Doushantuo Formation

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Doushantuo Formation
Ma[1]
A purported Ediacaran embryo contained within an acritarch from the Doushantuo Formation
UnderliesDengying Formation
OverliesNantuo Formation
ThicknessUp to 400 m; usually around 200 to 250 m
Lithology
PrimaryShale
OtherMudstone, marl, carbonate or phosphate minerals
Location
RegionSouth China
CountryChina
Type section
Named forDoushantuo, Hubei
Named byLi Siguang and Zhao Yazeng [zh]
Year defined1924[2]
Part of a series on
The Cambrian explosion
Fossil localities
Key organisms
Ediacaran biota
Burgess-type
Small shelly fauna
Evolutionary concepts
Trends
Themes

The Doushantuo Formation (formerly

Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic
.

Sedimentology

The whole sequence sits on an

glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Marinoan glaciation (also known as Varangian glaciation, this is the second and last of a series of very extensive glaciations during a period called the Cryogenian -- named because 'Snowball Earth
' conditions at the time). This latest Cryogenian glacial level is tentatively dated ca 654 (660 ± 5) — 635 Ma (million years ago).

The Doushantuo formation itself has three layers representing aquatic sediments that formed as sea levels rose with the melting of worldwide glaciation. Biomarkers indicate highly saline conditions, such as might be found in a lagoon, low oxygen levels, and very little sediment that had been washed off land surfaces.

The richest finds (the Lagerstätte itself) lie at the bottom of the middle stratum, with a date about 570 Ma, thus from some time after the great

Gaskiers glaciation
of [585 ± 1 - 582.1 ± 0.4 Ma].

Fossils

Further information:
Fossil embryos

Doushantuo fossils are all aquatic, microscopic, and preserved to a great degree of detail. The latter two characteristics mean that the structure of the organisms that made them can be studied at the cellular level, and considerable insight has been gained into the embryonic and larval stages of many early creatures. One contentious claim is that many of the fossils show signs of

sponges and corals) makes these claims difficult to prove: some argue that their lack suggests these finds are not larval and embryonic forms at all; supporters contend that some unidentified process "filtered out" all but the smallest forms from fossilization. An alternative interpretation suggests that it was created by non-biological rock-forming processes.[8] The team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found ten specimens (not illustrated) of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.[9]

The discovery was made when the rich phosphate deposits were being mined, and was first reported in 1998. The finds offer direct evidence that confirms expectations that major evolutionary diversification of animals already had occurred before the onset of the Cambrian period, with its apparent 'explosion' of

metazoan life-forms and, therefore, that more remote ancestral forms of the phyla recognizable in Cambrian macrofossils must have existed previously.[citation needed
]

The documented

cnidarians
, as well as adult sponges and a variety of adult cnidarians.

An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur

phosphatic fossilization through microbially mediated phosphate precipitation by the bacteria, which has been observed in modern environments. If dark spots in the fossil transpire to be fossilised nuclei - an unlikely claim[12] - this would refute the Thiomargarita hypothesis. That being said, recent comparisons of the Doushantuo fossils to modern decaying Thiomargarita and expired sea urchin embryos shows little similarity between the fossils and decaying bacterial cells.[13]

Only about one-twentieth of the site's fossils have been excavated. The fossil beds are threatened by increasing intensity of

phosphate mining operations in the area. A workshop led in protest by local paleontologists resulted in a temporary halt to the mining in 2017.[14]

Palaeogeography

The formation was laid down on a carbonate shelf, whose rim enclosed a lagoon between tidal flats on the shore, and the deeper ocean. This lagoon was periodically anoxic or

euxinic (containing hydrogen sulfide); variations in the chemistry in the lagoon can be detected in isotopic and elemental abundance cycles in the rock and possibly contributed to the fossil preservation.[1]

Geochemistry

The most recent Doushantuo rocks show a sharp decrease in the 13C/12C carbon isotope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a

metazoans (multi-celled organisms) removed carbon from the water, which increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans.[15]

See also

  • Phosphatic fossilization

Footnotes

  1. ^
    doi:10.1016/j.gr.2011.01.006
    .
  2. ^
    doi:10.1111/j.1755-6724.1924.mp33-4004.x
    .
  3. S2CID 85771456
    .
  4. doi:10.1016/j.palaeo.2007.03.028
    . Retrieved 16 October 2022.
  5. S2CID 129367730
    .
  6. doi:10.1016/j.precamres.2009.03.009
    . Retrieved 16 October 2022.
  7. PMID 15178752
    .
  8. PMID 15550644
    .
  9. S2CID 84743928
    .
  10. OCLC 156823511
  11. S2CID 4346066
    .
  12. doi:10.1130/G32546.1
    .
  13. ^ Reardon, Sara (6 December 2011). "Dead Fossils Tell the Best Tales". ScienceNOW. Archived from the original on 8 June 2013.
  14. ^ Cyranoski, David. "Mining threatens Chinese fossil site that revealed planet's earliest animals". Nature. Retrieved 20 April 2017.
  15. S2CID 11673032
    .

References

External links
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