Doushantuo Formation
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The Doushantuo Formation is a lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils. The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran geological period and the more famous Cambrian Explosion. Taken as a whole, the Doushantuo Formation ranges from about 590 Ma at its base to about 565 Ma at its top, predating by perhaps five million years (Ma) the earliest of the 'classical' Ediacaran faunas from Mistaken Point, Newfoundland, and recording conditions a good forty to fifty million years before the Cambrian explosion.
The whole sequence sits on an unconformity with the underlying Liantuo Formation, which is free of fossils, an unconformity usually being interpreted as a period of erosion. On that lie tillites, (the Nantuo Formation) cemented glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Varangian glaciation ('Snowball Earth'). This glacial level is tentatively dated ca 610 - 590 Ma.
The Doushantuo formation itself has three layers representing marine 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.
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[edit] Fossils
Doushantuo fossils are all marine, microscopic, and highly preserved. 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 bilateral symmetry, a common feature in many modern-day animals which is usually assumed to have evolved during the later Cambrian Explosion. A nearly microscopic fossil animal, Vernanimalcula ("springtime micro-animal") was announced in October 2005, with the claim that it was the oldest known bilateral animal. However, the absence of adult forms of almost all animal types in the Doushantuo (there are microscopic adult 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.
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.
Documented biota now includes phosphatized microfossils of algae, multicellular thallophytes (seaweeds), acritarchs, and cyanophytes, besides adult sponges and adult cnidarians, which may include early forms of tabulate corals. There also seem to be what scientists cautiously report as bilateral animal embryos. Some of the possible animal embryos are in an early stage of cellular division that was first interpreted as spores or algal cells, including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges, a variety of adult cnidarians, and putative embryos of bilateral animals.
Recently, Baily, et. al., (2007) has offered evidence that Parapandorian, the putative embryos, and Megasphaera, putative eggs, could be fossils of giant sulfur bacteria like Thiomargarita, a bacterium so large that it is visible to the naked eye. The interpretation would also provide a mechanism for phosphatic fossilization through microbially mediated phosphate precipitation by the bacteria, which has been observed in modern environments. However, possible nuclei in the fossils would refute this bacterial interpretation.
[edit] See also
[edit] References
- Xiao, S., Zhang, Y. & Knoll, A. H. Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite. Nature 391, 553–558 (1998).
- Hagadorn, J. W. et al. Cellular and Subcellular Structure of Neoproterozoic Animal Embryos Science. 314, 291–294 (2006).
- Bailey, J. V., et. al. Evidence of giant sulphur bacteria in Neoproterozoic phosphorites Nature. 445, 198–201 (2007).
