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Doushantuo Formation

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. While its age has not been pinned down exactly, only one other lagerstätte in the world is known for certain to be older.

The formation is of particular interest because it appears to cover the boundary between the problematic Ediacaran organisms and the more famous Cambrian Explosion. The formation ranges from early Vendian age – about 590 Ma at its base to about 565 Ma at its top, predating by perhaps 5 million years (Ma) the earliest of the 'classical' Ediacaran faunas from Mistaken Point, Newfoundland, and recording conditions a good 40 to 50 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 represents marine sediments that formed as sea levels rose with the melting of the worldwide glaciation. Biomarkers indicate highly saline conditions, 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 lagerstatt itself) lies at the bottom of the middle stratum, with a date about 570 Ma.
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 bilaterality, a common feature in many modern-day animals which is usually assumed to have evolved during the later Cambrian Explosion. However, the absence of adult forms of all animal types (there are microscopic adult sponges and corals) makes these claims difficult to prove: some argue that their lack suggests they are not larval and embryonic forms at all; supporters contend that some process "filtered out" all but the smallest forms from fossilization.

The discovery was made when the rich phosphate deposits were being mined, and 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 'explosion' of metazoan life-forms and, therefore, that more remote ancestral forms must have existed.

Documented biota now includes phosphatized microfossils of algae,multicellular thallophytes (seaweeds), acritarchs, and cyanophyte[[s. Besides adult sponges and adult cnidarians, which may include early forms of [[tabulate coralsthere 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.

See also Phosphatic fossilization

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