STABLE ISOTOPIC COMPOSITION, PALEOECOLOGY, AND HABITAT OF THE AMMONITE SPHENODISCUS LOBATUS IN THE UPPER CRETACEOUS (MAASTRICHTIAN) WESTERN INTERIOR SEAWAY, USA

The abundantly fossiliferous Late Cretaceous marine sediments of the epicontinental U.S. Western Interior Seaway (WIS) preserve an excellent record of the biotic evolution of ammonoid cephalopods (ammonites). One of the most common ammonites in Maastrichtian (72-66 Ma) deposits is the oxyconic genus Sphenodiscus. The mode of life, and ecology of ammonites is an area of ongoing debate due to the lack of soft tissue preservation in the fossil record. We analyzed the oxygen (δ¹⁸O) and carbon (δ¹³C) isotopic composition of well-preserved shell material from different growth stages of Sphenodiscus lobatus from the Pierre Shale and Fox Hills Formation of South Dakota. Pairing isotopic data with analysis of fossil distribution allows us to reconstruct the paleoecology and habitat of this species. Distribution within the Fox Hills Formation indicates that smaller Sphenodiscus lobatus are consistently preserved in the offshore silts of the Little Eagle lithofacies of the Trail City Member, with larger specimens found in the nearshore equivalent Irish Creek lithofacies and the overlying sandy Timber Lake Member. When plotted ontogenetically using whorl height (WH) as a proxy for growth, δ¹⁸O values in early ontogeny of Sphenodiscus lobatus are depleted (-3 to -4‰), irrespective of facies and stratigraphic position. These data suggest a warm surface water habitat, and/or an environment with lowered salinity. A positive shift at a WH of ~40 mm in all specimens indicates a change to waters with a different isotopic composition – perhaps a cooler, more nektobenthic habitat. Oxygen isotope values remain highly variable (>3‰ within individual specimens) throughout adulthood, indicating either high seasonality or migration of S. lobatus through different environments (vertically within the water column or spatially). Shell δ¹³C values show no pattern and are variable throughout ontogeny, a result of differential incorporation of metabolic carbon through the lifetime of the animal.