Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.

https://science.sciencemag.org/content/sci/334/6059/1091.full.pdf

The Cambrian conundrum: Early divergence and later ecological success in the early history of animals

Data are presented that support the idea of an oxygenation event in the immediate aftermath of the Marinoan glaciation, pre-dating previous estimates for post-Marinoan oxygenation by more than 50 million years. Macroscopic metazoans first appeared in the fossil record shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago. It has been suggested that an oxygenation event at about this time was the driving factor behind the rise of the metazoans, but current estimates suggest that oxygenation occurred between 580 million and 550 million years ago, well after initial animal diversification. New geochemical data from early Ediacaran organic-rich black shales of the basal Doushantuo Formation in South China now suggest that the oxidation event occurred more than 50 million years earlier, in the immediate aftermath of the Marinoan glaciation. The data provide evidence for a significant postglacial oxygenation and support a link between the most severe glaciations in Earth's history, the oxygenation of Earth's surface and the earliest emergence of complex animals. Metazoans are likely to have their roots in the Cryogenian period1,2,3, but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago4,5,6. It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity7,8. But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general9. Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial10. Here we report geochemical data from early Ediacaran organic-rich black shales (∼635–630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (Δ34S values ≥65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation11,12,13 by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth’s history, the oxygenation of the Earth’s surface environments, and the earliest diversification of animals.

Ocean oxygenation in the wake of the Marinoan glaciation

Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany)

Geochimica et Cosmochimica Acta

Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600–1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification 800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.

/pdf/paleobiological-perspectives-on-early-eukaryotic-evolution-2m3fkf9ix5.pdf

Paleobiological Perspectives on Early Eukaryotic Evolution

Abstract Much of our understanding of the history of life on Earth comes from fossil sites with exceptional preservation. Here, we use microscopy and chemical analysis of spiders found in the Oligocene Aix-en-Provence Formation, France, to unravel the taphonomic pathway responsible for the preservation of these arachnids. Microscopy revealed the presence of diatom mats and a black polymer on the body of the spiders, while elemental analysis revealed the polymer was composed of co-localized carbon and sulfur. We hypothesize that the extracellular polymeric substances produced by the diatoms found widely in this deposit could have helped promote the sulfurization of the spiders. Additionally, examination of similar Fossil-Lagerstätten reveals that this preservation pathway may be widespread across the Cenozoic and, if so, would be responsible for much of our understanding of insect, arachnid, amphibian, and plant life preserved in lacustrine settings. 

/pdf/the-exceptional-preservation-of-aix-en-provence-spider-1w25fmml.pdf

The exceptional preservation of Aix-en-Provence spider fossils could have been facilitated by diatoms

The rise of eukaryotic macroalgae in the late Mesoproterozoic to early Neoproterozoic was a critical development in Earth's history that triggered dramatic changes in biogeochemical cycles and benthic habitats, ultimately resulting in ecosystems habitable to animals. However, evidence of the diversification and expansion of macroalgae is limited by a biased fossil record. Non-mineralizing organisms are rarely preserved, occurring only in exceptional environments that favor fossilization. Investigating the taphonomy of well-preserved macroalgae will aid in identifying these target environments, allowing ecological trends to be disentangled from taphonomic overprints. Here we describe the taphonomy of macroalgal fossils from the Tonian Dolores Creek Formation (ca. 950 Ma) of northwestern Canada (Yukon Territory) that preserves cm-scale macroalgae. Analytical microscopy, including scanning electron microscopy and tomographic x-ray microscopy, was used to investigate fossil preservation, which was the result of a combination of pyritization and aluminosilicification, similar to accessory mineralization observed in Paleozoic Burgess Shale-type fossils. These new Neoproterozoic fossils help to bridge a gap in the fossil record of early algae, offer a link between the fossil and molecular record, and provide new insights into evolution during the Tonian Period, when many eukaryotic lineages are predicted to have diversified. 

https://www.nature.com/articles/s41598-022-10223-x.pdf

Preservation of early Tonian macroalgal fossils from the Dolores Creek Formation, Yukon

A contiguous record of the SPICE event, sea-level change and the first appearance of Fenghuangella laevis in Shandong Province, North China

With the 1859 publication of On the Origin of Species , Charles Darwin posed contention against his synthesis on the history of life. His dilemma specifically regarded that the geologically sudden appearance of complex shelly invertebrates at the Cambrian Explosion followed an incomprehensible absence of a long-standing gradual transition to such forms. Indeed, as quoted from Chapter 10 of the sixth edition, “To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer… the difficulty of assigning any good reason for the absence of vast piles of strata rich in fossils beneath the Cambrian system is very great… The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained” (p. 286–288). In the 155 years since this assertion, paleontologists focusing on the strata of the Ediacaran–Cambrian transition have uncovered a rich evolutionary history prior to the radiation of animals, but our resulting discoveries have neither been without debate nor unraveled the intricacies suggested by Darwin's dilemma. While we are continuing to learn from both geological and paleontological records, the organisms, their expanding ecosystem intricacy, and the …

https://bioone.org/journals/Journal-of-Paleontology/volume-88/issue-2/13-000I/An-Examination-of-Life-History-and-Behavioral-Evolution-Across-the/10.1666/13-000I.pdf

James D. Schiffbauer

Papers

The exceptional preservation of Aix-en-Provence spider fossils could have been facilitated by diatoms

Preservation of early Tonian macroalgal fossils from the Dolores Creek Formation, Yukon

A contiguous record of the SPICE event, sea-level change and the first appearance of Fenghuangella laevis in Shandong Province, North China

An Examination of Life History and Behavioral Evolution Across the Ediacaran–Cambrian Transition

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