Shell microstructures of the helcionelloid mollusc Anabarella australis from the
lower Cambrian (Series 2) Xinji Formation of North China
Luoyang Lia, b, Xingliang Zhanga, *, Christian B. Skovsteda, b, *, Hao Yuna, Guoxiang
Lic, Bing Panb, c a State Key Laboratory of the Continental Dynamics, Shaanxi Key Laboratory of
Early Life and Environments, Department of Geology, Northwest University, Xi’an
710069, PR China; b Department of Palaeobiology, Swedish Museum of Natural
History, Box 50007, SE-104 05 Stockholm, Sweden; c State Key Laboratory of
Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology,
Chinese Academy of Sciences, Nanjing 210008, PR China.
*Corresponding authors. Email: [email protected]; [email protected]
Although various types of shell microstructures are uncovered from Cambrian molluscs, precise organization and mineralogical composition of Terreneuvian molluscs are rarely known. Anabarella was one of the first helcionellid molluscs to appear in the Terreneuvian, and ranged up to Cambrian Epoch 3 in age. Here, shell microstructures of Anabarella australis have been studied based on new collections from the lowermost Cambrian (Series 2) Xinji Formation of the North China Block.
Results show that A. australis has a laminar inner shell layer that consists of crossed foliated lamellar microstructure (CFL). Nacreous, crossed-lamellar and foliated
1 aragonite microstructures previously documented in Terreneuvian A. plana are here revised as preservational artefacts of the CFL layers. This complex skeletal organization of Anabarella suggests that mechanisms of molluscan biomineralization evolved very rapidly. Morphologically, specimens from the Chaijiawa section show a distinct “pseudo-dimorphism” pattern as external coatings are obviously identical to
Anabarella, while associated internal moulds are similar to the helcionelloid genus
Planutenia. In contrast, internal moulds from the Shangzhangwan section show considerable morphological variation owing to preservational bias, which are more similar to specimens from South Australia, Northeast Greenland and Germany. These observations demonstrate that the wide morphological variants of internal moulds of
Anabarella worldwide are in large part preservational artefacts, and are unlikely to represent the real intra- and interspecific variability of the animal. In these cases,
Planutenia is confirmed to be a subjective synonym of Anabarella.
Keywords: Cambrian; Helcionelloida; Shell Microstructure; Taxonomy
Introduction
One outstanding innovation in the evolutionary history of early life is the advent of biomineralized skeletons among metazoans during the Cambrian explosion, with many phylum-rank clades first acquiring the capacity to produce a variety of biominerals (e.g. carbonate, phosphate, and silica minerals) in an astounding range of
2 organizations and functions (Bengtson 2005; Zhuravlev & Wood 2008; Wood &
Zhuravlev 2012). Molluscs were among the first metazoans to construct mineralized external shells under exquisite controls over biomineralization by underlying tissues
(the mantle). They produce polymorphs of calcium carbonate (calcite and/or aragonite, vaterite) in highly organized macro-micro architectures within the shells (Checa 2000;
Addadi et al. 2006). At present, 40 types of shell microstructures (prismatic, nacre, crossed lamellar, foliated, homogeneous, etc.) have been determined in mollusc shells in terms of hierarchical organization, biocompositional nature, crystallographic orientation, etc. (Carter et al. 1990; Sato & Sasaki 2015).
As a successful animal group in Cambrian marine environments, molluscs constituted conspicuous elements among the Cambrian small shelly fossil assemblages (Bengtson et al. 1990; Gravestock et al. 2001). In recent years, intensive studies on their shell microstructures indicated that Cambrian molluscs evolved very rapidly in their shell organizations in addition to morphological diversity and disparity.
Various types of microstructural fabrics have been recognized such as lamello-fibrillae (Feng & Sun 2003), foliated calcite (Vendrasco et al. 2010a, b), and foliated aragonite (Li et al. 2017). Notwithstanding, shell microstructures of
Cambrian molluscs are rarely known, especially the Terreneuvian members that are evolutionarily closer to ancestral forms. The Fortunian maikhanellid shells, characterized by cap-shaped morphology with scale-like ornamentations, have been assigned to the Mollusca by many authors (e.g. Bengtson 1992; Ponder et al. 2007).
