PALEOPROTEROZOIC LIFE ON LAND AND THE RISE OF ATMOSPHERIC OXYGEN

The recently described problematicum Diskagma buttonii includes small (0.3-1.8 mm long), locally abundant, urn-shaped fossils attached by rhizomorphs and runners to surface horizons of a paleosol in the 2.2 Ga Hekpoort Formation near Waterval Onder, South Africa. Despite greenschist facies metamorphic recystallization, total organic carbon of the fossils is 0.04 % and its isotopic composition (δ¹³C) is –25.6 ± 0.08 ‰ vs VPDB. The fossils are locally clumped within surface swales of a Vertisol paleosol, identified from characteristic penecontemporaneous deformation (clastic dikes between swales of mukkara structure) and from pronounced geochemical differentiation (phosphorus and copper strain-corrected mass-depletion characteristic of an oxidized biologically active soil). This paleosol’s chemical composition is evidence of temperate humid climate (mean annual temperature 11.3 ± 4.4^oC, and mean annual precipitation 1489 ± 182 mm). Observation of hundreds of specimens in thin section reveals substantial variation in growth (elongation) and decay (shredding and deflation). They had a hollow ellipsoidal interior that is unusually devoid of opaque debris, unlike the matrix. Although larger and morphologically more complex than known Paleoproterozoic microfossils, Diskagma predates the oldest previously reported fossil eukaryotes (1.9 Ga) and fungi (1.5 Ga), and current molecular clock estimates for eukaryotes (1.6 Ga) and fungi (1.1 Ga). Lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks. Diskagma is superficially comparable with lichens such as Cladonia (Ascomycota with hollow thallus and cortical phycobionts) and Geosiphon (Glomeromycota with endosymbiotic cyanobacteria in central hollow), and photosynthetic capability of Diskagma is compatible with evidence from paleosols for atmospheric CO₂ of 6640 ⁺¹²⁸⁸⁰/_-4293 ppm (0.6%) and 0.9-5 % atmospheric O₂. Diskagma lived during the Great Oxidation Event, which terminated anoxic conditions of the Archean. Although biological affinities of Diskagma are uncertain, these fossils reveal that there was Paleoproterozoic life on land, its general size and shape, and a significant role for life in terrestrial weathering.