Ediacaran Oceanic Oxygenation may be Older Than Previously Thought

Stratigraphic position of the Ediacaran Miaohe biota and its constrains on the age of the upper Doushantuo δ13C anomaly in the Yangtze Gorges area, South China

Authors:

An et al

Abstract:

The siliceous shale unit that hosts the Ediacaran Miaohe biota in the Yangtze Gorges area, commonly referred to as the Miaohe Member, has an age of 551 ± 0.7 Ma. This unit is thought to be time-equivalent with the Doushantuo Member IV, which marks the top of the upper Doushantuo negative δ13C excursion. New bio-, sequence- and chemostratigraphic studies presented here demonstrate that the Miaohe Member is significantly younger than the Doushantuo Member IV, most likely time-equivalent with the lower Shibantan Member of the Dengying Formation. The new findings indicate that the top of the Doushantuo Formation, i.e., the top of the upper Doushantuo/Shuram δ13C excursion, should be much older than 551 Ma (likely ≥ 560 Ma). The ocean oxygenation event documented from the Doushantuo Member IV black shales should also be older than 560 Ma and predate the Miaohe biota for more than 10 Ma.

New Ediacaran Doushantuo Embryo-like fossils ARE Early Metazoans, Specifically Megasphaera

Are the new Ediacaran Doushantuo embryo-like fossils early metazoans? A reply

Authors:

Chen et al

Abstract:

Chen et al. (2014) described a suite of one-celled, Parapandorina-stage, Megaclonophycus-stage (some with dyads and tetrads), and matryoshka-stage fossils from the Doushantuo Formation, and interpreted them as representing a sequence of ontogenetic stages of the animal embryo-like fossil Megasphaera. Tang (2015) argues that the matryoshkas might have been parasites or symbionts, rather than developmental products of dyads and tetrads in Megaclonophycus-stage fossils. Assessing Tang's (2015) arguments against available evidence, we conclude that the matryoshkas likely represent an ontogenetic stage of Megasphaera. As such, they have the potential to illuminate the developmental biology, life cycle, and phylogenetic affinity of the enigmatic fossil Megasphaera.

Are the new Ediacaran Doushantuo Megasphaera-like Acritarchs Really Animals? Or Something Else?

Are the new Ediacaran Doushantuo Megasphaera-like acritarchs early metazoans?

Author:

Tang

Abstract:

A recent report by Chen et al. (Chen, L., Xiao, S., Pang, K., Zhou, C.M., Yuan, X.L., 2014. Cell differentiation and germ-soma separation in Ediacaran animal embryo-like fossils. Nature 516, 238-241) describes the new Ediacaran Doushantuo acritarchs with fascinating features. Nested within Megaclonophycus-like fossils that exhibit palintomic cell division are multicellular spheroid structures termed matryoshkas, which are interpreted to be germ cells. Such distinctive features of cellular differentiation and germ-soma separation suggest affinities of these with early stem group metazoans. In this commentary, I suggest some alternative interpretations to those elaborated by Chen et al. (2014).

Ediacaran Doushantuo Embryo-like Fossils are Only Either Algae or Animals

Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossils

Authors:

Chen et al

Abstract:

Phosphorites of the Ediacaran Doushantuo Formation (~600 million years old) yield spheroidal microfossils with a palintomic cell cleavage pattern. These fossils have been variously interpreted as sulphur-oxidizing bacteria, unicellular protists, mesomycetozoean-like holozoans, green algae akin to Volvox and blastula embryos of early metazoans or bilaterian animals. However, their complete life cycle is unknown and it is uncertain whether they had a cellularly differentiated ontogenetic stage, making it difficult to test their various phylogenetic interpretations. Here we describe new spheroidal fossils from black phosphorites of the Doushantuo Formation that have been overlooked in previous studies. These fossils represent later developmental stages of previously published blastula-like fossils, and they show evidence for cell differentiation, germ–soma separation, and programmed cell death. Their complex multicellularity is inconsistent with a phylogenetic affinity with bacteria, unicellular protists, or mesomycetozoean-like holozoans. Available evidence also indicates that the Doushantuo fossils are unlikely crown-group animals or volvocine green algae. We conclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely but more data are needed to constrain further the exact phylogenetic affinity of the Doushantuo fossils.

