Fig 1.
The geological setting and stratigraphy of the study area.
(A) Paleogeography of the North China Platform in the early period of Cambrian Epoch 3 (modified after Feng [21]). (B) Simplified geological map of the Mianchi area. The white area corresponds with the Quaternary. (C) Stratigraphic column of M3 Fm. in the Rencun section. The black arrow indicates fossil beds.
Fig 2.
Photograph of section and thrombolites.
(A) Overview of the Cambrian succession in the study area. The white arrow denotes the thrombolite horizon (B) Sharp contact between microbialite and overlying oolitic limestone (dashed line). (C) Stratiform thrombolite comprising yellow and black clots. (D) Enlargement of the rectangular area in (C) to show bifurcating clots perpendicular to bedding surfaces and truncated by the upper argillaceous stylolite. (E) Thrombolites comprise domed mounds and argillaceous interstitial matter. Hammer length = 33 cm. (F) Domed mounds showing the grey infillings and black clots that stack perpendicularly to bedding. Gel pen length = 14 cm.
Fig 3.
Photomicrographs of clot and calcified cyanobacteria.
(A), (B), (C), and (D) show polished blocks of thrombolites viewed under reflected light, while (E), (F), (G) and (H) show calcified cyanobacteria viewed under transmitted light. (A) Clear boundary between a black clot and grey infilling. The positions of the figure represent geochemical sampling points that are listed in Table 1. (B) Brown patches in the dendritic clot suggest non-homogeneous internal components. (C) Enlargement of the rectangular (dashed line) area in (B) showing brown dendritic branches. (D) Enlargement of the rectangular (full line) area in (B) showing spherical colonies. (E) The fine calcite in branches (yellow arrows) and spheres (white arrows) has darker outlines compared to peripheral coarse calcite. Co-occurrence of spheres and branches may indicate their affinity. (F) Dichotomous branches of Epiphyton. (G) Tubomorphophyton composed of thin micritic walls and filled with microsparite. (H) Close up of the rectangle in (G) showing tubiform structure.
Fig 4.
Scanning electron microscope images of Epiphyton within Cambrian thrombolites.
(A) Regular rectangles (white arrow). (B) Spheres (white arrow) with fine grained calcite and external coatings, back scattered electron mode.
Fig 5.
Scanning electron microscope images of rectangular and spherical capsules from Cambrian thrombolites.
(A) Rectangular capsules (marked with dotted line) and smaller rectangles (marked with arrow). (B) Two rectangles connected end-to-end and sharing the same envelope. These capsules comprise micritic calcite in contrast to the surrounding sparry calcite. (C) Close-up view of envelopes that separate two capsules. (D) Some acicular or fibrous clay minerals in pits, marked with white arrows. (E) Irregular spherical capsules. (F) Close up of capsule showing pits arrayed equidistantly within calcite. (G) Hollow and round pits containing scattered lamellar minerals. (H) Lamellar envelopes around the capsule.
Fig 6.
EDS spectra of mineralized spherical capsule (A, spot analyses) and rectangular capsule (B, panel analyses). Scale bars: A– 50 μm; B– 70 μm.
Table 1.
δ13C and δ18O for black clots (B) and greyish fillings (G) of the Cambrian thrombolites (Sampling points are shown in Fig 3A and 3B), and the Late Jurassic calcified counterparts (shadow).
Fig 7.
Archean and modern analogues of Epiphyton.
(A–C) Archean calcified coccoid cyanobacteria (modified after Kazmierczak et al. [26]) showing honeycomb patterns, layered envelopes, pits with silicified cocci and identical elements. (D) A modern coccoid cyanobacterial mat from Lake Van (Turkey) (modified after Kazmierczak et al. [26]) showing honeycomb structure and similar elements. (E) Examples of degradation cocci and remaining pits from Lake Van [26]. (F) Colonies with a number of cells contained within an outer membrane, which represent a prototype of mineralized capsules [39]. (G) Model to show the correspondence between cocci colonies in vivo and the mineralized capsule.
Fig 8.
(A) In vivo calcification for elevated alkalinity via photosynthesis. (B) Post-mortem calcification via sulfate reduction which leads to the alkalinity produced by the metabolism of sulfate reducing bacteria. Chemical equation showing how rising alkalinity contributes to precipitation. (C) Authigenic Al-Mg-Fe silicate minerals that encase the capsule.
Fig 9.
The cell division and assemble model for Epiphyton.
(A) Cell division pattern showing two new daughter cells and gelatinous envelopes that are reproduced (modified after Golubic and Hofmann [39]). Envelopes in sequence are marked as 1°, 2°, and 3°. (B) A slightly calcified colony of modern coccoidal cyanobacteria (Entophysalis sp.) from Lake Van (Turkey) [27] showing how thicker mucilage envelopes stack vertically to be branches. Scale bars: 50 μm. (C) An apical growth model for Epiphyton implying that dendritic thalli formed by successive growth and the calcification of similar-sized colonies.