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Increased nitrate availability may have enabled ecological dominance of marine eukaryotes 800 million years ago. 

Sections of the Gabbs Formation exposed near New York Canyon, Nevada, have long been recognized as important sites for Late Triassic and Early Jurassic stratigraphy, and the Norian-Rhaetian parts of these sections continue to be important for defining this boundary (NRB). The two candidate sections for the base of the Rhaetian are in Tethys; both sections utilize the first occurrence of the conodont species Misikella posthernsteini as a proxy for the boundary. Although not a candidate section, data from New York Canyon will help to determine the most suitable position for the NRB, especially in Panthalassa. Previous reports of conodonts from New York Canyon recognized a fauna with Mockina englandi, Mo. bidentata and morphotypes of Mo. mosheri in the Nun Mine Member, below isolated occurrences of Zieglericonus rhaeticum and Mi. posthernsteini in the Mount Hyatt and Muller Canyon members. The first occurrence of Mi. posthernsteini in the section occurs well above the first occurrence of Rhaetian ammonoids (Paracochloceras amoenum) and together with late Rhaetian radiolarians. It is also above excursions in Sr- and C-isotopes, both of which correlate with Tethyan NRB excursions. Therefore, the NRB has previously been placed much lower in North America than Tethys, at the first occurrences of the radiolarian Proparvicingula moniliformis and the conodont Mo. mosheri morphotype C. To help reconcile the biochronological and geochemical data from New York Canyon, new conodont samples have been collected from the Nun Mine and Mt Hyatt members at the New York Canyon Road and Luning Draw sections. These samples contain: Mo. englandi, Mo. bidentata, and Mo. mosheri morphotypes B and C, all previously reported from New York Canyon, although this is the first record of Mo. mosheri morphotype C from the Nun Mine Member; Parvigondolella spp. B and C, from much lower in the Nun Mine Member than previously reported; and Pa. andrusovi, which has not previously been recorded from North America. Overall, this fauna represents the Mo. bidentata and Mo. mosheri zones of North America, equivalent to the Sevatian Mo. bidentata and Pa. andrusovi zones of Tethys. This would be consistent with a higher placement of the NRB at New York Canyon; however, if the NRB is to be recognized at the first occurrence of Mo. mosheri morphotype C, then the boundary must be lower than previously thought, within the Nun Mine Member. 

Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera. 

The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic.