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The role of oxygen as a driver for early animal evolution is widely debated. During the 21 Cambrian explosion, episodic radiations of major animal phyla occurred coincident with 22 repeated carbon isotope fluctuations. However, the driver of these isotope fluctuations and 23 potential links to environmental oxygenation are unclear. Here, we report high-resolution 24 carbon and sulphur isotope data for marine carbonates from the southeastern Siberian 25 Platform that document the canonical explosive phase of the Cambrian radiation from ~524 26 to ~514 Myr ago. These analyses demonstrate a strong positive covariation between 27 carbonate  13 C and carbonate-associated sulphate  34 S through five isotope cycles. 28 Biogeochemical modelling suggests that this isotopic coupling reflects periodic oscillations 29 in atmospheric O2 and the extent of shallow ocean oxygenation. Episodic maxima in the 30 biodiversity of animal phyla directly coincided with these extreme oxygen perturbations. 31 Conversely, the subsequent Botoman Toyonian animal extinction events (~514 to ~512 32Myr ago) coincided with decoupled isotope records that suggest a shrinking marine 33 sulphate reservoir and expanded shallow marine anoxia. We suggest that fluctuations in 34 oxygen availability in the shallow marine realm exerted a primary control on the timing and 35 tempo of biodiversity radiations at a crucial phase in the early history of animal life. 36 37The early Cambrian witnessed a dramatic diversification of animal body plans and 38 behaviours 1 , as well as between-species interactions and palaeocommunity innovations 2,3 , 39 ultimately leading to modern animal ecosystems. Ocean oxygenation is a commonly invoked 40 environmental pre-requisite 4 6 . However, some recent studies suggest that despite probable 41 low-oxygen conditions, the oceans exceeded requisite oxygen thresholds for simple animals, 42 such as sponges, well before the Cambrian Period 7,8 . Many of the new animal body plans and 43 lifestyles that appeared during the early Cambrian were associated with considerably higher 44 oxygen demands 9,10 . Fluctuations in the maximum dissolved oxygen content of surface 45 waters, or the extent of shallow ocean oxygenation, could therefore have played an important 46 role in regulating the pattern of Cambrian radiations. This brings into question the role of 47 55 single, large oxidised oceanic reservoir (dissolved sulphate and inorganic carbon), the isotopic 56 composition of which is governed by isotope fractionation during microbially-mediated 57 reduction to sulphide (ultimately preserved as pyrite) and organic carbon. Burial of these 58 reduced species represents the two main net sources of oxygen to the surface environment 12 59 14 , and also imprints on both the seawater sulphate sulphur isotope ( 34 S, as recorded by 60 carbonate-associated sulphate) and carbon isotope ( 13 C, as recorded in carbonate) records, 61 allowing redox changes in the surface environment to be traced through geologic time.62 63 Here we present...

A current paradigm accepts the presence of weakly biomineralized animals only, barely above a low metazoan grade of organization in the terminal

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Four sections from the mid-Atdabanian to lowest Toyonian (middle Cambrian) along the Lena River of Siberia were sampled for carbon isotope stratigraphy. These show a mainly heavy but highly oscillatory δ13C signature for the Atdabanian to mid-Botomian interval, coincident with the major phase of invertebrate innovation. A prolonged interval of negative δ13C followed until late Toyonian times, coincident with Botomian-Toyonian mass extinctions. Eleven carbon isotope cycles are identified through the lower Cambrian, which should now be tested for their utility in global correlation and relationship to bioevents in the Cambrian explosion.

Much uncertainty remains as to the temporal relationship between the Ediacaran and Cambrian biotas, yet this is critical to our understanding of the rise of metazoans. Here we present new high resolution carbon isotope chemostratigraphy and biostratigraphy for a terminal Ediacaran to Cambrian succession on the eastern Siberian Platform, Russia, which shows the presence of a succession of diverse fossil assemblages before the start of the basal Cambrian negative carbon isotope excursion (BACE). Softbodied Ediacaran biota (Beltanelliformis) occur before the start of the late Ediacaran positive carbon isotope plateau (EPIP), a mixed Ediacaran and Cambrian skeletal biota (Cloudina, Anabarities, Cambrotubulus) appear within the EPIP, and diverse Cambrian-type small shelly fossils including Protohertzina and other protocondonts, halkieriids, chancelloriids, hyoliths, hyolithelminthes and the burrowing trace fossil (Diplocraterion) appear at the beginning of the BACE. These integrated data show that taxa attributed to so-called Ediacaran and earliest Cambrian skeletal biotas in fact overlap without notable biotic turnover, and thus refute the presence of a large isotope excursion coincident with mass extinction of all Ediacaran biota. We propose a new biozone, the Cloudina-Namacalathus-Sinotubulites Assemblage Zone, to precede the known small shelly fossil (SSF) zones. These observations raise doubts as to whether there is any true separation between the Ediacaran and Cambrian skeletal biotas, and suggest that there is a deep root for the Cambrian Explosion of metazoans. Response to Reviewers:Many thanks for support from three reviewers that our work will be an excellent contribution to GEOLOGY, and their helpful comments and corrections on our manuscript. We considered the comments carefully and made necessary revisions and Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporationcorrections in text and figures. Major changes and responses to reviewers' comments are listed here, you will find more detail responses to reviews as attached file.

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