Figure 1.Shell architecture of Latusobolus xiaoyangbaensis gen. et sp. nov. from the Cambrian Series 2 Shuijingtuo Formation in southern Shaanxi, South China. A-C, ELI-XYB S5-1 BS01. A, cross section of a ventral lateral margin, note post-metamorphosis pustules by arrow, primary layer 1 and stacked sandwich columnar units 2-7, box indicates area in B. B, enlarged view of A, showing top primary-underlying 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 underlying secondary layer columns. H, one unit of dorsal stacked columnar architecture with the exfoliation of top primary layer, noting column canals on the stratiform lamella surface by arrows, ELI-XYB S4-2 BO08. I, apatite spherules of granule aggregations of ventral columnar shell structure, note granule rods by arrow and thin gap left by the degradation of organic counterparts 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.Figure 2.Shell architecture of ventral Eoobolus acutulus sp. nov. from the Cambrian Series 2 Shuijingtuo Formation in Three Gorges areas, South China. A-C, ELI-AJH S05 BT12. A, cross section of a ventral lateral margin, note primary layer 1 and stacked sandwich columnar units 2-11, box indicates area in B. B, enlarged view of A. C, enlarged view of B, show thin gap left by the degradation of organic counterparts (tailed arrow) between two stratiform lamellae by dashed lines, the fusion point of two columnar units 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 columnar architecture. F, poorly phosphatised columns, note the opening of organic canals along the organic membrane by arrows, and space between two stratiform lamellae by tailed arrows. G-H, ELI-AJH 8-2-3 BT03. G, note organic canals on the cross section and surface of stratiform lamella 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 architecture (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.Figure 3.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 surfaces beneath by arrows, 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 dorsal 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, epithelial cell moulds on stratiform lamella surfaces of successive three stacked sandwich columnar units developed on cardinal muscle areas with columns between (arrow). Scale bars: A, D, 50 µm; B, E, F, H, 10 µm; C, G, I, 20 µm.Figure 4.Biomineralized columnar architecture 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 primary layer 1 and stacked sandwich 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 stacked columnar units into one by arrow. D, imbricated growth pattern of stacked columnar units. E, Palaeotreta zhujiahensis, note column openings (arrow) on eight successive columnar units by tailed arrows, ELI-AJH 8-2-1 AE09. F-J, Eohadrotreta zhenbaensis. F, relatively taller columns (ca. 20 µm), ELI-AJH 8-2-1 acro16. G, apatite spherules of granule aggregations in one columnar unit, note column openings (arrows) on both stratiform lamella surfaces, ELI-AJH S05 E18. H, cross section show column openings on 4 successive units by tailed arrows, ELI-WJP 7 AB98. I, poorly phosphatised columns (arrows), note openings of organic canals on surface of stratiform lamella by tailed arrows, ELI-AJH S05 I76. J, stacked columnar units 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; D, J, 50 µm.Figure 5.Biomineralization process of typical columnar architecture by the stacked sandwich model of phosphatic-shelled brachiopods. Abbreviation: D=Diameter; H=height; T=Thickness. (modified from (Williams and Holmer, 1992; Z. L. Zhang et al., 2016))Figure 6.The evolution of stacked sandwich columnar architecture in early Eoobolidae Eoobolus incipiens, Latusobolus xiaoyangbaensis gen. et sp. nov., Eoobolus acutulus sp. nov., Eoobolus variabilis, Eoobolus? aff. priscus, Lingulellotretidae Lingulellotreta malongensis, Lingulellotreta ergelievi, Acrotretida Palaeotreta shannanensis, Palaeotreta zhujiahensis, Eoohadrotreta zhenbaensis, Hadrotreta primaeva, Canthylotreta crista and stem group Setatella significans. The height and diameter data of column are from the literatures (Skovsted and Holmer, 2003; Streng et al., 2007; Streng and Holmer, 2006; Ushatinskaya and Korovnikov, 2014; Z. L. Zhang et al., 2016, 2020a, 2020b).