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The primarily carbonate Gabbs Formation of west-central Nevada, U.S.A., remains an important, rare example of Panthalassan shallow-marine environments from the Late Triassic and through the Triassic–Jurassic boundary. Its relevance as a locality persists, particularly as the end-Triassic mass extinction interval is increasingly recognized as a carbonate crisis, evidenced by the global decline of carbonate facies and calcareous marine organisms. However, important transitions across key stratigraphic boundaries in this region have tended to be evaluated in isolation, producing an incomplete picture of sedimentation through the duration of the Late Triassic leading up to the geochemical crises and mass extinction at the close of the Triassic. In this work, multiple correlative measured sections are used to describe the facies in the mixed carbonate-siliciclastic ramp and characterize controls on carbonate facies. The outer ramp transition to inner ramp facies of the Gabbs represent a transgressive–regressive–transgressive cycle culminating in a transition to terrigenous inputs and a gradual decline of carbonate sediments stratigraphically below the mass extinction interval. This predictable loss of carbonate facies and the near continuous deposition of the Gabbs Formation allow for evidence of facies changes and acidifying conditions in the latest Triassic to be considered independently, in contrast to other global Late Triassic sections where depositional hiatuses or abrupt facies changes often compound these records. Establishing a baseline sedimentation for the Late Triassic demonstrates that the final loss of carbonate facies can be decoupled from the onset of acidifying conditions, resulting in a more precise timeline of latest Triassic environmental upheaval.

The latest Triassic was an interval of prolonged biotic extinction culminating in the end-Triassic Extinction (ETE). The ETE is now associated with a perturbation of the global carbon cycle just before the end of the Triassic that has been attributed to the extensive volcanism of the Circum-Atlantic Magmatic Province (CAMP). However, we attribute the onset of declining latest Triassic diversity to an older perturbation of the carbon cycle (delta13Corg) of global extent at or very close to the Norian/Rhaetian boundary (NRB). The NRB appears to be the culmination of stepwise biotic turnovers that characterize the latest Triassic and includes global extinctions of significant marine and terrestrial fossil groups. These biotic events across the NRB have been largely under- appreciated, yet together with a coeval disturbance of the carbon cycle were pivotal in the history of the Late Triassic. Here, we present new and published delta13Corg data from widespread sections (Italy, Greece, ODP, Australia, New Zealand, USA, Canada). These sections document a previously unknown perturbation in the carbon cycle of global extent that spanned the NRB. The disturbance extended across the Panthalassa Ocean to both sides of the Pangaean supercontinent and is recorded in both the Northern and Southern Hemispheres. The onset of stepwise Late Triassic extinctions coincides with carbon perturbation (delta13Corg) at the NRB, indicating that a combination of climatic and environmental changes impacted the biota at a global scale. The NRB event may have been triggered either by gas emissions from the eruption of a large igneous province pre-dating the NRB, by a bolide impact of significant size or by some alternative source of greenhouse gas emissions. As yet, it has not been possible to clearly determine which of these trigger scenarios was responsible; the evidence is insufficient to decisively identify the causal mechanism and merits further study.

The Norian Stage of the Late Triassic represents a long interval from which the benthic faunal succession is poorly understood, particularly from eastern Panthalassa. Fossiliferous bulk samples of shallow marine carbonates were collected from the Luning and Gabbs formations in west-central Nevada to evaluate the changes in faunal composition and paleoecological structure during the Norian Stage. Stationary epifauna dominated the early Norian faunal assemblages but gradually became less common by the late Norian, with the exception of cementing bivalves, which were common in the middle and late Norian. After the early Norian, mobile infauna also became increasingly abundant and diverse. These paleoecological trends are similar to those observed in Tethyan level-bottom carbonate deposits from the Lombardian Basin in northern Italy. Reclining epifauna remained uncommon in the local area until the Early Jurassic, and this Mesozoic decline in recliners preceded the end-Triassic mass extinction in Nevada.

The end-Triassic mass extinction was global, severe, and accompanied by worldwide disturbance to carbonate ramp and platform sedimentation. We examine the earliest known Jurassic carbonate ramp produced in the back-arc basin along NE Panthalassa following the extinction event to determine the biotic constituents and timing of local ecological recovery. Field observations, fossil surveys, and microfacies analysis focused on the Ferguson Hill Member (Hettangian and Sinemurian) of the Sunrise Formation in the New York Canyon area of west central Nevada, USA. In the Hettangian, post-extinction biosiliceous sedimentation extended to the inner ramp, where an ooid and grapestone shoal marked the outermost extent of a narrow belt of carbonate sedimentation. An early recovery phase in the late Hettangian is characterized by widespread, laterally homogeneous, demosponge-dominated level-bottom sedimentation across the mid- to inner-ramp, in addition to limited trophic tiering (sessile epifaunal suspension feeding and mobile infaunal deposit feeding), substantial ramp aggradation, and poor settling conditions for competitive benthic colonizers (e.g., corals, crinoids, infaunal bivalves). Within 1.6–2 Myr after the extinction (in the early Sinemurian), a late recovery phase is recognized by the appearance of epifaunal grazers (gastropods, echinoids) and suspension feeders (crinoids, solitary scleractinian corals), phototrophic microbialites (oncoids, and possibly photosymbionts within corals), and infaunal deposit or suspension feeders (bivalves). Although the late recovery faunas included more trophic levels than pre-extinction carbonate ramp habitats, development and progradation of the first Jurassic carbonate ramp still relied heavily on sponge, microbialite, and abiotic mineralization.

