LIFE AFTER THE “BORING BILLION”: THE FOSSIL RECORD IN THE TONIAN PERIOD (Invited Presentation)

The Tonian Period (~1000–720 Ma) has become a focus of recent geobiological investigations. Various molecular clock estimates suggest that animals may have diverged in the Tonian. Emerging geochemical data indicate that major environmental changes may have occurred in the Tonian: atmospheric oxygen content may have reached moderate levels to support animal metabolism, predominantly euxinic mid-depth seawaters may have transitioned to mostly ferruginous ones, and fundamental changes may have happened to the global phosphorus and nitrogen cycles. What does the fossil record tell us about life and its environment after the “boring billion”? To address this question, we carried out a systematic paleobiological investigation of Tonian successions in the southern margin of North China, and also analyzed the global taxonomic and morphological diversity of Tonian eukaryotes, including acritarchs and macroalgae. Although the taxonomic diversity of eukaryotes shows a notable increase in the Tonian Period compared with preceding geological ages, morphological disparity shows only a modest increase and is dwarfed by Ediacaran eukaryotes. Thus, the geobiological significance of Tonian fossils must be sought in their specific traits that are relevant to global biogeochemical cycles, rather than overall diversity. In this regard, the demonstration of the multicellular nature of certain Tonian fossils such as Chuaria, which was traditionally regarded as unicellular, and the discovery of filamentous cyanobacteria with differentiated akinetes (and, by implication, differentiated diazotrophic or nitrogen-fixing heterocysts) from Tonian strata in North China, are important. In particular, the rise of mutlicellular diazotrophic cyanobacteria with specialized nitrogen-fixing heterocysts may have been an evolutionary response to the combined geobiological effects of the increasing availability of bioessential metals and the rise in atmospheric oxygen levels in the Tonian Period.