| Diskagma Temporal range:Paleoproterozoic,2200 Ma | |
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| Diskagma buttonii holotype, in thin section of 2200 million year old Waterval Onder paleosol, South Africa | |
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| Genus: | Diskagma Retallack (2013) |
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| Diskagma buttonii Retallack (2013) | |
Diskagma ("disc-like fragment") is agenus of problematicfossil from aPaleoproterozoic (2200 million years old)paleosol fromSouth Africa, and significant as one of the oldest likelyeukaryotes and some of the earliest evidence for megascopic life on land.[1]
Diskagma buttonii is a small fossil less than 1mm in length found within the surface horizon of avertisolpaleosol above theHekpoort Basalt dated to 2200 million years old. The opacity of the matrix and the size of the fossil meant that its three dimensional structure required imaging by computer-assistedx-raytomography using acyclotron source[1] The fossils are shaped like an urn with an apical cup, which is filled with filamentous structures whose exact nature is uncertain due to recrystallization of the matrix undergreenschist facies metamorphism. The base of these hollow urns is a system of hollow tubes running over the paleosol and connecting the urns into groups. The walls ofDiskagma have scattered spiny or tubular extensions.
Diskagma buttonii is a problematic fossil that has been named before its biological affinities have been understood. Its size and complexity suggest that it had the degree ofcytoskeletal complexity found ineukaryotes, but it predates the other fossil candidate for the oldest eukaryoteGrypania, now known to be 1800 million years old,[2] and at 2200 million years old is much older thanmolecular clock estimates for eukaryotes of 1600 million years.[3] Another similar fossil isHorodyskia. The size and hollow shape ofDiskagma are similar to the living fungusGeosiphon, which is endosymbiotic with thecyanobacteriumNostoc. However, the apical cup and filaments are not seen in modernGeosiphon.
Diskagma buttonii dates to thePaleoproterozoicGreat Oxygenation Event, a time of marked increase in atmospheric oxygenation compared with that of the Archean.[4] If, like the livingGeosiphon, the central cavity ofDiskagma housed aphotosymbiont, it may have contributed to atmospheric oxygenation.
Although Precambrian landscapes are customarily regarded as barren as the surface ofMars,Diskagma is evidence for very early life on land. Furthermore, at 2200 million years old,Diskagma was larger than coeval marine microbes of theGunflint Chert, and more complex thanstromatolites.