THE RECIPE FOR BURGESS SHALE-TYPE FOSSIL PRESERVATION

Exceptionally preserved biotas found in the Burgess Shale and other similar Cambrian deposits, provide rare but critical insights into the early diversification of the Metazoa. The extraordinary preservation of the labile tissues of these soft-bodied fossils has remained unexplained since Walcott’s initial discovery in 1909, although several hypotheses have been offered. The mechanism of Burgess Shale-type preservation is demonstrated using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and other principal Burgess Shale-type deposits. Extraordinary fossilization of organic remains as carbonaceous compressions resulted from early restriction of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability, while rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. As demonstrated clearly by δ³⁴S_pyrite, these combined effects resulted in early restriction of microbial decomposition and the conservation of soft-bodied assemblages as carbonaceous remains.