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The Norian–Rhaetian boundary (Late Triassic) represents an important precursor extinction event to the end- Triassic mass extinction, but the biotic and geochemical shifts are not well-understood due to poor stratigraphic constraints. Here we examine the microfossil record for metazoans and protists on a Panthalassan carbonate ramp (Gabbs Formation, Nevada, U.S.A.) during the late Norian to mid-Rhaetian, and correlate changes in these assemblages with macrofossil shifts and geochemical data (strontium and carbon isotopes). In the latest Norian, demosponge spicules represent a small proportion of shallow marine biosediments. Demosponges are joined in the earliest Rhaetian by increasingly abundant hypersilicified sponge spicules and silica-limited hexactinellid sponge spicules synchronous with a negative strontium isotope excursion indicating increased hydrothermal or volcanic activity. Common carbonate microfossils such as echinoderm ossicles and ostracods are typically silicified in these deposits as well, suggesting increased silicic porewater. The source for increased dissolved silica in shallow marine systems is suggested to be hydrothermal vent degassing, likely associated with increased tectonic rifting activity. Mid-Rhaetian microfossil assemblages exhibit evidence for intermitted anoxia in reducing conditions, supporting a scenario in which environmental stress was a prolonged feature of much of the Rhaetian Stage, rather than a short-term event in the terminal Rhaetian. While there is no marine sedimentary record of volcanism recognized for this interval, biosedimentary assemblages may serve as proxies for geochemical conditions associated with rifting. 

The Late Triassic was a time of major evolutionary transition for marine vertebrates, with the emergence of important new clades and the expansion of durophagy. The diversity record of shallow marine vertebrates has been limited by poor preservation, and palaeogeographic ranges of common marine vertebrates is poorly constrained. Isolated ichthyoliths are an important resource for documenting taxonomic and ecological diversity in shallow marine environments and have a relatively high preservation potential. Here, we report the first Upper Triassic elasmobranchs, neopterygians and chondrosteans from Nevada, and contribute 14 new generic occurrences. The vertebrates represented in this survey include teeth of specialised durophages and piscivores, and a tentative reconstruction of the shallow marine ecosystem trophic niches based on dietary interpretations and known shelly macrofossils from this locality. The ichthyolith assemblage exhibited greater similarity to low-latitude Europe than high-latitude Canada localities, and the results of this study suggest that several taxa previously considered to be endemic to Europe may be cosmopolitan. The present study highlights the need for additional research in early Mesozoic ichthyoliths in order to establish diversity patterns, paleobiogeographic ranges, and timing of adaptive radiations among key groups of marine vertebrates in the Triassic Period in suboptimal preservation conditions.