Various types of fibrous microstructures, broadly assigned to lamello-fibrillae, were
3 previously described in some maikhanellid taxa, e.g. Maikhanella Zhegallo, 1982 and
Canopoconus Jiang, 1982. Such fibrous microstructures have also been widely reported in Terreneuvian helcionelloid molluscs, e.g. Latouchella Cobbold, 1921 and
Archaeospira Yu, 1979 (Feng et al. 2002; Feng & Sun 2003).
The helcionelloid mollusc Anabarella Vostokova, 1962 has been known to range stratigraphically from Cambrian Terreneuvian to Series 3. Its shell is highly compressed laterally, with apex tightly coiled, almost connecting to the apertural margin (Gubanov et al. 1999; Gravestock et al. 2001; Gubanov & Peel 2003;
Gubanov et al. 2004; Parkhaev 2004a, b). Although shell microstructures of
Terreneuvian molluscs are mainly preserved as fibrous forms (Feng & Sun 2003;
Vendrasco et al. 2015; Li et al. 2017), to our knowledge, Anabarella and its relative
Watsonella Grabau, 1900 are the only known two taxa recording blade or lath-like crystallites of aragonite with a stepwise growth pattern among Cambrian Terreneuvian molluscs (Runnegar 1985; Kouchinsky 1999). Those observations imply that shell microstructures of Anabarella, Watsonella, and probably other Terreneuvian molluscan taxa might have developed different types of crystal forms (rod, lath, blade, tablet, etc.), and evolutionarily advanced organizations in addition to fibers of lamello-fibrillae.
The new materials of Anabarella australis Runnegar in Bengtson et al., 1990 from the lowermost Cambrian Xinji Formation (upper Stage 3 or lower Stage 4) of North
China allow us to recognize the complex hierarchical shell microstructure of this characteristic helcionelloid species. More interestingly, owing to preservational bias,
4 internal mould specimens of Anabarella are quite different from its external coatings, but greatly resemble the helcionelloid genus Planutenia Elicki, 1994, and hence enable a taxonomic revision.
Geological setting, material and methods
The Xinji Formation, deposited along the south margin of North China Block during the global transgression event in Cambrian Series 2, is particularly well-exposed in
Longxian County, Shaanxi Province (Fig. 1A). In the Chaijiawa section, the Xinji
Formation rests disconformably on the Ediacaran Dongpo Shale, and is conformably overlain by massive dolostones of the Zhushadong Formation. This rock unit, approximately 34m thick, is deposited in a shoal or back-shoal environment, and can be subdivided into three parts: the basal sandstones with phosphatic intraclasts (0.65m thick), the mid bioclastic limestones (4.8m thick), and the upper calcareous sandstones (27.7m thick) (Li et al. 2016; Yun et al. 2016; Fig. 1B).
Rock samples were treated with buffered 5% acetic acid to retrieve acid-resistant microfossils. Selected specimens were mounted, sputter-coated with gold and examined with a Scanning Electron Microscope FEI Quanta 400 FEG in Northwest
University. Microfossils described below are all reposited at the Department of
Geology, Northwest University, Xian, China.
A total of 160 internal moulds from Chaijiawa section were imaged in lateral view with their apex posterior oriented to the left, while oppositionally positioned
5 steinkerns were mirror imaged (Fig. 4A). Two landmarks and 48 sliding semi-landmarks were digitized by LYL with the software TpsDig.232. The two landmarks respectively corresponded to the sharp curvatures in the umbilicus and ad-apertural dorsum of the shell, while semi-landmarks were digitized in a clockwise direction (Fig. 4B). The software TpsUtil.64 was applied to transfer curves to landmarks and create the sliders file defining the semi-landmarks. The superimposition method applied here is the Generalized Procrustes Analysis (GPA), computed by the software TpsRelw32. It calculates the consensus configuration or mean shape, which serves as the reference for the superimposition of all specimens.