Ediacaran NeoProterozoic Doushantuo Biota Fossilization

Fossil preservation through phosphatization and silicification in the Ediacaran Doushantuo Formation (South China): A comparative synthesis

Authors:

Muscente et al

Abstract:

Phosphatized and silicified microfossils—preserved through replication of organic templates by authigenic calcium phosphate and silica, respectively—form through mechanistically similar mineralization processes but under notably different conditions. In order to characterize the environments and taphonomic processes associated with these preservational pathways, we compare phosphatized and silicified fossil assemblages in the Ediacaran Doushantuo Formation (South China) in terms of stratigraphic and paleogeographic distribution, preservation, and biodiversity. Several differences are recognized: (1) phosphatization and silicification both occur in sub-tidal Doushantuo sediments, but phosphatized fossils generally occur in shallower sub-tidal facies than silicified fossils; (2) although both phosphatized and silicified fossils are preserved three dimensionally, the latter are typically more compressed; and (3) whereas phosphatized assemblages are numerically dominated by multicellular acritarchs (putative metazoan eggs and embryos), silicified assemblages are dominated by prokaryotes. These differences can be explained by environmental and taphonomic distinctions. Phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates, and were likely fossilized near the suboxic/anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to P remineralization, phosphate burial via iron-pumping, and calcium phosphate precipitation. Reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial. In contrast, silicified fossils typically occur in chert nodules, which formed deeper in the sediment column—in ferruginous pore waters and perhaps under ferruginous bottom waters—where bacterial sulfate reduction and pyrite formation created low-pH conditions conducive to calcite dissolution and silica precipitation. Once formed, chert nodules mostly remained in situ, but some were reworked with turbidites and olistostromes developed in slope facies. Despite these distinctions, phosphatized and silicified fossil assemblages in the Doushantuo Formation generally overlap in time and space, and representatives of all major groups (and many common genera) of Doushantuo organisms are present in both phosphorites and cherts. Consequently, although both preservational pathways have paleoenvironmental and taphonomic biases, their relative biases on taxonomic presence/absence data are minimal given sufficient sampling efforts, and paleobiologic and biostratigraphic information based on presence/absence (but not abundance) data from the Doushantuo Formation appears to be robust.

Evidence Ediacaran NeoProterozoic Doushantuo Embryo Fossils may be Stem-Group Animals

Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossils

Authors:

Chen et al

Abstract:

Phosphorites of the Ediacaran Doushantuo Formation (~600 million years old) yield spheroidal microfossils with a palintomic cell cleavage pattern. These fossils have been variously interpreted as sulphur-oxidizing bacteria, unicellular protists4, mesomycetozoean-like holozoans green algae akin to Volvox and blastula embryos of early metazoans or bilaterian animals. However, their complete life cycle is unknown and it is uncertain whether they had a cellularly differentiated ontogenetic stage, making it difficult to test their various phylogenetic interpretations. Here we describe new spheroidal fossils from black phosphorites of the Doushantuo Formation that have been overlooked in previous studies. These fossils represent later developmental stages of previously published blastula-like fossils, and they show evidence for cell differentiation, germ–soma separation, and programmed cell death. Their complex multicellularity is inconsistent with a phylogenetic affinity with bacteria, unicellular protists, or mesomycetozoean-like holozoans. Available evidence also indicates that the Doushantuo fossils are unlikely crown-group animals or volvocine green algae. We conclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely but more data are needed to constrain further the exact phylogenetic affinity of the Doushantuo fossils.