Experimental evidence for the microbial origin of Precambrian discoidal fossils: insights from cultivation and taphonomy of fungal colonies

Macroscopic fossils of microbial colonies, particularly those unrelated to cyanobacterial mats, remain underrepresented in studies of the Precambrian fossil record. Although microbial life was ubiquitous in early Earth environments, its role in generating colonial macroscopic structures has received little attention from paleontolgy. In this study, we cultured three fungal species, Penicillium copticola , Penicillium polonicum , and Cladosporium halotolerans , under varying temperature and pH conditions to evaluate the morphological stability of their colonies. To simulate burial and fossilization processes, mature colonies were embedded in fine-grained sediment and subjected to static pressure for two weeks, enabling assessment of their preservation potential. Our results demonstrate that fungal colonies, particularly those of Cladosporium halotolerans , exhibit remarkable morphological stability during growth and exceptional resistance to deformation during compaction. Diagnostic features, including radial furrows, concentric rings, crater-like elevations, and sharply defined colony margins, remained well-preserved after burial. These structures closely resemble discoidal fossils reported from the Paleoproterozoic Francevillian Series (∼2.1 Ga, Gabon) as well as from various Ediacaran assemblages. These findings suggest that some Precambrian discoidal fossils, commonly interpreted as medusoids, holdfasts, or early multicellular organisms, may alternatively represent fossilized microbial colonies produced through morphological convergence. This study provides an experimentally validated taphonomic model for the preservation of microbial colonies and highlights the need to critically reassess the biological affinities of Precambrian discoidal fossils. Before assigning such fossils to complex multicellular lineages, their potential of microbial origin should be rigorously evaluated.