Preservation and Taphonomy.

http://en.wikipedia.org/wiki/Maotianshan_Shales

Fossils occur in a section of mudstone fifty meters thick in the Yuanshan Member of the Qiongzhusi Formation. The Yuanshan Member is extensive, covering tens of thousands of square kilometers of eastern Yunnan Province, where there are numerous, scattered outcrops yielding fossils. Studies of the strata are consistent with a tropical environment with sea level changes and tectonic activity. The region is believed to have been a shallow sea with a muddy bottom. The preserved fauna is primarily benthic and was likely buried by periodic turbidity currents, since most fossils do not show evidence of post-mortem transport. Like the younger Burgess Shale fossils, the paleo-environment enabled preservation of non-mineralized, soft body parts. Fossils are found in thin layers less than an inch thick. The soft parts are preserved as aluminosilicate films, often with high oxidized iron content, and often exhibiting exquisite details.

The Chengjiang beds are very deeply weathered, as evidenced by their low specific gravity (i.e., they are very lightweight).Trace fossils are abundant.