220-10 Stratigraphy of Marine Reptile Horizons in the Anisian Fossil Hill Member (Middle Triassic, Nevada, USA) Reveals Trends in Panthalassic Ichthyosaur Diversity

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology (Posters)

Poster Booth No.: 136

Presenting Author:

Nicholas Counts

Authors:

Counts, Nicholas T.¹, Blouet, Jean-Philippe², Klein, Nicole³, Li, Qiang⁴, Li, Jun⁵, Sander, Martin P.⁶

(1) Southern Methodist University, Dallas, TX, USA, (2) Blouet Explorations, Nancy, USA, (3) State Museum of Natural History Stuttgart, Stuttgart, USA, (4) Division of Geology, Hefei University of Technology, Hefei, USA, (5) Division of Geology, Hefei University of Technology, Hefei, USA, (6) Section Paleontology, Bonn Institute of Organismic Biology, Univ Bonn, Bonn, , USA,

Abstract:

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 8 named ichthyosaur species and 3 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. This species represents 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, deep-water 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 P/Tr extinctions at 252 ma, the Fossil Hill fauna offers crucial insight into the biotic recovery, being the only early Middle Triassic lagerstätte in Panthalassa. A census based on field and collection records from >3 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 (total length: small <2.5 m; medium, 2.5 - 5 m; large, 5 - 15 m; giant: >15 m) assigned to each specimen reveal a clear trend. Moving upsection, small-bodied ichthyosaurs become more common while larger ichthyosaur occurrence decreases, although it is unclear if these trends have an evolutionary or ecological background. The dominance of large ichthyosaurs in the lowest horizons of the Fossil Hill Member reflects the ongoing recovery of pelagic marine communities in Panthalassa. Full recovery is documented by the third horizon, yielding the giant Thalattoarchon and C. youngorum, 9 million years into the Triassic. The trends we have identified will aid in focusing further field efforts in the Augusta Mountains and other Fossil Hill Member outcrops. Furthermore, studying these earliest ocean-going reptiles informs on how life in the World’s Ocean recovered from catastrophic collapse.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11227

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