Re-evaluation of cryptoendolithic microfossils in pillow basalt of two Variscan orogens

The presence of fossilised fungi within deep crustal rock formations has been established based on fossil evidence from 400 Ma continental crust and 81 Ma oceanic basaltic crust. Moreover, the Palaeoproterozoic Ongeluk Formation contains putative fungal remains reaching 2.4 Ga. The resulting gap of 2 billion years raises questions regarding the history of fungi in marine subsurface environments, in particular the lack of bona fide fossils in ophiolites, sections of layered basalts from mid-ocean ridges. Devonian examples of cryptoendolithic microorganisms preserved in marine pillow basalt stem from the Arnstein locality, Rheinisches Schiefergebirge, and the Kahlleite locality, Thüringer Wald, Germany, and have previously been described as filaments of microorganisms with uncertain biological affinity. The filamentous fossils were investigated using environmental scanning electron microscopy, Raman spectroscopy, confocal microscopy, widefield microscopy, and optical light microscopy. Energy dispersive spectroscopy analyses of several of the inferred microfossils revealed a presence of clay minerals, pointing to a mode of mineralisation in association with organic matter and agreeing with a biological origin. Raman spectroscopy showed carbon localised within the studied filaments and revealed that particularly iron oxide minerals are associated with carbon. Element compositions similar to younger mineralised fungal remains and morphologies resembling sporophores and hyphae agree with the interpretation of the Arnstein and Kahlleite fossils as marine fungi, shedding new light on many of the previously undetermined fossils and plausibly narrowing the fossil gap of oceanic deep subsurface fungi by at least 300 million years.