A diverse Ediacara assemblage survived under low-oxygen conditions

Abstract

The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ238U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.

The role of iron in the formation of Ediacaran ‘death masks’

“…where bonds between the mineral and the substrate are stronger than those between the mineral and the local solution phase (Abraham, 1974;Chernov, 1984;De Yoreo & Vekilov, 2003;Hamm et al, 2014;Mullin, 1992;Mutaftschiev, 1993;Neilsen, 1967) or where mineral deposition reduces the exposure of an energetically unfavorable substrate-solution interface (Giuffre et al, 2013).Even in the absence of significant bacterial sulfate reduction at the onset of burial, we speculate that iron begins to form associations with (what we assume to be) organic sulfhydryl groups associated with buried organisms,presentingafirstlayerofFe-Sbeforecontinuedveneer growth fueled by rising HS − from the decay activity of SRB. These results show agreement with the preservation of some organic tubedwelling Ediacaran fossils (for example Schiffbauer et al, 2014Schiffbauer et al, , 2020.…”

“…Support for this model has been interpreted from Fe speciation and trace fossil data of Namibia (Wood et al, 2015). More recently, geochemical data from several end Ediacaran basins support widespread euxinic conditions concentrated around productive continental margins with associated fossils suggesting that some sessile, soft‐bodied Ediacara biota were capable of thriving in low‐oxygen environments (Cherry et al, 2022). Under this euxinic model, greater oxygenation of the ocean to more modern levels may have occurred well into the Phanerozoic (Lu et al, 2018; Wallace et al, 2017).…”

The Cambrian Explosion: macroevolution and biomineralization

“…The long-standing debate over the feeding strategies of the Ediacaran creatures has recently focused on four possible solutions to the problem: osmotrophy, chemoautotrophy, filter feeding, and mixotrophies (that may have included photosymbiosis). The recent discovery that a diverse Ediacaran assemblage could survive in low oxygen conditions indicates that at least some Ediacarans had a lifestyle that was facultatively anaerobic [38]. Biogeochemical challenges such as sulfide accumulation in tissue could have been solved in Ediacarans by symbiosis with sulfur-oxidizing bacteria [39].…”

New Ediacaran biota from the oldest Nama Group, Namibia (Tsaus Mountains), and re-definition of the Nama Assemblage

“…The Kanies and lower Mara members may pre-date deposition of the Khatyspyt Formation of the northern Siberian Platform, which has a maximum depositional age of ca. 550 Ma based on detrital zircon analyses of the underlying Maastakh Formation (Cherry et al 2022). The Kanies and lower Mara members may also pre-date the Shibantan Mb of the middle Dengying Formation, South China, based on sedimentation rate estimates in the Nama succession, chemostratigraphic correlation between the lower Nama Group and Dengying Formation, and a zircon U-Pb age of 550.1 ± 0.6 Ma from an ash bed within the upper Doushantuo Fm (Mb IV), which immediately underlies the Dengying Formation (Yang et al 2021).…”