Paleoenvironments of the earliest animal fossils

The Ediacaran period (635 to 543 Ma) represents an evolutionary juncture between the Precambrian world-dominated by microbial single-celled organisms, and the Phanerozoic Era, which saw the expansion of macroscopic multicellular organisms and the development and diversification of animals. The Doushantuo Formation (635 to 551 Ma) of South China provides a unique fossil record of early animal evolution and developmental biology in this period. In this study, the mineralogy and geochemistry of Doushantuo Formation sediments deposited in an intrashelf basin in the Yangtze Gorges area, which host the oldest animal embryo fossils, are examined to determine paleoenvironmental conditions in which these fossils lived.;A suite of neoformed saponite, diagenetic corrensite and chlorite dominates the clay fraction that makes up to 30% of the sediments from the Yangtze Gorges area. Textural and compositional analysis of saponitic clays show that they are the transformation products of detrital precursors, a process that does not occur under modern oceanic conditions and typically require high pH conditions (∼9 or more) commonly found in alkaline lakes. These clays are not found in mudstones from temporally equivalent basin sections from the craton margin >200 km to the south, or in the upper part of the succession in the Yangtze Gorges, indicating that alkaline conditions were temporally and spatial restricted. Other independent geochemical indicators are consistent with idea that this basin was isolated from the ocean and developed a unique chemistry, including; the absence of enrichment of redox-sensitive elements in anoxic saponitic mudstones, as well as a 10‰ difference in the carbon isotope record of coeval sediments from shelf and basin sections.;These findings suggest that non-marine conditions may have been hospitable for the earliest animals. The alkaline chemistry favoring transformation of clays also provides a mechanism for concentrating silica, a source for the permineralization and preservation of fossils. The results also highlight the importance of understanding the affects of local conditions on geochemical proxies before attempting to interpret them in terms global environmental change.