Early animal evolution and highly oxygenated seafloor niches hosted by microbial mats.

The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550-541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fe), to quantify the seafloor oxygenation. Fe of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO levels. The highly oxygenated seafloor might be attributed to O production of the microbial mats. Despite the moderate atmospheric pO level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.