We describe three species of large scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (upper Campanian–lower Maastrichtian) of the Western Interior of North America. Each species occurs as two dimorphs, referred to as macroconch and microconch. All three species share a similar pattern of ornamentation consisting of long, thin, nonbifurcating ribs on the adoral part of the phragmocone, suggesting that they constitute a single monophyletic clade. Macroconchs of Hoploscaphites crassus (Coryell and Salmon, 1934) are characterized by a globose whorl section, with closely spaced ventrolateral tubercles on the body chamber, usually persisting to the aperture. Macroconchs of Hoploscaphites plenus (Meek and Hayden, 1860) differ from those of H. crassus in having a more subquadrate whorl section with flatter flanks, and fewer, larger, and more widely spaced ventrolateral tubercles. Macroconchs of Hoploscaphites peterseni, n. sp., closely resemble those of H. crassus, but differ in being nearly circular in side view with a more compressed whorl section. All three species lived at approximately the same time in the same general area and depositional environment. They are abundant in the Baculites baculus Zone but also occasionally occur in the B. eliasi Zone and possibly lower part of the B. grandis Zone. They are present in the Pierre Shale of east-central Montana and east-central Wyoming, the Lewis Shale of south-central Wyoming, and the Bearpaw Shale of northeast Montana. It is possible that these three species represent subspecies within a single species or a “flock” of very closely related species, similar to the “species flocks” observed in modern cichlid fishes.

The lower Member of the Wood Canyon Formation contains a variety of simple late Precambrian–early Cambrian dwelling and grazing traces, typical of early bioturbators. The traces, dominated by Planolites and Palaeophycus, are heavily abundant throughout sections comprised of higher energy siliciclastics. Two of which, Chicago Pass and Boundary Canyon (California, USA), also contain evidence for targeted microbial mat penetration by undermat mining organisms below and into the PC–C boundary. The co-occurence of penetrative burrows within microbial microlaminae, surficial trails, and dwelling structures is characteristic of the complex Ediacaran trace fossil, Lamonte trevallis, as described by Meyer et al., (2014) from the Dengying Formation of South China. The specimens from the Wood Canyon Formation present similar structures resulting from altering behaviors of epibenthic locomotion, undermat mining, and dwelling ( Meyer et al., 2014 ). Principally, grain size analyses confirmed the presence of microbial mat-targeted mining, seemingly resistant to overprinting by co-occurring horizontal traces. Therefore, we have classified the traces present in the California sections as Lamonte trevallis, broadening the geographic extent of this particular mining behavior in the Precambrian and the instances of complex bioturbating behaviors preceding the Cambrian substrate revolution.

The stratigraphic context and geochemical proxy potential are described for a newly reported early Jurassic marine iron ooid deposit in west central Nevada, U.S.A. Sedimentary analysis of the Ferguson Hill Member in the Sinemurian-aged Sunrise Formation was conducted in the Gabbs Valley Range of Nevada, U.S.A. Iron ooids were examined by transmitted light microscopy and electron scanning microscopy and found to consist of chamosite/berthierine and amorphous silica in cortical layers of the Sunrise Formation. Sedimentological characteristics of the uppermost beds of the Ferguson Hill Member show iron ooids are associated with transgressive deposits on the earliest-known first post-extinction metazoan carbonate factory identified in Eastern Panthalassa. The Ferguson Hill Member iron ooids are unique among most iron ooid deposits for being distributed grains within the carbonate. The formation of such a deposit required concentrations of iron and silica and fluctuating oxidation/reduction conditions sufficient to produce chamosite/berthierine and silica while not completely shutting down the shallow marine carbonate factory. For this reason, iron ooids observed in shallow carbonate sections serve as a useful indicator of shallow water geochemical conditions in the sedimentary record.

The Luning and Gabbs formations in west-central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Muller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index-species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare 87Sr/86Sr measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained 87Sr/86Sr record from Tethys, where a negative excursion in 87Sr/86Sr is noted across the Norian–Rhaetian boundary. Our new 87Sr/86Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in 87Sr/86Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.

The recently expanded Norian stage now encompasses nearly half of the entire Triassic period, making it the longest single stage of the Phanerozoic. Very little is known about the paleoecological dynamics of shallow marine level-bottom benthos within the stage, however. Successive bulk samples from the Lombardian Alps, Italy, reveal gradual changes in dominant taxa throughout the Norian, and paleoecological transitions consistent with the Mesozoic Marine Revolution (MMR) hypothesis. At the expense of stationary epifauna, mobile infauna diversified and became dominant by the end of the Norian. In addition to increases in stationary semi-infauna, abundant mobile epifauna were perhaps early alternatives to increasing both mobility and infaunality during the middle Norian. The particular suite of trends observed suggests that an adaptive response to epifaunal and demersal predators was operating in this system, and is coincident with the known taxonomic radiations of shell-crushing predators from this interval (lobsters, fish). Thus, the MMR was gradually intensifying as early as 25 myr before the Triassic-Jurassic boundary.