Stratigraphy of Marine Reptile Horizons in the Anisian Fossil Hill Member (middle Triassic, Nevada, Usa)

The Fossil Hill Member is a marine rock unit dating to the middle and late Anisian (Middle Triassic) located in Central Nevada, USA. The ichthyosaur-dominated assemblage preserved in the Fossil Hill Member is notable for its diversity, with eight named ichthyosaur species and three further species awaiting description. This ichthyosaur biota is especially significant for its large morphological and size disparity, with small-bodied mixosaurs (1–2 m) co-occurring with the 17-meter Cymbospondylus youngorum, the earliest giant marine vertebrate and the largest known animal to that point in time. The macrophagous predatory ichthyosaur Thalattoarchon, the first marine tetrapod to occupy this niche, was of similar body size. The dark muddy limestones and shales of the Fossil Hill Member indicate an anoxic, deepwater environment open to the global Panthalassic Ocean. The base of the section in the Augusta Mountains is ca. 244 Ma old, and the top ca. 242 Ma. Given its close chronological proximity to the Permian-Triassic extinction at ca. 252 Ma, the Fossil Hill fauna offers crucial insight into the biotic recovery in the open ocean, being the only early Middle Triassic Lagerstätte in Panthalassa. A census based on field and collection records from more than three decades of field work includes 91 ichthyosaur and four other reptile skeletons from the Fossil Hill Member’s 250-meter outcrop, but their distribution in the section has remained unclear. In a composite stratigraphic section, we found six stratigraphic horizons producing skeletons and plotted their abundance and size distribution. Broad size categories assigned to each specimen reveal a clear trend. Moving upsection, small-bodied ichthyosaurs become more common while larger ichthyosaur occurrence decreases. More detailed study of these early ocean-going tetrapods will inform on how life in the open ocean recovered from catastrophic collapse.