Semi-landmarks are slid by minimizing Procrustes distances between individual specimens and the average configuration. Relative warp analysis (RWA) was carried out to obtain a lower-dimensional ordination of individuals in the morphospace. The thin-plate splines (TPS) were used to depict the morphological variation corresponding to the extremes of PC1 and PC2 as TPS-grids.
Systematic palaeontology
Phylum Mollusca Cuvier, 1797
Class Helcionelloida Peel, 1991
Order Helcionellida Geyer, 1994
Family Helcionellidae Wenz, 1938
Genus Anabarella Vostokova, 1962
6
1994 Planutenia Elicki: 82, figs 5/2, 13. 1996 Planutenia (Elicki); Elicki: 166, pl. 5, figs 1–9. 2003 Planutenia (Elicki); Elicki: 386, fig. 16. 2007 Planutenia (Elicki); Skovsted & Peel: 735, figs 4G, H. 2010 Planutenia (Elicki); Skovsted & Peel: 756, fig. 2.7.
Diagnosis. From Gubanov et al. (2004). Small, bilaterally symmetrical univalve mollusc with advolute or slightly involute, rapidly expanding, laterally compressed shell, and coiled through about one whorl. Shell smooth or slightly ornamented by transverse growth lines and regularly spaced comarginal rugaes.
Type species. Anabarella plana Vostokova, 1962.
Species composition. See review by Parkhaev in Gravestock et al. (2001), Gubanov
& Peel (2003), Gubanov et al. (2004) and Parkhaev (2004a).
Anabarella australis Runnegar in Bengtson et al. 1990.
(Figs 2–5)
1990 Anabarella australis Runnegar in Bengtson et al.: 240, 241, figs 163A, 164A– G. 1990 Anabarella argus Runnegar in Bengtson et al.: 241, figs 164J–N. 1994 Planutenia flectata Elicki: 82, figs 5/1, 12. 1994 Planutenia inclinata Elicki: 82, figs 5/2, 13. 1996 Anabarella australis (Runnegar); Elicki: 152, pl. 6, figs 1, 2. 1996 Planutenia flectata (Elicki); Elicki: 166, pl. 5, figs 1, 2. 1996 Planutenia inclinata (Elicki); Elicki: 166, pl. 5, figs 3–9. 2000 Anabarella australis (Runnegar); Parkhaev: 397, figs 13–15. 2001 Anabarella australis (Runnegar); Gravestock et al.: 185–187, pl. 42, figs 1–14. 2003 Anabarella cf. argus (Runnegar); Wrona: 204, fig. 13B. 2003 Planutenia inclinata (Elicki); Elicki: 386, fig. 16. 2004a Anabarella australis (Runnegar); Parkhaev: 255, pl. 2, figs 3, 4.
7 2004 Anabarella australis (Runnegar); Gubanov et al.: 722, figs 2.1–17. 2004 Anabarella australis (Runnegar); Skovsted: 27, fig. 7N. 2007 Planutenia flectata (Elicki); Skovsted & Peel: 735, figs 4G, H. 2009 Anabarella australis (Runnegar); Topper et al.: 230, figs 10E–G. 2010 Planutenia flectata (Runnegar); Skovsted & Peel: 756, fig. 2.7. 2016 Anabarella australis (Runnegar); Betts et al.: 196, figs 18I–M. 2017 Anabarella australis (Runnegar); Parkhaev: 456, pl. 1, figs 15, 16.
Holotype. SAMP29017 from the base of Mernmerna Formation, Lower Cambrian,
Horse Gully, Yorke Peninsula, South Australia.
Materials. More than 400 specimens from the Chaijiawa Section at Longxian County,
Shaanxi Province, North China.
Occurrences in North China. Cambrian upper Stage 3 or lower Stage 4, Xinji
Formation, South margin of North China Block. Longxian, Shaanxi Province; Luonan,
Shaanxi Province; Yexian, Henan Province; Huoqiu, Anhui Province.
Description. Phosphatized coating specimens are identical to Anabarella australis.
