{{Short description|Genus of problematic fossil}} {{Speciesbox | fossil_range = Paleoproterozoic, {{long fossil range|2200}} | image = Diskagma holotype.tif | image_caption = ''Diskagma buttonii'' holotype, in thin section of 2.2 billion year old Waterval Onder paleosol, South Africa | genus = Diskagma | parent_authority = Retallack (2013) | species = buttonii | authority = Retallack (2013) }}
'''''Diskagma''''' ("disc-like fragment") is a genus of problematic fossil from a Paleoproterozoic (2.2 billion years old) paleosol from South Africa, and significant as one of the oldest likely eukaryotes and some of the earliest evidence for life on land.<ref name="Retallacketal.2013">{{cite journal| journal=Precambrian Research| title=Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa|author1=Retallack, G.J. |author2=Krull, E.S. |author3=Thackray, G.D. |author4= Parkinson, D. | volume=235| pages=71–87|year=2013| doi=10.1016/j.precamres.2013.05.015| bibcode=2013PreR..235...71R}}</ref>
thumb|left|Reconstruction of ''Diskagma buttonii''.
==Description== ''Diskagma buttonii'' is a small fossil less than 1mm in length found within the surface horizon of a vertisol paleosol above the Hekpoort Basalt dated to 2.2 billion years old. The opacity of the matrix and the size of the fossil meant that its three dimensional structure required imaging by computer-assisted x-ray tomography using a cyclotron source.<ref name="Retallacketal.2013" /> 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 under greenschist 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 of ''Diskagma'' have scattered spiny or tubular extensions. thumb|left|Poorly preserved filamentous structures in the apex of ''Diskagma buttonii''
==Biological affinities==
thumb|3D reconstruction of ''Diskagma buttonii'' ''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 of cytoskeletal complexity found in eukaryotes, but it predates the other fossil candidate for the oldest eukaryote ''Grypania'', now known to be 1.8 billion years old,<ref name="Schneideretal.2002">{{cite journal| journal=Canadian Journal of Earth Sciences| title=Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region|author1=Schneider, D.A. |author2=Bickford, M.E. |author3=Cannon, W.F. |author4=Schulz, K.J. |author5=Hamilton, M.A. | volume=39| pages=999–1012|year=2002| bibcode=2002CaJES..39..999S| doi=10.1139/e02-016| issue=6}}</ref> and at 2.2 billion years old is much older than molecular clock estimates for eukaryotes of 1.6 billion years.<ref>{{cite book| title=The time tree of life|author1=Hedges, S.B. |author2=Kumar, S.| year=2009 | publisher=Oxford University Press, New York}}</ref> Another similar fossil is ''Horodyskia''. The size and hollow shape of ''Diskagma'' are similar to the living fungus ''Geosiphon'', which is endosymbiotic with the cyanobacterium ''Nostoc''. However, the apical cup and filaments are not seen in modern ''Geosiphon''.
==Paleoenvironmental significance==
''Diskagma buttonii'' dates to the Paleoproterozoic Great Oxygenation Event, a time of marked increase in atmospheric oxygenation compared with that of the Archean.<ref name="Murakamietal.2011">{{cite journal| journal=Geochimica et Cosmochimica Acta| title=Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics|author1=Murakami, T. |author2=Sreenivas, B. |author3=Sharma, S.D. |author4=and Sugimori, H. | volume=75| pages=3982–4004|year=2011| bibcode=2011GeCoA..75.3982M| doi=10.1016/j.gca.2011.04.023| issue=14}}</ref> If, like the living ''Geosiphon'', the central cavity of ''Diskagma'' housed a photosymbiont, it may have contributed to atmospheric oxygenation.
Although Precambrian landscapes are customarily regarded as barren as the surface of Mars, ''Diskagma'' is evidence for very early life on land. Furthermore, at 2.2 billion years old, ''Diskagma'' was larger than coeval marine microbes of the Gunflint Chert, and more complex than stromatolites.
==References== {{Reflist}}
==External links== *[http://www.sci-news.com/paleontology/science-fossils-early-terrestrial-life-01249.html news coverage] *''[http://www.indexfungorum.org/Names/genusrecord.asp?RecordID=20238 Geosiphon]'' at Index Fungorum *[http://www.geosiphon.de The ''Geosiphon pyriformis'' symbiosis - fungus 'eats' cyanobacterium] Schuessler lab *[http://www.amf-phylogeny.com Glomeromycota phylogeny] {{Eukaryota}} {{Taxonbar|from1=Q21369073|from2=Q16981390}}
Category:Monotypic eukaryote genera Category:Paleoproterozoic Category:Controversial taxa Category: Proterozoic life