1A) sclerite, NIGPAS 169631. L. 4+1A sclerite, NIGPAS 169626, top view. M. Basal view of L. N. 3+1A sclerite, LZ02019, basal view. O. 3+1A sclerite, LC1202080. Scale bars: 200 mm.

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four-ray (Onychia Jiang in Luo et al., 1982) sclerites, have yet to be identified in Allonnia scleritomes and so are currently excluded from this taxon concept. Allonnia tripodophora Dore´ et Reid, 1965 Fig. 6(A–K) 1990. Allonnia cf. tripodophora Dore´ and Reid - Bengtson et al., p. 57, fig. 26L–N. 2001. Allonnia sp. Form B - Ferna´ndez-Remolar, pp. 56, 58, figs. 3b–d, 7m–o, q–s, 8a, b. ?2004. Allonnia ex gr. A. tripodophora Dore´ and Reid - Wrona, p. 26, fig. 5J. 2014. Allonnia sp. - Devaere et al., p. 182, pl. 2, figs. F–H. Emended diagnosis: Allonnia scleritome characterized by sclerites composed of three slender, tapering rays; 2+1A sclerites are the dominating form. Description: Sclerites with 3 rays, two lateral rays approximately parallel to the basal plane and one ascending ray vertical or recurved strongly over the basal plane (2+1A form; Fig. 6(A–I)). In a few sclerites, all three rays are drastically curved away from the basal plane (3+0 form; Fig. 6(J, K)), and consequently no specific ascending ray can be identified. Rays generally 0.5 mm in length, though distal tips are often broken. Articulatory facets of sclerites mostly obscured by adhering fine-grained sediments. Basal foramina of rays usually large, preserved as three rounded projections on the basal surface. In some sclerites, foramina are sub-circular shaped, with raised peripheral walls (Fig. 6(B, D, G, I)). Remarks: Seven species of Allonnia are currently recognized. Five are scleritome-based, including Allonnia phrixothrix Bengtson et Hou, 2001, Al. tintinopsis Bengtson et Collins, 2015, Al. erjiensis Yun et al., 2018, Al. nuda Cong et al., 2018, and Al. tenuis Zhao et al., 2018. Two are sclerite-based, including Al. tripodophora and Al. erromenosa Jiang in Luo et al., 1982. The sclerite structure of all Allonnia species are broadly similar. The scleritome-based species are mostly differentiated by the development and variation of the apical tuft, the ornaments on the integument and density of sclerites (Yun et al., 2018; Cong et al., 2018). The two sclerite-based species are usually difficult to distinguish from each other. It was stated that Al. erromenosa likely differs from Al. tripodophora in possessing more robust rays and foramina closer to the center of the sclerites (Moore et al., 2014), but the taxonomic significance of these features is questionable. Genus Archiasterella Sdzuy, 1969 Type species: Archiasterella pentactina Sdzuy, 1969.