Shells are small, univalve, rapidly expanding, bilaterally symmetrical and strongly compressed laterally (Fig. 2C). They are coiled throughout about one whorl, with initial shell tightly coiled, and almost connecting to the apertural margin (Fig. 3D, G–
K). Protoconch of the shell is globular, clearly separated from the teleoconch by a distinct circumferencial ridge and initially identified growth lines (Fig. 3I, J). In contrast, internal moulds are very similar to Planutenia in general morphology (Fig.
2A). The initial part of internal mould specimen is blunt, slightly bent posteriorly, with apex extremely overhanging over the rear margin of the aperture, and coiled about half one whorl (Fig. 3A–C, E, F). Interestingly, these two distinctive morphtypes (Anabarella-type coating and Planutenia-type internal mould) co-occur
8 in the same specimen, which form a distinct void chamber in the initial part of the shell (Fig. 2B, 3H).
Landmark-based geometric morphometric analysis. In total, 96 relative wrap scores were obtained from the analysis (see Supplementary Table 1). The first identified wrap score (RW1) accounts for 44.72% of the total morphological variance followed by 27.11%, 10.80%, 5.50% for RW2, RW3, RW4, respectively. The first two RWs jointly account for 71.83% of the total variance. Internal moulds with positive value of RW1 generally show an extremely concave subapical wall, which results in a strongly overhanging apex. A negative score for the RW1 corresponds to a relative convex subapical wall in contrast to that of the mean shape of the internal moulds. RW2 score mainly concerns the height of the shell. A positive RW2 score reflects a comparably lower mould, while those with negative scores have a much higher mould with well-rounded large subapical area. In general, internal moulds from the Chaijiawa section show narrow morphological range of variabilities and consistent in the Planutenia morphotype (Fig. 4).
Microstructure. Shell microstructures are observable in ad-apertural areas of internal moulds owing to preservational bias, which consist of mould of aragonitic crossed foliated lamellae (CFL) (Fig. 5A, B). The first-order lamellae are comarginally arranged, and more or less overlapping each other (Fig. 5B). Second-order folia grow incrementally, parallel to the shell surface, margined with straight growth front, and arranged in distinct stepwise pattern (Fig. 5C, D). Third-order laths and sub-structural elements, however, can hardly be observed due to the limits of preservation.
9
Discussion
Preservation, Taxonomy and Comparison
Preservation of Cambrian mollusc shells is strongly influenced by rock type, diagenesis, and the method of fossils extraction (Gubanov & Peel 2003). In studies based on small shelly fossil assemblages, Cambrian calcareous shells have most often been preserved as phosphatized internal moulds, which not only show considerable morphological variations, but also can look strikingly different from their external moulds or coatings owing to preservational bias (Bengtson et al. 1990; Skovsted
2004). New materials of Anabarella australis from the Chaijiawa section show a similar trend. However, the principle shape variation of internal moulds mainly focuses on sub-apical areas that vary in size and shape. Otherwise, they are consistent in morphology with mean shape strikingly resembling Planutenia, as illustrated in the
TPS-grids by the method of landmark based geometric morphometrics. In these cases, most of our specimens from the Chaijiawa section express a distinct
“pseudo-dimorphism” pattern, as external coatings are obviously identical to
Anabarella, while associated internal moulds are more similar to Planutenia.
For comparison, specimens of A. australis were collected from equivalent units of the Xinji Formation along the south margin of North China Block. Internal moulds specimens of A. australis from Sanjianfang section of Henan Province (Skovsted et al.
2016) share most similarities with those from the Chaijiawa section of Shaanxi
10 Province in gross appearance. While, internal moulds from Shangzhangwan section of
Shaanxi Province (Li et al. 2014) have distinctive morphology, and are more similar to internal moulds from the Mernmerna Formation of South Australia and Bastion
Formation of Northeast Greenland (Personal observation; Bengtson et al. 1990;
Gravestock et al. 2001; Gubanov et al. 2004). This new data provide convincing evidence that the wide morphological variations of internal moulds of Anabarella australis are mainly ascribed to preservational artefacts rather than real intra- and interspecific variation of the animal. In particular, the co-occurrence of
Anabarella-type coating and Planutenia-type internal mould in the same specimen indicates that Planutenia is a subjective synonym of Anabarella.
