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

Abstract

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 O2 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 (Fecarb), to quantify the seafloor oxygenation. Fecarb 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 O2 fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO2 levels. The highly oxygenated seafloor might be attributed to O2 production of the microbial mats. Despite the moderate atmospheric pO2 level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O2 barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.

The coupling of Phanerozoic continental weathering and marine phosphorus cycle

“…The Fe redox cycle in the ocean and continental weathering are related in that, high riverine flux of dissolved P would stimulate high surface ocean productivity 15 , which in turn enhances organic matter supply for MIR. On the other hand, high productivity also lowers O 2 fugacity at the sediment-water interface (SWI), enhancing the benthic Fe 2+ flux from sediment porewater to seawater [37][38][39][40] . Therefore, high P in favors repeated Fe reduction-oxidation cycles between seawater and sediment, enhancing the shuttling of seawater P into sediment.…”

Decoupling of morphological disparity and taxonomic diversity during the end-Permian mass extinction

“…Furthermore, a cyanobacteria-dominated microbial mat would be anoxic or euxinic below the surface, which is not an ideal environment for ostracods. Unlike the Ediacaran biota, which had some special adaptive abilities to withstand regular exposure to anoxia and daily cycles in oxygen production (Gingras et al 2011; Ding et al 2019), ostracods did not have the same adaptive capabilities. In addition, in modern thrombolites, metazoans that are well adapted to microbial habitats are typically motile and move around the surface of microbial mats to exploit spatial variance in oxygen availability (Tarhan et al 2013).…”

“…Implications for the Microbialite-Refuge Hypothesis.-Microbial mat refugia were suggested to have played a critical role in the evolution of life during the Ediacaran, in that they provided oxygenated oases in otherwise anoxic environments and would have been a source of nutrients for grazing metazoans (Gingras et al 2011;Ding et al 2019). This concept has also been applied to ostracod faunas from the end-Permian mass extinction interval to explain their unusually high diversity and abundance, as well as the survival of other metazoans in microbialite successions (Forel et al 2013b(Forel et al , 2015Crasquin and Forel 2014;Forel 2014).…”

Late Ediacaran organic microfossils from Finland

“…Chen et al 2013). This association has led to the hypothesis that Ediacaran bilaterians exploited such matgrounds as sources of nutrient concentration (Stanley, 1973;Seilacher 1999;Jensen et al 2005;Seilacher et al 2005;Buatois et al 2011;Gingras et al 2011;Meyer et al 2014;Evans et al 2019;Ivantsov et al 2019b), or as oxygen-rich microenvironments if photosynthetic (Canfield & Des Marais, 1993;McIlroy & Logan 1999;Gingras et al 2011;Ding et al 2019). Current accounts of Ediacaran matground habitats are almost entirely based on records from MISS, castand-mould fossils and trace fossils.…”