Cryptic terrestrial fungus-like fossils of the early Ediacaran Period

The colonization of land by fungi had a significant impact on the terrestrial ecosystem and biogeochemical cycles on Earth surface systems. Although fungi may have diverged ~1500–900 million years ago (Ma) or even as early as 2400 Ma, it is uncertain when fungi first colonized the land. Here we report pyritized fungus-like microfossils preserved in the basal Ediacaran Doushantuo Formation (~635 Ma) in South China. These micro-organisms colonized and were preserved in cryptic karstic cavities formed via meteoric water dissolution related to deglacial isostatic rebound after the terminal Cryogenian snowball Earth event. They are interpreted as eukaryotes and probable fungi, thus providing direct fossil evidence for the colonization of land by fungi and offering a key constraint on fungal terrestrialization.


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in South China is well constrained to be ca. 635 Ma by a U-Pb zircon age of 634.6 ± 0. 9 Ma from the topmost Nantuo Formation 12 and a U-Pb zircon age of 635.2 ± 0. 6 Ma from the top of cap dolostone in South China 13 . In the Weng'an area, a whole-rock Pb-Pb isochron age of 599.3 ± 4.2 Ma was reported from the D4 Member at Weng'an 14  sheet-cavity walls. The basal part of ID is characterized by fine-grained radial or granular dolomite crystals, and the distal part typically consists of coarser rhombic dolomite. Zhou et al. (2010) proposed that ID cement in sheet-cavities was formed during a second transgression of regional or global scale after the Marinoan deglaciation 11 . Zhao et al. (2018), on the basis of rare earth element geochemistry, suggested that ID precipitated from the mixture of contemporaneous seawater and pore water near the water-sediment interface 18 . Considering the evidence for karstification related to post-glacial rebound and the termination of ID consistently followed by karstic FC, we infer that physically formed sheet-cracks may have been augmented by karstic dissolution in the phreatic zone and that ID precipitation probably occurred also in the phreatic zone by comparison with the vertical zoning of modern karst 19,20 ( Supplementary Fig. 2c)  Based on the above discussion, the sequence of mineral precipitation and 7 replacement in sheet-cavities is summarized in Supplementary Figure 2, emphasizing the relationship between post-glacial rebound, karstification, and dissolution and precipitation in the phreatic and vadose zones. The fungus-like microfossils reported in this study are preserved mostly in the silicified botryoidal structures with microcrystalline quartz core and radial chalcedony fabrics (RC), and we infer that these fungus-like micro-organisms colonized the cryptic cavities during the karstification.

Supplementary Note 3: U-Pb radiometric dating of isopachous dolomite (ID)
Isopachous dolomite cement in Doushantuo sheet-cavities was chosen for in-situ radiometric dating. After optical examination and LA-ICP-MS elemental analyses, suitable domains with high U/Pb ratios and no fractures were targeted for isotopic analysis using LA-ICP-MS to provide the best achievable precision and accuracy.
Uranium concentration of analyzed samples ranges from 0.81 to 9.66 ppm, total lead from 0. 17 Fig. 8), which is interpreted as the age of isopachous dolomite cementation in Doushantuo sheet-cavities.

Supplementary Note 4: Taphonomic discussion
Cyanobacteria, bacteria, fungi, and algae have been found in modern speleothems [33][34][35][36][37][38][39][40][41] , and culture experiments have shown that microbial activities can effectively promote the growth of carbonate minerals [42][43][44] . Speleothems are a suitable habitat for subaerial biofilms, and this is particularly true for certain speleothems such as moonmilk and flowstone, the formation of which are considered to be related to microbial activities 45 . Some microbial fossils, including bacterial and fungal fossils, have been known to be preserved in Jurassic calcareous crusts 46 , and dendritic cyanobacteria fossils in early Cretaceous micro-karst 47  modified from ref. 6 . c-e, based on modern mixing dolomitization model 19 and karst zone models 20,48 . Maximum distance between sheet-cavities and karstic surface atop the cap dolostone is 4 meters (i.e., the thickness of the cap dolostone), and sometimes sheet-cavities are exposed on and physically connected with the karstic surface ( Supplementary Fig. 1e).

Supplementary Figure 3 | Type A filaments and associated small spheres. a,
Aggregate of Type A filaments with small hollow spheres (white arrows). Filaments are embedded in but disrupted by chalcedony botryoids (yellow arrows). b, Multiple orders of branches and a short lateral branch (arrow). c, Short and bent lateral branch (arrow).  Table 2  Supplementary Tables   Supplementary Table 1