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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.

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 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.