Shell ultrastructure of Latusobolus xiaoyangbaensis gen. et sp. nov. from the Cambrian Series 2 Shuijingtuo formation in southern Shaanxi. A-C, ELI-XYB S5- 1 BS01. A, cross section of a ventral lateral margin, note post-metamorphosis pustules by arrow and shell layers 1-7, box indicates area in B. B, enlarged view of A, showing primary-secondary layer boundary by dashed line. C, enlargement of shell layers 1-4 of A, note organic canals of columns (arrows), organic layer (tailed arrows) between two stratiform lamellae by dashed lines. D, poorly phosphatised columns of ventral valve, note organic canals by tailed arrows, ELI-XYB S5-1 BR06. E, columns of dorsal valve, ELI-XYB S5-1 BS17. F, cross section of a ventral lateral margin, show developed post-metamorphosis pustules, box indicates the area in G, ELI-XYB S4-2 BO06. G, enlarged primary layer pustules and secondary layer columns underneath. H, one layer of dorsal columnar shell with the exfoliation of above primary layer, noting column canals on the layer surface by arrows, ELI-XYB S4-2 BO08. I, apatite spherules of granule aggregations of ventral columnar shell, note granule rods by arrow and organic membrane by tailed arrow, ELI-XYB S4-2 BO06. Scale bars: A, 50 µm; B, E, G, 20 µm; C, H, 10 µm; D, 5 µm; F, 100 µm; I, 1 µm.

Ultrastructure of ventral Eoobolus acutulus sp. nov. from the Cambrian Series 2 Shuijingtuo formation in Three Gorges areas. A-C, ELI-AJH S05 BT12. A, cross section of a ventral lateral margin, note stacked columnar units 2-11, box indicates area in B. B, enlarged view of A, showing shell layers 2-11. C, enlarged view of B, show thin organic layer (tailed arrow) between two stratiform lamellae by dashed lines, the fusion of two columnar layers by arrow. D-F, ELI-AJH 8-2-3 BT02. D, cross section of shell margin, box indicates area in E. E, different preservation condition of columns. F, poorly phosphatised columns, note the opening of organic canals along the organic lamellae by arrows, and space between two stratiform laminae by tailed arrows. G-H, ELI-AJH 8-2-3 BT03. G, note organic canals on the cross section and surface of columnar layer by arrows, and partly exfoliated primary layer by tailed arrow. H, magnified columns composed of granule spherules with canal in G by arrow. I, cross section of shell margin, box indicates area in (J), ELI- AJH 8-2-3 BT04. J, enlarged short columns. K, imbricated columnar layers (arrows), ELI-AJH S05 BT12. Scale bars: A, E, 50 µm; B, I, 20 µm; C, 5 µm; D, 200 µm; F, G, J, K, 10 µm; H, 1 µm.

Shell ornamentation, ultrastructure and epithelial cell moulds of Cambrian Series 2 brachiopods. A, post-metamorphic pustules of Latusobolus xiaoyangbaensis gen. et sp. nov., ELI-XYB S4-3 AU11. B-D, Eoobolus acutulus sp. nov. B, metamorphic hemispherical pits, ELI-AJH 8-2-2 Lin01. C, epithelial cell moulds, note column openings on layer surface beneath by arrow, ELI-AJH S05 N31. D, enlarged column openings on layer surface, ELI-WJP 7 CE05. E-I, Eohadrotreta zhenbaensis. E, partly broken columns, note organic canals by arrows, ELI-AJH F36. F, polygonal epithelial cell moulds on valve floor, dash lines indicate margin of one epithelial cell, ELI-WJP 6 R79. G-I, ELI-AJH Acro 053. G, epithelial cell moulds on median septum with columns between. H, enlarged view of G, note rudiment of columns by tailed arrows and one column on epithelial cell margin by arrow. I, three layers of epithelial cell moulds on cardinal muscles with columns between (arrow). Scale bars: A, D, 50 µm; B, E, F, H, 10 µm; C, G, I, 20 µm.

Columnar layers of Cambrian Series 2 brachiopods. A, Eoobolus incipiens, AJXM-267.5 DT12. B-D, Lingulellotreta ergalievi, ELI-AJH 8-2-3 CI11. B, cross section of shell margin, box indicates area in C, note stacked columnar units 2-22, and raised pseudointerarea (tailed arrow). C, enlarged view of thin organic layer (tailed arrow) between two stratiform lamellae by dashed lines, the fusion of two columnar units into one by arrow. D, imbricated growth pattern of columnar layers. E, Palaeotreta zhujiahensis, note column openings (arrow) on eight successive columnar units by tailed arrow, ELI-AJH 8-2-1 AE09. F-J, Eohadrotreta zhenbaensis. F, relatively higher columns (ca. 20 µm), ELI-AJH 8-2-1 acro16. G, Apatite spherules of granule aggregations in columnar layer, note column openings (arrows) on both stratiform lamella surface, ELI-AJH S05 E18. H, cross section show column openings on 4 successive units by tailed arrow, ELI-WJP 7 AB98. I, poorly phosphatised columns (arrows), note openings of organic canals on stratiform lamella by tailed arrows, ELI-AJH S05 I76. J, stacked columnar layers in imbricated pattern, ELI-WJP 6 R47. Scale bars: A, E, I, 10 µm; B, 100 µm; C, F, 20 µm; G, 2 µm; H, 5 µm; J, 50 µm.

Biomineralization process of typical columnar layer by the conveyor-belt system of phosphatic-shelled brachiopods. Abbreviation: D=Diameter; H=height; T=Thickness. (modified from (Williams and Holmer, 1992; Z. L. Zhang et al., 2016))

The evolutionary growth of stacked columnar shells in early Eoobolidae Eoobolus incipiens, Latusobolus xiaoyangbaensis gen. et sp. nov., Eoobolus acutulus sp. nov., Eoobolus variabilis, Eoobolus? aff. priscus, Lingulellotretidae Lingulellotreta malongensis, L. ergelievi, Acrotretida Palaeotreta shannanensis, Palaeotreta zhujiahensis, Eoohadrotreta zhenbaensis, Canthylotreta crista and stem group Mickwitzia cf. occidens. The height and diameter of column is from the literature (Skovsted and Holmer, 2003; Streng et al., 2007; Streng and Holmer, 2006; Ushatinskaya and Korovnikov, 2014; Z. L. Zhang et al., 2020a, 2020b, 2016).