Owing to the limits of preservation and uncertainties regarding the species-level classification of Anabarella based on internal moulds, a number of valid and invalid
“species” have been named in previous literature (Gravestock et al. 2001; Parkhaev
2004a). The type species A. plana has a global distribution in Cambrian Terreneuvian
Epoch, and is characterized by growth lines and regularly spaced comarginal rugae on external shells. In contrast, A. australis from South Australia, Northeast Greenland,
Germany and North China show smooth or comarginal growth lines. Internal moulds of the middle Cambrian (Miaolingian Epoch) species A. simesi Mackinnon, 1985 from New Zealand and A. chelata Skovsted, 2006 from Nevada, which were transferred to Mellopegma by Vendrasco et al. (2011b) and Peel et al. (2016) respectively, however, are strikingly similar to the Planutenia-type form in general morphology. Notwithstanding, one confusing specimen of A. simesi with rugose
11 growth lines reported by Peel et al. (2016 Fig. 14L) is distinguishable from A. plana and A. australis by more slowly expanding and less tightly coiled shell. Therefore, we reconsider A. simesi as a valid species of Anabarella, but also need further taxonomic investigation. The situation of the remaining species such as Anabarella tshitaensis
Parkhaev, 2004a, known only from internal moulds, is hard to evaluate, except that internal moulds of Planutenia flectata and P. inclinata from the upper Ludwigsdorf
Member of Germany, which share strikingly similarities with the Planutenia morphotype from the Chaijiawa section, and thus are very likely to be Anabarella australis.
Shell microstructures of Anabarella
Polygonal texture, on average ranging from 5μm to 30μm in size, concave, convex or with tubercles, was common among early Cambrian molluscs (Ushatinskaya &
Parkhaev 2005). Similar polygonal texture was previously described from the apical area and dorsal margin of Anabarella plana, and was interpreted representing a prismatic outer shell layer (Fig. 6A; Kouchinsky 1999). However, there are other possibilities regarding the polygonal network on the surface of molluscan internal moulds, which were variously interpreted as imprints of muscle scars (Fig. 6C, D;
Parkhaev 2004b, 2006, 2014), intra-prismatic membrane (Fig. 6B; Vendrasco et al.
2010a, b), cells or cell aggregates of the outer mantle epithelium (Ushatinskaya &
Parkhaev 2005), nacre or intra-nacreous membrane (Fig. 6E; Vendrasco et al. 2015).
In addition, Vendrasco et al. (2015) argued that similar polygonal texture can also be
12 formed during diagenetic alteration (Fig. 6F). It is hitherto difficult to confirm the assumption that polygonal texture merely corresponds to prism-like units of outer shell layers. Consequently, the occurrence of prismatic shell microstructure in
Anabarella and other Cambrian molluscan shells remains doubtful until further evidence is available.
In contrast with the dubious prismatic microstructure of the outer shell layer of A. plana, the microstructural fabrics of the inner shell layer were much better preserved.
The new specimens show that the inner shell layer of Anabarella australis consists of crossed foliated lamellar microstructure and is exclusively aragonite in mineralogy
(Fig. 5A–D). The lamellar microstructures with a stepwise pattern, which were previously regarded as “nacreous” (Runnergar 1985), crossed lamellar (Kouchinsky
1999) and foliated aragonite (Vendrasco et al. 2011a) in A. plana, are very likely partially preserved CFL layer as each second-order folia of CFL grows in a distinct stepwise pattern.
