Doushantuo Formation

The Doushantuo Formation is a recently discovered lagerstätte in Guizhou Province, China that is most notable for its scientific contributions in the hunt for Precambrian life. Doushantuo is of particular interest because its fossils, dating from about 565 to 590 million years ago, predate the Cambrian Explosion by at least 20 million years. The most fossiliferous zones are estimated to be 570 million years old.

Some Doushantuo fossils are so exquisitely preserved through Phosphatic fossilization that cellular structures are visible under magnification, including soft tissues of putative metazoan embryos and algae. Perhaps the most significant and still contentious finding from the Doushantuo fossils is evidence of bilateral symmetry, a key characteristic in many modern animals. Just when bilateral symmetry appears (Precambrian or during the Cambrian) is an important question in evolution. Vernanimalcula (meaning springtime micro-animal) has been suggested as the oldest known bilateral animal in the fossil record. If so, the fossils would validate the widely held view that the major evolutionary diversification of animals already had occurred prior to the Cambrian period where a explosion of metazoan life-forms apparently occurred. Other researchers such as Bengtson and Budd (2004) offer evidence that Vernanimalcula may be a taphonomic artifact produced by growth of phosphate within a spherical object such as an acritarch.

The biota of Doushantuo comprises algae, multicellular thallophytes (seaweeds), acritarchs, and cyanophytes, poriferans (sponges), cnidarians, including possible tabulate corals. Most important are the putative bilateral animal embryos. Some of these possible animal embryos apparently record the early stage of cellular division. There are also putative bilateral animal embryos, some of which appear to be in an early stage of cellular division. This is also in dispute, as Baily and co-workers, (2007) argued that the putative embryos, Parapandorian, and putative eggs, Megasphaera, could be fossils of giant sulfur bacteria similar to Thiomargarita, a bacterium so large that it is visible to the naked eye. Baily's interpretation also provides a mechanism for phosphatic fossilization found at Doushantuo through microbially mediated phosphate precipitation by the bacteria.

Of particular interest is that putative embryos from Doushantuo Formation can be digitally dissected into their component cells, and even the sub-cellular organelles examined. The fossils bear similarities to modern organisms at early stages of development. However, they lack The absence of ectodermal and mesodermal sheets of tissue at later stages, suggesting that these animals are very primitive, and that they are preserved remains of the embryos of a stem group of metazoans.

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