Emended diagnosis (after Bengtson and Collins, 2015): Chancelloriids with scleritome dominated by m+nA (m is usually 3 or 4, n is usually 1) sclerites. Remarks: In this emended diagnosis, the form (lateral rays + ascending rays) of Archiasterella sclerites is very similar to the Allonnia sclerites. The difference between Archiasterella and Allonnia then lies in the lateral ray number of the sclerites, the composition of sclerites in the scleritome (Table 2), and the development of the apical tuft (Allonnia usually has a more developed tuft) (Bengtson and Collins, 2015). However, there are only two species of Archiasterella with scleritomes: Ar. fletchergryllus Randell et al., 2005, based on several incomplete specimens from the Sekwi Fm., and Ar. coriacea Bengtson et Collins, 2015, based on Burgess Shale specimens. Further investigation on the scleritome composition of this genus is required to clearly distinguish it from Allonnia. Archiasterella pentactina Sdzuy, 1969 Fig. 6(L, M) 1998. Archiasterella sp. - Mehl, p. 1176, pl. 7, fig. 11. ?2006. Archiasterella sp. - Clausen and A´lvaro, p. 228, fig. 3K, L. 2014. Archiasterella cf. pentactina Sdzuy - Devaere et al., pp. 181–182, pl. 2, figs. A–D. 2014. Archiasterella cf. pentactina Sdzuy - Moore et al., pp. 859– 861, 863, figs. 3I, K, 12. Description: Sclerites bilaterally symmetrical, composed of four lateral rays and one ascending, recurved ray (4+1A from). Rays are cone-shaped, generally 0.5 to 1 mm in length. Lateral rays can be divided into two groups based on their relative positions to the ascending ray. Two rays (abapical lateral rays) are adjacent and articulated to the ascending ray. Their size is similar to the ascending ray and curve slightly away from the basal plane and point abaxially distally. The other two rays (adapical lateral rays), which are both parallel to the basal plane and the longest and most robust rays in the sclerites, are articulated to abapical lateral rays but separated from the ascending ray. Foramina on the basal facets are prominent, 20–30 mm in diameter and possessing a subrounded, raised wall. Remarks: The 4+1A form of Archiasterella pentactina sclerites is typical and easy to distinguish from other species in isolated chancelloriid sclerite assemblages. The scleritome-based species Ar. fletchergryllus is also characterized by 4+1A sclerites (Randell et al., 2005), though the type and only specimens are not well

Table 2 Sclerite components of the known chancelloriid scleritomes. Scleritome

Chancelloria eros Chancelloria cf. eros Chancelloria eros Chancelloria eros Chancelloria pentacta Chancelloria cruceana Chancelloria australilonga Allonnia phrixothrix Allonnia phrixothrix Allonnia tintinopsis Allonnia erjiensis Allonnia nuda Allonnia tenuis Allonnia sp. Archiasterella fletchergryllus Archiasterella coriacea

Sclerite Type Main Component

Minor Component

6+1C, 6+1C, 6+1C, 6+1C, 5+1C 6+1C 6+1C, 2+1A 2+1A 2+1A 2+1A 2+1A 2+1A 2+1A 4+1A 3+1A

8+1C, 5+1C, 4+0 ?5+1C ?4+0 8+1C, 5+1C, 4+0, 3+0

7+1C 7+1C 5+1C 7+1C

7+1C ?3+0 3+0 3+0 3+0 ?3+0 ?3+0 ?3+0

Locality

Reference

Wheeler Shale, USA Sekwi Formation, Canada Kaili Biota, China Burgess Shale, Canada Wheeler Shale, USA San Esidro Formation, Argentina Emu Bay Shale, Australia Chengjiang Biota, China Chengjiang Biota, China Burgess Shale, Canada Chengjiang Biota, China Chengjiang Biota, China Chengjiang Biota, China Kaili Biota, China Sekwi Formation, Canada Burgess Shale, Canada

Janussen et al. (2002) Randell et al. (2005) Zhao et al. (2011) Bengtson and Collins (2015) Rigby (1978) Beresi and Rigby (2013) Yun et al. (2019) Bengtson and Hou (2001) Janussen et al. (2002) Bengtson and Collins (2015) Yun et al. (2018) Cong et al. (2018) Zhao et al. (2018) Zhao et al. (2011) Randell et al. (2005) Bengtson and Collins (2015)

