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1. New multicellular marine macroalgae from the early Tonian of northwestern Canada

   https://doi.org/10.1130/G48508.1  

   Maloney, Katie M. ; Halverson, Galen P. ; Schiffbauer, James D. ; Xiao, Shuhai ; Gibson, Timothy M. ; Lechte, Maxwell A. ; Cumming, Vivien M. ; Millikin, Alexie E.G. ; Murphy, Jack G. ; Wallace, Malcolm W. ; et al ( March 2021 , Geology)

   Abstract Molecular phylogenetic data suggest that photosynthetic eukaryotes first evolved in freshwater environments in the early Proterozoic and diversified into marine environments by the Tonian Period, but early algal evolution is poorly reflected in the fossil record. Here, we report newly discovered, millimeter- to centimeter-scale macrofossils from outer-shelf marine facies of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores Creek Formation in the Wernecke Mountains, northwestern Canada. These fossils, variably preserved by iron oxides and clay minerals, represent two size classes. The larger forms feature unbranching thalli with uniform cells, differentiated cell walls, longitudinal striations, and probable holdfasts, whereas the smaller specimens display branching but no other diagnostic features. While the smaller population remains unresolved phylogenetically and may represent cyanobacteria, we interpret the larger fossils as multicellular eukaryotic macroalgae with a plausible green algal affinity based on their large size and presence of rib-like wall ornamentation. Considered as such, the latter are among the few green algae and some of the largest macroscopic eukaryotes yet recognized in the early Neoproterozoic. Together with other Tonian fossils, the Dolores Creek fossils indicate that eukaryotic algae, including green algae, colonized marine environments by the early Neoproterozoic Era. 

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2. The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an “Explosion”

   https://doi.org/https://doi.org/10.1093/icb/icy088  

   Sperling, E.A. ; Stockey, R.G. ( January 2018 , Integrative and comparative biology)

   Animals originated and evolved during a unique time in Earth history—the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560–540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity—or at least lower quantities of organic carbon reaching the seafloor—compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the “fire triangle” metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event. 

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3. A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: Implications for its taphonomy and paleoecology

   https://doi.org/10.1111/gbi.12240  

   Bykova, N. ; Gill, B. C. ; Grazhdankin, D. ; Rogov, V. ; Xiao, S. ( April 2017 , Geobiology)

   The Ediacara biota features the rise of macroscopic complex life immediately before the Cambrian explosion. One of the most abundant and widely distributed elements of the Ediacara biota is the discoidal fossilAspidella, which is interpreted as a subsurface holdfast possibly anchoring a frondose epibenthic organism. It is a morphologically simple fossil preserved mainly in siliciclastic rocks, which are unsuitable for comprehensive stable isotope geochemical analyses to decipher its taphonomy and paleoecology. In this regard, three‐dimensionally preservedAspidellafossils from upper Ediacaran limestones of the Khatyspyt Formation in the Olenek Uplift of northern Siberia offer a rare opportunity to leverage geochemistry for insights into their taphonomy and paleoecology. To take advantage of this opportunity, we analyzed δ¹³C_carb, δ¹⁸O_carb, δ¹³C_org, δ³⁴S_pyr, and iron speciation of the KhatyspytAspidellafossils and surrounding sediment matrix in order to investigate whether they hosted microbial symbionts, how they were fossilized, and the redox conditions of their ecological environments.Aspidellaholdfasts and surrounding sediment matrix show indistinguishable δ¹³C_orgvalues, suggesting they did not host and derive significant amount of nutrients from microbial symbionts such as methanogens, methylotrophs, or sulfide‐oxidizing bacteria. δ¹³C_carb, δ¹⁸O_carb, and δ³⁴S_pyrdata, along with petrographic observations, suggest that microbial sulfate reduction facilitated the preservation ofAspidellaby promoting early authigenic calcite cementation in the holdfasts before matrix cementation and sediment compaction. Iron speciation data are equivocal, largely because of the low total iron concentrations. However, consideration of published sulfur isotope and biomarker data suggests thatAspidellalikely lived in non‐euxinic waters. It is possible thatAspidellawas an opportunistic organism, colonizing the seafloor in large numbers when paleoenvironments were favorable. This study demonstrates that geochemical data of Ediacaran fossils preserved in limestones can offer important insights into the taphonomy and paleoecology of these enigmatic organisms living on the eve of the Cambrian explosion.

    

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4. Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

   https://doi.org/10.1130/G49593.1  

   Millikin, Alexie E.G. ; Strauss, Justin V. ; Halverson, Galen P. ; Bergmann, Kristin D. ; Tosca, Nicholas J. ; Rooney, Alan D. ( February 2022 , Geology)

   Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history. 

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5. Common origin of sterol biosynthesis points to a feeding strategy shift in Neoproterozoic animals

   https://doi.org/10.1038/s41467-023-43545-z  

   Brunoir, T. ; Mulligan, C. ; Sistiaga, A. ; Vuu, K. M. ; Shih, P. M. ; O’Reilly, S. S. ; Summons, R. E. ; Gold, D. A. ( December 2023 , Nature Communications)

   Steranes preserved in sedimentary rocks serve as molecular fossils, which are thought to record the expansion of eukaryote life through the Neoproterozoic Era ( ~ 1000-541 Ma). Scientists hypothesize that ancient C₂₇steranes originated from cholesterol, the major sterol produced by living red algae and animals. Similarly, C₂₈and C₂₉steranes are thought to be derived from the sterols of prehistoric fungi, green algae, and other microbial eukaryotes. However, recent work on annelid worms–an advanced group of eumetazoan animals–shows that they are also capable of producing C₂₈and C₂₉sterols. In this paper, we explore the evolutionary history of the24-C sterol methyltransferase(smt) gene in animals, which is required to make C₂₈₊sterols. We find evidence that thesmtgene was vertically inherited through animals, suggesting early eumetazoans were capable of C₂₈₊sterol synthesis. Our molecular clock of the animalsmtgene demonstrates that its diversification coincides with the rise of C₂₈and C₂₉steranes in the Neoproterozoic. This study supports the hypothesis that early eumetazoans were capable of making C₂₈₊sterols and that many animal lineages independently abandoned its biosynthesis around the end-Neoproterozoic, coinciding with the rise of abundant eukaryotic prey.

    

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