Although the limited preservability of shell microstructures replicated on the surface of internal moulds hampers further investigations from various longitudinal and transverse directions, new data shed a new light on the evolution and biomineralization of Cambrian molluscs. Shell microstructure of Anabarella australis is strikingly similar to the initial shell layer of crossed foliated lamellar (CFL) of
Pelagiella madianensis in their general hierarchical organizations, as the first-order lamellae overlap slightly with each other, and thus form a pattern of irregularly alternating sequences in comarginal direction. However, a comparable mature layer of
13 CFL with a characteristic regular sequence as in P. madianensis (Li et al. 2017, Figure
7) has not yet been observed in Anabarella. It can be assumed that the CFL in
Anabarella represented an early evolutionary stage and retained a more primitive organization. Notwithstanding, Anabarella was one of the earliest helcionelloid molluscs, with its first appearance in the regional Purella Zone of the Siberian
Platform (equivalent to Upper Fortunian Stage; Kouchinsky et al. 2017). The complex skeletal organization of Anabarella suggests that mechanism of molluscan biomineralization evolved very rapidly.
Acknowledgements
We are grateful to Cong Liu and Wenrui Pei for picking up specimens. Yanlong Chen and Yu Wu are particularly thanked for their assistance in Geometric Morphometric analysis. Li, L-Y is supported by the China Scholarship Council (CSC, No.
201706970037) for one year research stay with Christian B. Skovsted in Swedish
Museum of Natural History. This research was supported by funds from the National
Key Research and Development Program (Grant No. 2017YFC0603101), Natural
Science Foundation of China (Grant nos. 41621003), the Strategic Priority Research
Program of Chinese Academy of Sciences (Grant No. XDB26000000), 111 Project
(D17013), and Swedish Research Council (Grant VR2016-04610).
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Figure Captions
Figure 1. Geological setting and stratigraphic column of studied section. A, the Xinji
Formation deposits along the south margin of North China Block, with the Chaijiawa section located at Longxian, Shangzhangwan section at Luonan, Sanjianfang section at Yexian; B, composite stratigraphic column of the Xinji Formation, with abundant specimens collected from the middle unit of bioclastic carbonate. NP: Neoproterozoic;
21 DP: Dongpo; ZSD: Zhushadong.
Figure 2. “Pseudo-dimorphism” preservational pattern of Anabarella. All specimens were collected from the Xinji Formation of Chaijiawa section, Longxian, Shaanxi
Province, North China. A, LC0623-04, internal mould showing the Planutenia morphotype, lateral view; B, LC0625-04, showing the pseudo-dimorphism pattern of
Anabarella, lateral view; C, LC0624-14, external coating showing the Anabarella morphtype. Scale bars are 100μm.
Figure 3. Anabarella australis from the Chaijiawa section of North China. A,
LC0629-29, internal mould; B, LC06-A-76, internal mould; C, LC0624-03, internal mould; D, LC0627-03, external coating; E, LC0622-05, internal mould; F,
LC0629-02, internal mould; G, LC0626-04; H, LC0621-12; I, LC0603-02; J, amplification of I, showing the protoconch; K, LC0621-22, showing distinct growth lines. Scale bars are: A–I, 100μm; J, K, 10μm.
Figure 4. Landmark based geometric morphometric analysis. A, LC0605-15. SEM image of Anabarella australis; B, consensus configuration (i.e. mean shape) of all specimens; C, plots for RW 1, 2 of the relative warp analysis (RWA) of the shape of internal mould of A. australis; RW1=44.72%, RW2=27.11% of the total shape variance.
22 Figure 5. Crossed foliated lamellar microstructure of Anabarella australis. A, simplified reconstruction of the crossed foliated lamellar microstructure; B,
LC0609-04, lateral view; C, amplification of B, shows the first-order lamellae overlapping each other; D, LC0620-21, showing first-order lamellae in comarginal direction; E, LC0609-012, showing second-order folia with straight fronts; F,
LC0619-15, second-order folia and third-order laths. Scale bars are: C, 50μm; D, E,
10μm; F, 5μm.
Figure 6. Possible origin of polygonal texture on Cambrian molluscan steinkerns. A–
D, shell microstructures of living unidentified oyster mollusc, A, B, Sample-O1-3, A, prismatic shell microstructure; B, intra-prismatic membrane; C, D, Sample-O1-4, C, adductor muscle; D, polygonal texture within muscle attached zone; E, column nacre and intra-nacreous membrane of unidentified living gastropod; F, Ajax-M-1-01, shows digenetic polygonal texture on internal mould of Pelagiella subangulata. Scale bars are: C, F, 20μm; A, B, D, E, 10μm.