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preserved. Compared to Ar. pentactina sclerites, the sclerites of Ar. fletchergryllus have more robust rays in which the ascending ray is very prominent. Archiasterella tetraspina Vassiljeva et Sayutina, 1993 Fig. 6(N, O) ?1969. Chancelloria maroccana nov. sp. - Sdzuy, pp. 129–131, pl. 14, figs. 11, 15. ?1993. Allonnia sp. - Brock and Cooper, p. 764, fig. 6.9. 1993. Allonnia sp. aff. A. tetrathallus Jiang - Brock and Cooper, p. 764, fig. 6.13. ?1998. Chancelloria eros Walcott - Mehl, p. 1175, pl. 7, fig. 12. 2006 Archiasterella quadratina Lee - Lee, fig. 4.16, 4.17, 4.22. 2007. Archiasterella sp. - Skovsted and Peel, p. 741, fig. 6C, D. ?2011. Archiasterella cf. hirundo - Elicki, p. 161, fig. 5P, Q. ?2015. Archiasterella tetractina Duan – Yang et al., p. 1550, fig. 7J. Description: Sclerites bilaterally symmetrical, composed of three lateral rays and one ascending ray (3+1A form). Rays coneshaped, tapered distally. Ascending ray strongly recurves over the basal plane. Rays meet in the basal disc and their proximal parts are separated by narrow grooves (articulated facets). Foramina on basal surface are sub-rounded to oval and about 50 mm in diameter. Remarks: The 3+1A sclerite-based species include Archiasterella tetraspina (= ?Ar. tetractina Duan, 1984), Ar. hirundo Bengtson in Bengtson et al., 1990, and Ar. dhiraji Gilbert et al., 2016. Compared to Ar. tetraspina sclerites, the Ar. hirundo sclerites are characterized by large and flat basal area, large foramina and welldeveloped tuberculate ornament on the surface; sclerites of Ar. dhiraji possess slender rays, the bases of the ascending ray and adapical ray connect to each other, while the bases of two abapical rays are separated. In addition, though the sclerites of scleritomebased species Ar. coriacea also have a 3+1A form, the diagnosis of this species is mainly focused on the distinctive imbricating scaly surface of the integument and the inconspicuous apical tuft. 6. Discussion 6.1. Sclerite arrangement and distribution on the chancelloriid body Based on the articulated chancelloriid material described from the Cambrian Lagersta¨tten, the sclerite components of different chancelloriid scleritomes are listed (Table 2). Five key aspects of sclerite arrangement and distribution on the body wall of the scleritome are summarized, which provide pertinent references for chancelloriid sclerite recognition and classification:  There are usually dozens to hundreds of sclerites within a complete scleritome, with size gradually increasing from the abapical to the apical part (Yun et al., 2018) or increasing only to a certain distance from the base (Bengtson and Collins, 2015). The details of sclerites such as symmetry and ray thickness can also be variable;  The Chancelloria scleritome usually contains various m+nC (m from 4 to 10, n usually 1) sclerites. The m+0 (m is usually 4 or 3) sclerites distinguished by all rays approximately parallel to the basal plane occasionally occur;  The Allonnia scleritome contains 2+1A sclerites and rare 3+0 (all rays curved away from the basal plane) sclerites. The 2+1A sclerites associated with the Allonnia scleritome were once defined as Archiasterella charma by Moore et al. (2014) based on the form of isolated sclerites, who indicated that only those sclerites with all rays angled or recurved steeply abaxially from the basal plane (3+0 form herein) belonged to Allonnia. However, both 2+1A and 3+0 sclerites are actually distributed in Allonnia scleritomes (e.g., Bengtson and Collins, 2015: fig. 21). In a

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complete scleritome with curved outer surface, ascending rays and lateral rays of the sclerites generally point apically regardless of the various orientations of the basal plane, thus the angle between rays and the basal plane is not constant (e.g., Bengtson and Collins, 2015: fig. 20; Yun et al., 2018: fig. 4). From this perspective, lateral rays of sclerites on the regions around the apex likely curve away from the basal plane, which obscures the distinction between the ascending ray and the lateral ray. Consequently, rare 3+0 sclerites (with all rays curved notably away from their basal plane) can occur around the apical region of the Allonnia scleritome, which is otherwise dominated by 2+1A sclerites;  The Archiasterella scleritome is usually characterized by uniform m+nA (m is usually 3 or 4, n is usually 1) sclerites;  There is usually a palisade-like tuft formed by a series of modified single-ray sclerites around the apical orifice (Bengtson and Collins, 2015; Yun et al., 2018). The sclerites with only one ray in the SSF assemblages are probably represented by Eremactis Bengtson et Conway Morris in Bengtson et al., 1990. In addition, there are also scleritomes like Nidelric pugio Hou et al., 2014 and N. gaoloufangensis Zhao et al., 2018, composed mainly of singleray sclerites, though the definite position of Nidelric in the order Chancelloriida is still debated (Hu et al., 2013; Hou et al., 2014; Yun et al., 2018; Zhao et al., 2018).

