Stable isotopic composition, paleoecology, and habitat of the ammonite Sphenodiscus lobatus in the Late Cretaceous (Maastrichtian) Western Interior Seaway

Despite their abundance as fossils, the life histories of ammonites are still poorly understood. We analyzed the oxygen (δ18O) and carbon (δ13C) isotopic composition of well-preserved shell material taken from different growth stages of the streamlined oxyconic ammonite species Sphenodiscus lobatus from the Upper Cretaceous (Maastrichtian) Pierre Shale and Fox Hills Formation of South Dakota. Pairing isotopic data with an analysis of the distribution of this species through a range of shallow marine environments allows us to reconstruct the life history and habitat of this ammonite in the Western Interior Seaway (WIS). When plotted ontogenetically using whorl height (WH) as a proxy for growth stage, δ18O values in the early ontogeny of S. lobatus are consistently depleted (-3 to -4‰), irrespective of facies and stratigraphic position. These results suggest a warm surface water habitat with lowered salinity. A distinct positive shift at a WH of ~40 mm in all specimens indicates a change to waters with a different isotopic composition – potentially migration to a more nektobenthic habitat in cooler waters. Oxygen isotope values remain highly variable (> 3‰ within individual specimens) throughout adulthood, indicating either a high degree of seasonality in the WIS or the migration of S. lobatus through different environments (vertically or laterally within the water column). Values of δ13C show no pattern and are variable throughout ontogeny in all specimens, either a result of differential incorporation of metabolic carbon through the lifetime of the animal or reflecting changes to the dissolved inorganic carbon pool in shallow water environments. Distribution within the Fox Hills Formation indicates a consistent pattern in which smaller specimens of S. lobatus are more commonly preserved in the offshore silts of the Little Eagle lithofacies of the Trail City Member, and larger specimens are more commonly preserved in the nearshore, time-equivalent Irish Creek lithofacies and the overlying sandy lithofacies of the Timber Lake Member. These data are consistent with the hypothesis that adult Sphenodiscus were adapted to inhabit shallow water environments.