Ediacara biota flourished in oligotrophic and bacterially dominated marine environments across Baltica

Abstract

Middle-to-late Ediacaran (575–541 Ma) marine sedimentary rocks record the first appearance of macroscopic, multicellular body fossils, yet little is known about the environments and food sources that sustained this enigmatic fauna. Here, we perform a lipid biomarker and stable isotope (δ15Ntotal and δ13CTOC) investigation of exceptionally immature late Ediacaran strata (<560 Ma) from multiple locations across Baltica. Our results show that the biomarker assemblages encompass an exceptionally wide range of hopane/sterane ratios (1.6–119), which is a broad measure of bacterial/eukaryotic source organism inputs. These include some unusually high hopane/sterane ratios (22–119), particularly during the peak in diversity and abundance of the Ediacara biota. A high contribution of bacteria to the overall low productivity may have bolstered a microbial loop, locally sustaining dissolved organic matter as an important organic nutrient. These oligotrophic, shallow-marine conditions extended over hundreds of kilometers across Baltica and persisted for more than 10 million years.

Food sources for the Ediacara biota communities

“…This observation in the White Sea area is consistent with a recent spatial analyses of~570 Ma old Ediacaran ecosystems on the Avalon Peninsula, overlapping with the White Sea area taxonomically, that showed that competition for resources was not the driving factor for local Ediacaran communities 44 . Unlike previously predicted 17,19,20 , bacteria-dominated ecosystems in Ediacaran marine basins were generally not a cause of the unusual appearance and ecology of the Ediacara biota. Rather, the ecological and evolutionary bridge leading from a mid-Proterozoic bacteria-dominated world to the appearance of Phanerozoic animal-dominated ecosystems was paved with algae-rich environments akin to those preferred by modern eumetazoan animals.…”

“…2f). It has been suggested 17 that the elevated H/S values in the interior seaway of the EEP, just as those from the Tonian 15,16 , might reflect a strong predominance of bacteria among primary producers, potentially due to regionally oligotrophic conditions, thus highlighting a high spatial heterogeneity in primary producer communities in Ediacaran marine environments (though see Supplementary Note 2) 17 .…”

“…Based on the H/S data from global Ediacaran sediments, the Ediacara biota might have either inhabited newly established algae-rich environments 15,16 or, conversely, thrived in nutrientdepleted ecosystems dominated by bacterial primary productivity akin to the Tonian or Mesoproterozoic 17,20 . If latter hypothesis is correct, then the diet and ecology of organisms of the Ediacara biota must have been very distinct from most Phanerozoic animals 17,19,20 , and it would invalidate the premise that the emergence of abundant algal food sources was crucial for the ecological success of eumetazoan animals. Biomarker data from sediments directly associated with Ediacara biota fossils can shed light on this dispute.…”

“…The Ediacara biota represents the first global appearance of large heterotrophic organisms, including animals, in the fossil record, and thus reflects aspects of the ecology and evolution of early animals. Recent studies have reported biomarkers from Ediacaran sediments of the East European Platform (EEP) with H/S ratios that are high relative to typical Phanerozoic marine conditions 17,18 . Consequently, it has been suggested that Ediacaran macroorganisms preferentially inhabited oligotrophic environments and used bacteria as their main energy source, as either food or symbionts 17,[19][20][21] .…”

“…Recent studies have reported biomarkers from Ediacaran sediments of the East European Platform (EEP) with H/S ratios that are high relative to typical Phanerozoic marine conditions 17,18 . Consequently, it has been suggested that Ediacaran macroorganisms preferentially inhabited oligotrophic environments and used bacteria as their main energy source, as either food or symbionts 17,[19][20][21] . It is notable, however, that none of the biomarker data came from localities that preserve Ediacaran macrofossils (Fig.…”