''Diskagma'' ("disc-like fragment") is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of problematic

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

from a

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

(2200 million years old)

paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

from

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

, and significant as the oldest likely

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

and earliest evidence for megascopic life on land.

Description

''Diskagma buttonii'' is a small fossil less than 1mm in length found within the surface horizon of a

Vertisol A vertisol, or vertosol, is a soil type in which there is a high content of expansive clay minerals, many of them known as montmorillonite, that form deep cracks in drier seasons or years. In a phenomenon known as argillipedoturbation, alternat ...

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 An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

tomography Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantu ...

using a cyclotron source 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 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 The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...

complexity found in

s, but it predates the other fossil candidate for the oldest eukaryote ''

Grypania ''Grypania'' is an early, tube-shaped fossil from the Proterozoic eon. The organism, with a size over one centimeter and consistent form, could have been a giant bacterium, a bacterial colony, or a eukaryotic alga. The oldest probable ''Grypania' ...

'', now known to be 1800 million years old, and at 2200 million years old is much older than

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleoti ...

estimates for eukaryotes of 1600 million years. Another similar fossil is ''

Horodyskia ''Horodyskia'' is a fossilised organism found in rocks dated from to . Its shape has been described as a "string of beads" connected by a very fine thread. It is considered one of the oldest known eukaryotes. Biology Comparisons of differe ...

''. The size and hollow shape of ''Diskagma'' are similar to the living fungus ''

Geosiphon ''Geosiphon'' is a genus of fungus in the family Geosiphonaceae. The genus is monotypic, containing the single species ''Geosiphon pyriformis'', first described by Kützing in 1849 as ''Botrydium pyriforme''. In 1915, Von Wettstein characterized ...

'', which is endosymbiotic with the

cyanobacterium Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...

Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...

''. However, the apical cup and filaments are not seen in modern ''

''.

Paleoenvironmental significance

''Diskagma buttonii'' dates to the

Great Oxygenation Event The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...

, a time of marked increase in atmospheric oxygenation compared with that of the Archean. If, like the living ''

'', 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 Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

, ''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

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...

References

External links

news coverage
*
Geosiphon
' at

Index Fungorum ''Index Fungorum'' is an international project to index all formal names ( scientific names) in the fungus kingdom. the project is based at the Royal Botanic Gardens, Kew, one of three partners along with Landcare Research and the Institute of M ...

The ''Geosiphon pyriformis'' symbiosis - fungus 'eats' cyanobacterium
Schuessler lab
Glomeromycota phylogeny
{{Taxonbar, from=Q16981390 Glomeromycota Monotypic fungi genera Fungus genera Controversial taxa