23 Journal of Systematic Palaeontology Page 24 of 29
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 For Review Only 20 21 Figure 1. Geological setting and stratigraphic column of studied section. A, the Xinji Formation deposits 22 along the south margin of North China Block, with the Chaijiawa section located at Longxian, 23 Shangzhangwan section at Luonan, Sanjianfang section at Yexian; B, composite stratigraphic column of the 24 Xinji Formation, with abundant specimens collected from the middle unit of bioclastic carbonate. NP: 25 Neoproterozoic; DP: Dongpo; ZSD: Zhushadong. 26 27 75x32mm (300 x 300 DPI) 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp Page 25 of 29 Journal of Systematic Palaeontology
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Anabarella 16 Figure 2. “Pseudo-dimorphism” preservational pattern of . All specimens were collected from the Xinji Formation of Chaijiawa section, Longxian, Shaanxi Province, North China. A, LC0623-04, internal 17 mould showing the Planutenia morphotype, lateral view; B, LC0625-04, showing the pseudo-dimorphism 18 pattern of Anabarella, lateral view; C, LC0624-14, external coating showing the Anabarella morphtype. Scale 19 For Reviewbars are 100µm. Only 20 21 43x11mm (300 x 300 DPI) 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp Journal of Systematic Palaeontology Page 26 of 29
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 For Review Only 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Figure 3. Anabarella australis from the Chaijiawa section of North China. A, LC0629-29, internal mould; B, 41 LC06-A-76, internal mould; C, LC0624-03, internal mould; D, LC0627-03, external coating; E, LC0622-05, 42 internal mould; F, LC0629-02, internal mould; G, LC0626-04; H, LC0621-12; I, LC0603-02; J, amplification 43 of I, showing the protoconch; K, LC0621-22, showing distinct growth lines. Scale bars are: A–I, 100µm; J, 44 K, 10µm. 45 46 180x188mm (300 x 300 DPI) 47 48 49 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp Page 27 of 29 Journal of Systematic Palaeontology
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 For Review Only 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Figure 4. Landmark based geometric morphometric analysis. A, LC0605-15. SEM image of Anabarella 40 australis; B, consensus configuration (i.e. mean shape) of all specimens; C, plots for RW 1, 2 of the relative 41 warp analysis (RWA) of the shape of internal mould of A. australis; RW1=44.72%, RW2=27.11% of the total 42 shape variance. 43 174x176mm (300 x 300 DPI) 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp Journal of Systematic Palaeontology Page 28 of 29
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 For Review Only 20 21 22 23 24 25 26 27 28 29 30 31 Figure 5. Crossed foliated lamellar microstructure of Anabarella australis. A, simplified reconstruction of the 32 crossed foliated lamellar microstructure; B, LC0609-04, lateral view; C, amplification of B, shows the first- 33 order lamellae overlapping each other; D, LC0620-21, showing first-order lamellae in comarginal direction; 34 E, LC0609-012, showing second-order folia with straight fronts; F, LC0619-15, second-order folia and third- 35 order laths. Scale bars are: C, 50µm; D, E, 10µm; F, 5µm. 36 37 130x98mm (300 x 300 DPI) 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp Page 29 of 29 Journal of Systematic Palaeontology
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 For Review Only 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Figure 6. Possible origin of polygonal texture on Cambrian molluscan steinkerns. A–D, shell microstructures 44 of living unidentified oyster mollusc, A, B, Sample-O1-3, A, prismatic shell microstructure; B, intra- 45 prismatic membrane; C, D, Sample-O1-4, C, adductor muscle; D, polygonal texture within muscle attached 46 zone; E, column nacre and intra-nacreous membrane of unidentified living gastropod; F, Ajax-M-1-01, 47 shows digenetic polygonal texture on internal mould of Pelagiella subangulata. Scale bars are: C, F, 20µm; 48 A, B, D, E, 10µm. 49 173x197mm (300 x 300 DPI) 50 51 52 53 54 55 56 57 58 59 60 URL: http://mc.manuscriptcentral.com/tjsp