The Ediacaran Lantian and Miaohe biotas (South China) are among the earliest-known fossil assemblages with macroscopic and morphologically complex life forms. Preservation of these macrofossils has been described as carbonaceous compressions in black shales. However, detailed study of their mode of preservation is lacking. We investigated the preservation of unweathered and weathered macrofossils from the Lantian and Miaohe biotas. Our results show that although some Miaohe and Lantian macrofossils are preserved as carbonaceous compressions lacking any enrichment of aluminosilicate clays, many Lantian and Miaohe macrofossils are preserved as carbonaceous compressions associated with densely packed framboidal pyrite, the loss of which leaves clustered cavities in weathered specimens. Pyritization probably did not happen until after extensive degradation of organisms and loss of three-dimensional morphology. Pyrite framboids in Lantian fossils are significantly larger than those in the adjacent matrix, implying different redox conditions.

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