'steep mountain bay') is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China.

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 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 later, during the Cambrian Explosion.

An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur bacteria resembling Thiomargarita, a bacterium so large that it is visible to the naked eye.

Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent "possible feedback relationships between evolutionary innovation and seawater chemistry" in which 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.