Diskagma
| Diskagma Temporal range: Palaeoproterozoic | |
|---|---|
 | |
| Diskagma buttonii holotype, in thin section of 2200 million year old Waterval Onder paleosol, South Africa | |
| Scientific classification | |
| Kingdom: | Fungi |
| Division: | Glomeromycota |
| Order: | Archaeosporales |
| Genus: | Diskagma Retallack (2013) |
| Type species | |
| Diskagma buttonii Retallack (2013) | |
Diskagma ("disc-like fragment") is a genus of problematic fossil from a Paleoproterozoic (2200 million years old) paleosol from South Africa, and significant as the oldest likely eukaryote and earliest evidence for life on land.[1]
Description
Diskagma buttonii is a small fossil found within the surface horizon of a Vertisol paleosol above the Hekpoort 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-assisted x-ray tomography using a cyclotron 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 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.
Biological affinities
Diskagma buttonii is a problematicum, but its size and complexity suggest that it had the degree of cytoskeletal complexity found in eukaryotes. It predates the other fossil candidate for oldest eukaryote Grypania, now known to be 1800 million years old,[2] and also is older than molecular clock estimates for eukaryotes of 1600 million years.[3] Its exact biological affinities are unknown. Its size and hollow shape 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 PaleoproterozoicGreat Oxygenation Event, a time of marked increase in atmospheric oxygenation compared with that of the Archean.[4] If the central cavity of Diskagma housed a photosymbiont like living Geosiphon 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 2200 million years old, Diskagma was larger than coeval marine microbes of the Gunflint Chert, and more complex than stromatolites.
External links
- German Wikipedia article
- news coverage
- Geosiphon at Index Fungorum
- The Geosiphon pyriformis symbiosis - fungus 'eats' cyanobacterium Schuessler lab
- Glomeromycota phylogeny
References
- ↑ 1.0 1.1 Retallack, G.J., Krull, E.S., Thackray, G.D., and Parkinson, D. (2013). "Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa". Precambrian Research 235: 71–87. doi:10.1016/j.precamres.2013.05.015.
- ↑ Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., Hamilton, M.A. (2002). "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". Canadian Journal of Earth Sciences 39 (6): 999–1012. Bibcode:2002CaJES..39..999S. doi:10.1139/e02-016.
- ↑ Hedges, S.B., and Kumar, S. (2009). The time tree of life. Oxford University Press, New York.
- ↑ Murakami, T., Sreenivas, B., Sharma, S.D., and Sugimori, H. (2011). "Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics". Geochimica Cosmochimica Acta 75 (14): 3982–4004. Bibcode:2011GeCoA..75.3982M. doi:10.1016/j.gca.2011.04.023.