6.2. Chancelloriid sclerite identification and classification Chancelloriid species, genera and even families have previously been assigned based solely on the construction (the number and orientation of rays) and details of isolated sclerites (e.g., Dore´ and Reid, 1965; Sdzuy, 1969; Luo et al., 1982; Duan, 1984; Vassiljeva and Sayutina, 1988, 1993; Demidenko, 2000). The sclerite construction, summarized as the morphological series and form of sclerites (see Section 4.1.), was accepted as the most significant diagnosis of different chancelloriid families and genera. The abandoned family Archiasterellidae was originally defined by sclerites lacking central rays (Sdzuy, 1969); m+nC sclerites were distinguished as Chancelloria, m+nA as Archiasterella, and m+0 as Allonnia (e.g., Sdzuy, 1969; Li, 1999; Parkhaev and Demidenko, 2010; Moore et al., 2014). It is now clear that the taxonomic significance of ray number in a sclerite was previously overemphasized. For example, the 6+1C, 7+1C and 8+1C sclerites, now known to occur in the same scleritome, were previously assigned to different taxa including Eiffelia? hispanica Sdzuy, 1969, Chancelloria maroccana Sdzuy, 1969, and C. altaica Romanenko, 1968, respectively (Qian and Bengtson, 1989). Sclerite symmetry, ray thickness, degree of ray taper, basal facet flatness and presence of ornaments were also used to define taxa. For instance, the sclerite-based genus Chancelloriella is characterized by bilateral symmetry and rounded basal facets (Demidenko, 2000; Moore et al., 2014), and Allonnia erromenosa differs from the type species Al. tripodophora in having thick rays (Luo et al., 1982). Isolated sclerite structure is an important factor in chancelloriid classification, especially in differentiating species, but only in the context where the scleritome is at least partially known. Sole focus on isolated sclerites has also led to taxonomic confusion and misidentification of chancelloriid taxa (Qian, 1999). Consequently, Bengtson et al. (1990), Qian (1999) and Kouchinsky et al. (2011) stressed that the association of isolated sclerites in the original samples should be considered, based on the observation that sclerites of different shapes and types (forms) can occur in a single chancelloriid scleritome. In practice, the refinement of chancelloriid sclerite taxonomy requires statistically robust, large sample sizes to accurately capture the relative proportions and morphological variability of specific sclerite forms.

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In summary, previous difficulties in resolving chancelloriid taxonomy was the result of separate research of isolated sclerites in SSF assemblages derived from carbonate successions and articulated scleritome material recovered from siliciclastic dominated Konservat-Lagersta¨tten. It is thus mandatory to integrate and coordinate isolated sclerite data within the context of scleritome structure (including sclerite abundance and variability) to further understand the enigmatic chancelloriids. A three-step scheme is suggested to distinguish isolated sclerites taxonomically and effectively reconcile disconnected sclerite and scleritome investigations:  First, categorize sclerites into different series and forms based on their overall construction;  Second, figure out and compare the proportions of various sclerite forms in different rock samples by numeric analysis;  Third, integrate proportions with sclerite composition of the scleritomes and position the sclerites into the corresponding classification.

Acknowledgements Thanks go to our colleagues Yu Wu, Cong Liu, Wenrui Pei and Yunlong Pang for their assistances in the field works and related experiments. We are also grateful to the reviewers and editors for their constructive comments and language checking. This research was supported by funds from the National Key Research and Development Program (Grant No. 2017YFC0603101), National Natural Science Foundation of China (Grant Nos 41621003, 41890840 and 41720104002), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB26000000) and the 111 Project (D17013).

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