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Fossils, proxies, and models: Geobiology at critical transitions in the Proterozoic–Paleozoic

Data from paleontology, geochemistry, and sedimentology and stratig-raphy affirm that the geological record is punctuated by critical transitions. These transitions include oceanic and atmospheric oxygenation events, biological radiations and extinctions, and sedimentary environmental revolutions, which have significantly altered the evolutionary trajectories of the various life- forms and environments through our planet’s history. Given the importance of these transformative events for understanding geologic history and for predicting the response of the Earth system to major environmental change, the study of critical transitions represents an intellectually rich and impactful field of study. The geobiology of critical transitions continues to yield fun-damental knowledge regarding the diverse interactions that occur between the biosphere, lithosphere, and atmosphere. These interactions served as the theme for our topical session at the 2015 Annual Geological Society of America Meeting, wherein the papers presented in this special issue were initially reported. Collectively, the contributions in this special issue represent a cross- section of geologic time, ranging from the Mesoproterozoic (Manning- Berg & Kah, 2017) to the early Paleozoic (LoDuca, Bykova, Wu, Xiao, & Zhao, 2017). The articles investigate a scope of geobiolog-ical topics within this time frame, including proxies and models for the preservation of microbial mats (Manning- Berg & Kah, 2017; Newman al., 2017); new and unusual Cryogenian eukaryotic microfossils with agglutinated tests (Moore et al., 2017); Ediacaran paleoenviron-ments ranging in scale from ecosystems to localized sedimentary microenvironments